/*
 * GPL HEADER START
 *
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 only,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License version 2 for more details (a copy is included
 * in the LICENSE file that accompanied this code).
 *
 * You should have received a copy of the GNU General Public License
 * version 2 along with this program; If not, see
 * http://www.gnu.org/licenses/gpl-2.0.html
 *
 * GPL HEADER END
 */
/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 * Use is subject to license terms.
 *
 * Copyright (c) 2011, 2015, Intel Corporation.
 */
/*
 * This file is part of Lustre, http://www.lustre.org/
 * Lustre is a trademark of Sun Microsystems, Inc.
 */

#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>

#define DEBUG_SUBSYSTEM S_LLITE

#include "../include/obd_support.h"
#include "../include/lustre_dlm.h"
#include "llite_internal.h"

#define SA_OMITTED_ENTRY_MAX 8ULL

enum se_stat {
        /** negative values are for error cases */
        SA_ENTRY_INIT = 0,      /** init entry */
        SA_ENTRY_SUCC = 1,      /** stat succeed */
        SA_ENTRY_INVA = 2,      /** invalid entry */
};

/*
 * sa_entry is not refcounted: statahead thread allocates it and do async stat,
 * and in async stat callback ll_statahead_interpret() will add it into
 * sai_interim_entries, later statahead thread will call sa_handle_callback() to
 * instantiate entry and move it into sai_entries, and then only scanner process
 * can access and free it.
 */
struct sa_entry {
        /* link into sai_interim_entries or sai_entries */
        struct list_head              se_list;
        /* link into sai hash table locally */
        struct list_head              se_hash;
        /* entry index in the sai */
        __u64              se_index;
        /* low layer ldlm lock handle */
        __u64              se_handle;
        /* entry status */
        enum se_stat            se_state;
        /* entry size, contains name */
        int                  se_size;
        /* pointer to async getattr enqueue info */
        struct md_enqueue_info *se_minfo;
        /* pointer to the async getattr request */
        struct ptlrpc_request  *se_req;
        /* pointer to the target inode */
        struct inode       *se_inode;
        /* entry name */
        struct qstr          se_qstr;
};

static unsigned int sai_generation;
static DEFINE_SPINLOCK(sai_generation_lock);

/* sa_entry is ready to use */
static inline int sa_ready(struct sa_entry *entry)
{
        smp_rmb();
        return (entry->se_state != SA_ENTRY_INIT);
}

/* hash value to put in sai_cache */
static inline int sa_hash(int val)
{
        return val & LL_SA_CACHE_MASK;
}

/* hash entry into sai_cache */
static inline void
sa_rehash(struct ll_statahead_info *sai, struct sa_entry *entry)
{
        int i = sa_hash(entry->se_qstr.hash);

        spin_lock(&sai->sai_cache_lock[i]);
        list_add_tail(&entry->se_hash, &sai->sai_cache[i]);
        spin_unlock(&sai->sai_cache_lock[i]);
}

/*
 * Remove entry from SA table.
 */
static inline void
sa_unhash(struct ll_statahead_info *sai, struct sa_entry *entry)
{
        int i = sa_hash(entry->se_qstr.hash);

        spin_lock(&sai->sai_cache_lock[i]);
        list_del_init(&entry->se_hash);
        spin_unlock(&sai->sai_cache_lock[i]);
}

static inline int agl_should_run(struct ll_statahead_info *sai,
                                 struct inode *inode)
{
        return (inode && S_ISREG(inode->i_mode) && sai->sai_agl_valid);
}

/* statahead window is full */
static inline int sa_sent_full(struct ll_statahead_info *sai)
{
        return atomic_read(&sai->sai_cache_count) >= sai->sai_max;
}

/* got async stat replies */
static inline int sa_has_callback(struct ll_statahead_info *sai)
{
        return !list_empty(&sai->sai_interim_entries);
}

static inline int agl_list_empty(struct ll_statahead_info *sai)
{
        return list_empty(&sai->sai_agls);
}

/**
 * (1) hit ratio less than 80%
 * or
 * (2) consecutive miss more than 8
 * then means low hit.
 */
static inline int sa_low_hit(struct ll_statahead_info *sai)
{
        return ((sai->sai_hit > 7 && sai->sai_hit < 4 * sai->sai_miss) ||
                (sai->sai_consecutive_miss > 8));
}

/*
 * if the given index is behind of statahead window more than
 * SA_OMITTED_ENTRY_MAX, then it is old.
 */
static inline int is_omitted_entry(struct ll_statahead_info *sai, __u64 index)
{
        return ((__u64)sai->sai_max + index + SA_OMITTED_ENTRY_MAX <
                 sai->sai_index);
}

/* allocate sa_entry and hash it to allow scanner process to find it */
static struct sa_entry *
sa_alloc(struct dentry *parent, struct ll_statahead_info *sai, __u64 index,
         const char *name, int len)
{
        struct ll_inode_info *lli;
        struct sa_entry   *entry;
        int                entry_size;
        char             *dname;

        entry_size = sizeof(struct sa_entry) + (len & ~3) + 4;
        entry = kzalloc(entry_size, GFP_NOFS);
        if (unlikely(!entry))
                return ERR_PTR(-ENOMEM);

        CDEBUG(D_READA, "alloc sa entry %.*s(%p) index %llu\n",
               len, name, entry, index);

        entry->se_index = index;
        entry->se_state = SA_ENTRY_INIT;
        entry->se_size = entry_size;
        dname = (char *)entry + sizeof(struct sa_entry);
        memcpy(dname, name, len);
        dname[len] = 0;

        entry->se_qstr.hash = full_name_hash(parent, name, len);
        entry->se_qstr.len = len;
        entry->se_qstr.name = dname;

        lli = ll_i2info(sai->sai_dentry->d_inode);
        spin_lock(&lli->lli_sa_lock);
        INIT_LIST_HEAD(&entry->se_list);
        sa_rehash(sai, entry);
        spin_unlock(&lli->lli_sa_lock);

        atomic_inc(&sai->sai_cache_count);

        return entry;
}

/* free sa_entry, which should have been unhashed and not in any list */
static void sa_free(struct ll_statahead_info *sai, struct sa_entry *entry)
{
        CDEBUG(D_READA, "free sa entry %.*s(%p) index %llu\n",
               entry->se_qstr.len, entry->se_qstr.name, entry,
               entry->se_index);

        LASSERT(list_empty(&entry->se_list));
        LASSERT(list_empty(&entry->se_hash));

        kfree(entry);
        atomic_dec(&sai->sai_cache_count);
}

/*
 * find sa_entry by name, used by directory scanner, lock is not needed because
 * only scanner can remove the entry from cache.
 */
static struct sa_entry *
sa_get(struct ll_statahead_info *sai, const struct qstr *qstr)
{
        struct sa_entry *entry;
        int i = sa_hash(qstr->hash);

        list_for_each_entry(entry, &sai->sai_cache[i], se_hash) {
                if (entry->se_qstr.hash == qstr->hash &&
                    entry->se_qstr.len == qstr->len &&
                    memcmp(entry->se_qstr.name, qstr->name, qstr->len) == 0)
                        return entry;
        }
        return NULL;
}

/* unhash and unlink sa_entry, and then free it */
static inline void
sa_kill(struct ll_statahead_info *sai, struct sa_entry *entry)
{
        struct ll_inode_info *lli = ll_i2info(sai->sai_dentry->d_inode);

        LASSERT(!list_empty(&entry->se_hash));
        LASSERT(!list_empty(&entry->se_list));
        LASSERT(sa_ready(entry));

        sa_unhash(sai, entry);

        spin_lock(&lli->lli_sa_lock);
        list_del_init(&entry->se_list);
        spin_unlock(&lli->lli_sa_lock);

        if (entry->se_inode)
                iput(entry->se_inode);

        sa_free(sai, entry);
}

/* called by scanner after use, sa_entry will be killed */
static void
sa_put(struct ll_statahead_info *sai, struct sa_entry *entry)
{
        struct sa_entry *tmp, *next;

        if (entry && entry->se_state == SA_ENTRY_SUCC) {
                struct ll_sb_info *sbi = ll_i2sbi(sai->sai_dentry->d_inode);

                sai->sai_hit++;
                sai->sai_consecutive_miss = 0;
                sai->sai_max = min(2 * sai->sai_max, sbi->ll_sa_max);
        } else {
                sai->sai_miss++;
                sai->sai_consecutive_miss++;
        }

        if (entry)
                sa_kill(sai, entry);

        /*
         * kill old completed entries, only scanner process does this, no need
         * to lock
         */
        list_for_each_entry_safe(tmp, next, &sai->sai_entries, se_list) {
                if (!is_omitted_entry(sai, tmp->se_index))
                        break;
                sa_kill(sai, tmp);
        }

        wake_up(&sai->sai_thread.t_ctl_waitq);
}

/*
 * update state and sort add entry to sai_entries by index, return true if
 * scanner is waiting on this entry.
 */
static bool
__sa_make_ready(struct ll_statahead_info *sai, struct sa_entry *entry, int ret)
{
        struct list_head *pos = &sai->sai_entries;
        __u64 index = entry->se_index;
        struct sa_entry *se;

        LASSERT(!sa_ready(entry));
        LASSERT(list_empty(&entry->se_list));

        list_for_each_entry_reverse(se, &sai->sai_entries, se_list) {
                if (se->se_index < entry->se_index) {
                        pos = &se->se_list;
                        break;
                }
        }
        list_add(&entry->se_list, pos);
        entry->se_state = ret < 0 ? SA_ENTRY_INVA : SA_ENTRY_SUCC;

        return (index == sai->sai_index_wait);
}

/*
 * release resources used in async stat RPC, update entry state and wakeup if
 * scanner process it waiting on this entry.
 */
static void
sa_make_ready(struct ll_statahead_info *sai, struct sa_entry *entry, int ret)
{
        struct ll_inode_info *lli = ll_i2info(sai->sai_dentry->d_inode);
        struct md_enqueue_info *minfo = entry->se_minfo;
        struct ptlrpc_request *req = entry->se_req;
        bool wakeup;

        /* release resources used in RPC */
        if (minfo) {
                entry->se_minfo = NULL;
                ll_intent_release(&minfo->mi_it);
                iput(minfo->mi_dir);
                kfree(minfo);
        }

        if (req) {
                entry->se_req = NULL;
                ptlrpc_req_finished(req);
        }

        spin_lock(&lli->lli_sa_lock);
        wakeup = __sa_make_ready(sai, entry, ret);
        spin_unlock(&lli->lli_sa_lock);

        if (wakeup)
                wake_up(&sai->sai_waitq);
}

/* Insert inode into the list of sai_agls. */
static void ll_agl_add(struct ll_statahead_info *sai,
                       struct inode *inode, int index)
{
        struct ll_inode_info *child  = ll_i2info(inode);
        struct ll_inode_info *parent = ll_i2info(sai->sai_dentry->d_inode);
        int                added  = 0;

        spin_lock(&child->lli_agl_lock);
        if (child->lli_agl_index == 0) {
                child->lli_agl_index = index;
                spin_unlock(&child->lli_agl_lock);

                LASSERT(list_empty(&child->lli_agl_list));

                igrab(inode);
                spin_lock(&parent->lli_agl_lock);
                if (list_empty(&sai->sai_agls))
                        added = 1;
                list_add_tail(&child->lli_agl_list, &sai->sai_agls);
                spin_unlock(&parent->lli_agl_lock);
        } else {
                spin_unlock(&child->lli_agl_lock);
        }

        if (added > 0)
                wake_up(&sai->sai_agl_thread.t_ctl_waitq);
}

/* allocate sai */
static struct ll_statahead_info *ll_sai_alloc(struct dentry *dentry)
{
        struct ll_inode_info *lli = ll_i2info(dentry->d_inode);
        struct ll_statahead_info *sai;
        int                    i;

        sai = kzalloc(sizeof(*sai), GFP_NOFS);
        if (!sai)
                return NULL;

        sai->sai_dentry = dget(dentry);
        atomic_set(&sai->sai_refcount, 1);

        sai->sai_max = LL_SA_RPC_MIN;
        sai->sai_index = 1;
        init_waitqueue_head(&sai->sai_waitq);
        init_waitqueue_head(&sai->sai_thread.t_ctl_waitq);
        init_waitqueue_head(&sai->sai_agl_thread.t_ctl_waitq);

        INIT_LIST_HEAD(&sai->sai_interim_entries);
        INIT_LIST_HEAD(&sai->sai_entries);
        INIT_LIST_HEAD(&sai->sai_agls);

        for (i = 0; i < LL_SA_CACHE_SIZE; i++) {
                INIT_LIST_HEAD(&sai->sai_cache[i]);
                spin_lock_init(&sai->sai_cache_lock[i]);
        }
        atomic_set(&sai->sai_cache_count, 0);

        spin_lock(&sai_generation_lock);
        lli->lli_sa_generation = ++sai_generation;
        if (unlikely(!sai_generation))
                lli->lli_sa_generation = ++sai_generation;
        spin_unlock(&sai_generation_lock);

        return sai;
}

/* free sai */
static inline void ll_sai_free(struct ll_statahead_info *sai)
{
        LASSERT(sai->sai_dentry);
        dput(sai->sai_dentry);
        kfree(sai);
}

/*
 * take refcount of sai if sai for @dir exists, which means statahead is on for
 * this directory.
 */
static inline struct ll_statahead_info *ll_sai_get(struct inode *dir)
{
        struct ll_inode_info *lli = ll_i2info(dir);
        struct ll_statahead_info *sai = NULL;

        spin_lock(&lli->lli_sa_lock);
        sai = lli->lli_sai;
        if (sai)
                atomic_inc(&sai->sai_refcount);
        spin_unlock(&lli->lli_sa_lock);

        return sai;
}

/*
 * put sai refcount after use, if refcount reaches zero, free sai and sa_entries
 * attached to it.
 */
static void ll_sai_put(struct ll_statahead_info *sai)
{
        struct ll_inode_info *lli = ll_i2info(sai->sai_dentry->d_inode);

        if (atomic_dec_and_lock(&sai->sai_refcount, &lli->lli_sa_lock)) {
                struct ll_sb_info *sbi = ll_i2sbi(sai->sai_dentry->d_inode);
                struct sa_entry *entry, *next;

                lli->lli_sai = NULL;
                spin_unlock(&lli->lli_sa_lock);

                LASSERT(thread_is_stopped(&sai->sai_thread));
                LASSERT(thread_is_stopped(&sai->sai_agl_thread));
                LASSERT(sai->sai_sent == sai->sai_replied);
                LASSERT(!sa_has_callback(sai));

                list_for_each_entry_safe(entry, next, &sai->sai_entries,
                                         se_list)
                        sa_kill(sai, entry);

                LASSERT(atomic_read(&sai->sai_cache_count) == 0);
                LASSERT(list_empty(&sai->sai_agls));

                ll_sai_free(sai);
                atomic_dec(&sbi->ll_sa_running);
        }
}

/* Do NOT forget to drop inode refcount when into sai_agls. */
static void ll_agl_trigger(struct inode *inode, struct ll_statahead_info *sai)
{
        struct ll_inode_info *lli   = ll_i2info(inode);
        __u64            index = lli->lli_agl_index;
        int                rc;

        LASSERT(list_empty(&lli->lli_agl_list));

        /* AGL maybe fall behind statahead with one entry */
        if (is_omitted_entry(sai, index + 1)) {
                lli->lli_agl_index = 0;
                iput(inode);
                return;
        }

        /* Someone is in glimpse (sync or async), do nothing. */
        rc = down_write_trylock(&lli->lli_glimpse_sem);
        if (rc == 0) {
                lli->lli_agl_index = 0;
                iput(inode);
                return;
        }

        /*
         * Someone triggered glimpse within 1 sec before.
         * 1) The former glimpse succeeded with glimpse lock granted by OST, and
         *    if the lock is still cached on client, AGL needs to do nothing. If
         *    it is cancelled by other client, AGL maybe cannot obtain new lock
         *    for no glimpse callback triggered by AGL.
         * 2) The former glimpse succeeded, but OST did not grant glimpse lock.
         *    Under such case, it is quite possible that the OST will not grant
         *    glimpse lock for AGL also.
         * 3) The former glimpse failed, compared with other two cases, it is
         *    relative rare. AGL can ignore such case, and it will not muchly
         *    affect the performance.
         */
        if (lli->lli_glimpse_time != 0 &&
            time_before(cfs_time_shift(-1), lli->lli_glimpse_time)) {
                up_write(&lli->lli_glimpse_sem);
                lli->lli_agl_index = 0;
                iput(inode);
                return;
        }

        CDEBUG(D_READA, "Handling (init) async glimpse: inode = "
               DFID", idx = %llu\n", PFID(&lli->lli_fid), index);

        cl_agl(inode);
        lli->lli_agl_index = 0;
        lli->lli_glimpse_time = cfs_time_current();
        up_write(&lli->lli_glimpse_sem);

        CDEBUG(D_READA, "Handled (init) async glimpse: inode= "
               DFID", idx = %llu, rc = %d\n",
               PFID(&lli->lli_fid), index, rc);

        iput(inode);
}

/*
 * prepare inode for sa entry, add it into agl list, now sa_entry is ready
 * to be used by scanner process.
 */
static void sa_instantiate(struct ll_statahead_info *sai,
                           struct sa_entry *entry)
{
        struct inode *dir = sai->sai_dentry->d_inode;
        struct inode       *child;
        struct md_enqueue_info *minfo;
        struct lookup_intent   *it;
        struct ptlrpc_request  *req;
        struct mdt_body *body;
        int                  rc    = 0;

        LASSERT(entry->se_handle != 0);

        minfo = entry->se_minfo;
        it = &minfo->mi_it;
        req = entry->se_req;
        body = req_capsule_server_get(&req->rq_pill, &RMF_MDT_BODY);
        if (!body) {
                rc = -EFAULT;
                goto out;
        }

        child = entry->se_inode;
        if (!child) {
                /*
                 * lookup.
                 */
                LASSERT(fid_is_zero(&minfo->mi_data.op_fid2));

                /* XXX: No fid in reply, this is probably cross-ref case.
                 * SA can't handle it yet.
                 */
                if (body->mbo_valid & OBD_MD_MDS) {
                        rc = -EAGAIN;
                        goto out;
                }
        } else {
                /*
                 * revalidate.
                 */
                /* unlinked and re-created with the same name */
                if (unlikely(!lu_fid_eq(&minfo->mi_data.op_fid2, &body->mbo_fid1))) {
                        entry->se_inode = NULL;
                        iput(child);
                        child = NULL;
                }
        }

        it->it_lock_handle = entry->se_handle;
        rc = md_revalidate_lock(ll_i2mdexp(dir), it, ll_inode2fid(dir), NULL);
        if (rc != 1) {
                rc = -EAGAIN;
                goto out;
        }

        rc = ll_prep_inode(&child, req, dir->i_sb, it);
        if (rc)
                goto out;

        CDEBUG(D_READA, "%s: setting %.*s" DFID " l_data to inode %p\n",
               ll_get_fsname(child->i_sb, NULL, 0),
               entry->se_qstr.len, entry->se_qstr.name,
               PFID(ll_inode2fid(child)), child);
        ll_set_lock_data(ll_i2sbi(dir)->ll_md_exp, child, it, NULL);

        entry->se_inode = child;

        if (agl_should_run(sai, child))
                ll_agl_add(sai, child, entry->se_index);

out:
        /*
         * sa_make_ready() will drop ldlm ibits lock refcount by calling
         * ll_intent_drop_lock() in spite of failures. Do not worry about
         * calling ll_intent_drop_lock() more than once.
         */
        sa_make_ready(sai, entry, rc);
}

/* once there are async stat replies, instantiate sa_entry from replies */
static void sa_handle_callback(struct ll_statahead_info *sai)
{
        struct ll_inode_info *lli;

        lli = ll_i2info(sai->sai_dentry->d_inode);

        while (sa_has_callback(sai)) {
                struct sa_entry *entry;

                spin_lock(&lli->lli_sa_lock);
                if (unlikely(!sa_has_callback(sai))) {
                        spin_unlock(&lli->lli_sa_lock);
                        break;
                }
                entry = list_entry(sai->sai_interim_entries.next,
                                   struct sa_entry, se_list);
                list_del_init(&entry->se_list);
                spin_unlock(&lli->lli_sa_lock);

                sa_instantiate(sai, entry);
        }
}

/*
 * callback for async stat, because this is called in ptlrpcd context, we only
 * put sa_entry in sai_cb_entries list, and let sa_handle_callback() to really
 * prepare inode and instantiate sa_entry later.
 */
static int ll_statahead_interpret(struct ptlrpc_request *req,
                                  struct md_enqueue_info *minfo, int rc)
{
        struct lookup_intent     *it  = &minfo->mi_it;
        struct inode         *dir = minfo->mi_dir;
        struct ll_inode_info     *lli = ll_i2info(dir);
        struct ll_statahead_info *sai = lli->lli_sai;
        struct sa_entry *entry = (struct sa_entry *)minfo->mi_cbdata;
        wait_queue_head_t *waitq = NULL;
        __u64 handle = 0;

        if (it_disposition(it, DISP_LOOKUP_NEG))
                rc = -ENOENT;

        /*
         * because statahead thread will wait for all inflight RPC to finish,
         * sai should be always valid, no need to refcount
         */
        LASSERT(sai);
        LASSERT(!thread_is_stopped(&sai->sai_thread));
        LASSERT(entry);

        CDEBUG(D_READA, "sa_entry %.*s rc %d\n",
               entry->se_qstr.len, entry->se_qstr.name, rc);

        if (rc) {
                ll_intent_release(it);
                iput(dir);
                kfree(minfo);
        } else {
                /*
                 * release ibits lock ASAP to avoid deadlock when statahead
                 * thread enqueues lock on parent in readdir and another
                 * process enqueues lock on child with parent lock held, eg.
                 * unlink.
                 */
                handle = it->it_lock_handle;
                ll_intent_drop_lock(it);
        }

        spin_lock(&lli->lli_sa_lock);
        if (rc) {
                if (__sa_make_ready(sai, entry, rc))
                        waitq = &sai->sai_waitq;
        } else {
                entry->se_minfo = minfo;
                entry->se_req = ptlrpc_request_addref(req);
                /*
                 * Release the async ibits lock ASAP to avoid deadlock
                 * when statahead thread tries to enqueue lock on parent
                 * for readpage and other tries to enqueue lock on child
                 * with parent's lock held, for example: unlink.
                 */
                entry->se_handle = handle;
                if (!sa_has_callback(sai))
                        waitq = &sai->sai_thread.t_ctl_waitq;

                list_add_tail(&entry->se_list, &sai->sai_interim_entries);
        }
        sai->sai_replied++;

        if (waitq)
                wake_up(waitq);
        spin_unlock(&lli->lli_sa_lock);

        return rc;
}

/* finish async stat RPC arguments */
static void sa_fini_data(struct md_enqueue_info *minfo,
                         struct ldlm_enqueue_info *einfo)
{
        LASSERT(minfo && einfo);
        iput(minfo->mi_dir);
        kfree(minfo);
        kfree(einfo);
}

/**
 * prepare arguments for async stat RPC.
 */
static int sa_prep_data(struct inode *dir, struct inode *child,
                        struct sa_entry *entry, struct md_enqueue_info **pmi,
                        struct ldlm_enqueue_info **pei)
{
        const struct qstr      *qstr = &entry->se_qstr;
        struct md_enqueue_info   *minfo;
        struct ldlm_enqueue_info *einfo;
        struct md_op_data       *op_data;

        einfo = kzalloc(sizeof(*einfo), GFP_NOFS);
        if (!einfo)
                return -ENOMEM;

        minfo = kzalloc(sizeof(*minfo), GFP_NOFS);
        if (!minfo) {
                kfree(einfo);
                return -ENOMEM;
        }

        op_data = ll_prep_md_op_data(&minfo->mi_data, dir, child, qstr->name,
                                     qstr->len, 0, LUSTRE_OPC_ANY, NULL);
        if (IS_ERR(op_data)) {
                kfree(einfo);
                kfree(minfo);
                return PTR_ERR(op_data);
        }

        minfo->mi_it.it_op = IT_GETATTR;
        minfo->mi_dir = igrab(dir);
        minfo->mi_cb = ll_statahead_interpret;
        minfo->mi_cbdata = entry;

        einfo->ei_type   = LDLM_IBITS;
        einfo->ei_mode   = it_to_lock_mode(&minfo->mi_it);
        einfo->ei_cb_bl  = ll_md_blocking_ast;
        einfo->ei_cb_cp  = ldlm_completion_ast;
        einfo->ei_cb_gl  = NULL;
        einfo->ei_cbdata = NULL;

        *pmi = minfo;
        *pei = einfo;

        return 0;
}

/* async stat for file not found in dcache */
static int sa_lookup(struct inode *dir, struct sa_entry *entry)
{
        struct md_enqueue_info   *minfo;
        struct ldlm_enqueue_info *einfo;
        int                    rc;

        rc = sa_prep_data(dir, NULL, entry, &minfo, &einfo);
        if (rc)
                return rc;

        rc = md_intent_getattr_async(ll_i2mdexp(dir), minfo, einfo);
        if (rc)
                sa_fini_data(minfo, einfo);

        return rc;
}

/**
 * async stat for file found in dcache, similar to .revalidate
 *
 * \retval      1 dentry valid, no RPC sent
 * \retval      0 dentry invalid, will send async stat RPC
 * \retval      negative number upon error
 */
static int sa_revalidate(struct inode *dir, struct sa_entry *entry,
                         struct dentry *dentry)
{
        struct inode         *inode = d_inode(dentry);
        struct lookup_intent      it = { .it_op = IT_GETATTR,
                                         .it_lock_handle = 0 };
        struct md_enqueue_info   *minfo;
        struct ldlm_enqueue_info *einfo;
        int rc;

        if (unlikely(!inode))
                return 1;

        if (d_mountpoint(dentry))
                return 1;

        entry->se_inode = igrab(inode);
        rc = md_revalidate_lock(ll_i2mdexp(dir), &it, ll_inode2fid(inode),
                                NULL);
        if (rc == 1) {
                entry->se_handle = it.it_lock_handle;
                ll_intent_release(&it);
                return 1;
        }

        rc = sa_prep_data(dir, inode, entry, &minfo, &einfo);
        if (rc) {
                entry->se_inode = NULL;
                iput(inode);
                return rc;
        }

        rc = md_intent_getattr_async(ll_i2mdexp(dir), minfo, einfo);
        if (rc) {
                entry->se_inode = NULL;
                iput(inode);
                sa_fini_data(minfo, einfo);
        }

        return rc;
}

/* async stat for file with @name */
static void sa_statahead(struct dentry *parent, const char *name, int len)
{
        struct inode         *dir    = d_inode(parent);
        struct ll_inode_info     *lli    = ll_i2info(dir);
        struct ll_statahead_info *sai    = lli->lli_sai;
        struct dentry       *dentry = NULL;
        struct sa_entry *entry;
        int                    rc;

        entry = sa_alloc(parent, sai, sai->sai_index, name, len);
        if (IS_ERR(entry))
                return;

        dentry = d_lookup(parent, &entry->se_qstr);
        if (!dentry) {
                rc = sa_lookup(dir, entry);
        } else {
                rc = sa_revalidate(dir, entry, dentry);
                if (rc == 1 && agl_should_run(sai, d_inode(dentry)))
                        ll_agl_add(sai, d_inode(dentry), entry->se_index);
        }

        if (dentry)
                dput(dentry);

        if (rc)
                sa_make_ready(sai, entry, rc);
        else
                sai->sai_sent++;

        sai->sai_index++;
}

/* async glimpse (agl) thread main function */
static int ll_agl_thread(void *arg)
{
        struct dentry       *parent = arg;
        struct inode         *dir    = d_inode(parent);
        struct ll_inode_info     *plli   = ll_i2info(dir);
        struct ll_inode_info     *clli;
        struct ll_sb_info       *sbi    = ll_i2sbi(dir);
        struct ll_statahead_info *sai;
        struct ptlrpc_thread *thread;
        struct l_wait_info      lwi    = { 0 };

        sai = ll_sai_get(dir);
        thread = &sai->sai_agl_thread;
        thread->t_pid = current_pid();
        CDEBUG(D_READA, "agl thread started: sai %p, parent %pd\n",
               sai, parent);

        atomic_inc(&sbi->ll_agl_total);
        spin_lock(&plli->lli_agl_lock);
        sai->sai_agl_valid = 1;
        if (thread_is_init(thread))
                /* If someone else has changed the thread state
                 * (e.g. already changed to SVC_STOPPING), we can't just
                 * blindly overwrite that setting.
                 */
                thread_set_flags(thread, SVC_RUNNING);
        spin_unlock(&plli->lli_agl_lock);
        wake_up(&thread->t_ctl_waitq);

        while (1) {
                l_wait_event(thread->t_ctl_waitq,
                             !list_empty(&sai->sai_agls) ||
                             !thread_is_running(thread),
                             &lwi);

                if (!thread_is_running(thread))
                        break;

                spin_lock(&plli->lli_agl_lock);
                /* The statahead thread maybe help to process AGL entries,
                 * so check whether list empty again.
                 */
                if (!list_empty(&sai->sai_agls)) {
                        clli = list_entry(sai->sai_agls.next,
                                          struct ll_inode_info, lli_agl_list);
                        list_del_init(&clli->lli_agl_list);
                        spin_unlock(&plli->lli_agl_lock);
                        ll_agl_trigger(&clli->lli_vfs_inode, sai);
                } else {
                        spin_unlock(&plli->lli_agl_lock);
                }
        }

        spin_lock(&plli->lli_agl_lock);
        sai->sai_agl_valid = 0;
        while (!list_empty(&sai->sai_agls)) {
                clli = list_entry(sai->sai_agls.next,
                                  struct ll_inode_info, lli_agl_list);
                list_del_init(&clli->lli_agl_list);
                spin_unlock(&plli->lli_agl_lock);
                clli->lli_agl_index = 0;
                iput(&clli->lli_vfs_inode);
                spin_lock(&plli->lli_agl_lock);
        }
        thread_set_flags(thread, SVC_STOPPED);
        spin_unlock(&plli->lli_agl_lock);
        wake_up(&thread->t_ctl_waitq);
        ll_sai_put(sai);
        CDEBUG(D_READA, "agl thread stopped: sai %p, parent %pd\n",
               sai, parent);
        return 0;
}

/* start agl thread */
static void ll_start_agl(struct dentry *parent, struct ll_statahead_info *sai)
{
        struct ptlrpc_thread *thread = &sai->sai_agl_thread;
        struct l_wait_info    lwi    = { 0 };
        struct ll_inode_info  *plli;
        struct task_struct *task;

        CDEBUG(D_READA, "start agl thread: sai %p, parent %pd\n",
               sai, parent);

        plli = ll_i2info(d_inode(parent));
        task = kthread_run(ll_agl_thread, parent, "ll_agl_%u",
                           plli->lli_opendir_pid);
        if (IS_ERR(task)) {
                CERROR("can't start ll_agl thread, rc: %ld\n", PTR_ERR(task));
                thread_set_flags(thread, SVC_STOPPED);
                return;
        }

        l_wait_event(thread->t_ctl_waitq,
                     thread_is_running(thread) || thread_is_stopped(thread),
                     &lwi);
}

/* statahead thread main function */
static int ll_statahead_thread(void *arg)
{
        struct dentry       *parent = arg;
        struct inode         *dir    = d_inode(parent);
        struct ll_inode_info     *lli   = ll_i2info(dir);
        struct ll_sb_info       *sbi    = ll_i2sbi(dir);
        struct ll_statahead_info *sai;
        struct ptlrpc_thread *sa_thread;
        struct ptlrpc_thread *agl_thread;
        struct page           *page = NULL;
        __u64                pos    = 0;
        int                    first  = 0;
        int                    rc     = 0;
        struct md_op_data *op_data;
        struct l_wait_info      lwi    = { 0 };

        sai = ll_sai_get(dir);
        sa_thread = &sai->sai_thread;
        agl_thread = &sai->sai_agl_thread;
        sa_thread->t_pid = current_pid();

        CDEBUG(D_READA, "statahead thread starting: sai %p, parent %pd\n",
               sai, parent);

        op_data = ll_prep_md_op_data(NULL, dir, dir, NULL, 0, 0,
                                     LUSTRE_OPC_ANY, dir);
        if (IS_ERR(op_data)) {
                rc = PTR_ERR(op_data);
                goto out;
        }

        op_data->op_max_pages = ll_i2sbi(dir)->ll_md_brw_pages;

        if (sbi->ll_flags & LL_SBI_AGL_ENABLED)
                ll_start_agl(parent, sai);

        atomic_inc(&sbi->ll_sa_total);
        spin_lock(&lli->lli_sa_lock);
        if (thread_is_init(sa_thread))
                /* If someone else has changed the thread state
                 * (e.g. already changed to SVC_STOPPING), we can't just
                 * blindly overwrite that setting.
                 */
                thread_set_flags(sa_thread, SVC_RUNNING);
        spin_unlock(&lli->lli_sa_lock);
        wake_up(&sa_thread->t_ctl_waitq);

        while (pos != MDS_DIR_END_OFF && thread_is_running(sa_thread)) {
                struct lu_dirpage *dp;
                struct lu_dirent  *ent;

                sai->sai_in_readpage = 1;
                page = ll_get_dir_page(dir, op_data, pos);
                sai->sai_in_readpage = 0;
                if (IS_ERR(page)) {
                        rc = PTR_ERR(page);
                        CDEBUG(D_READA, "error reading dir "DFID" at %llu/%llu: opendir_pid = %u: rc = %d\n",
                               PFID(ll_inode2fid(dir)), pos, sai->sai_index,
                               lli->lli_opendir_pid, rc);
                        break;
                }

                dp = page_address(page);
                for (ent = lu_dirent_start(dp);
                     ent && thread_is_running(sa_thread) && !sa_low_hit(sai);
                     ent = lu_dirent_next(ent)) {
                        __u64 hash;
                        int namelen;
                        char *name;

                        hash = le64_to_cpu(ent->lde_hash);
                        if (unlikely(hash < pos))
                                /*
                                 * Skip until we find target hash value.
                                 */
                                continue;

                        namelen = le16_to_cpu(ent->lde_namelen);
                        if (unlikely(namelen == 0))
                                /*
                                 * Skip dummy record.
                                 */
                                continue;

                        name = ent->lde_name;
                        if (name[0] == '.') {
                                if (namelen == 1) {
                                        /*
                                         * skip "."
                                         */
                                        continue;
                                } else if (name[1] == '.' && namelen == 2) {
                                        /*
                                         * skip ".."
                                         */
                                        continue;
                                } else if (!sai->sai_ls_all) {
                                        /*
                                         * skip hidden files.
                                         */
                                        sai->sai_skip_hidden++;
                                        continue;
                                }
                        }

                        /*
                         * don't stat-ahead first entry.
                         */
                        if (unlikely(++first == 1))
                                continue;

                        /* wait for spare statahead window */
                        do {
                                l_wait_event(sa_thread->t_ctl_waitq,
                                             !sa_sent_full(sai) ||
                                             sa_has_callback(sai) ||
                                             !list_empty(&sai->sai_agls) ||
                                             !thread_is_running(sa_thread),
                                             &lwi);
                                sa_handle_callback(sai);

                                spin_lock(&lli->lli_agl_lock);
                                while (sa_sent_full(sai) &&
                                       !agl_list_empty(sai)) {
                                        struct ll_inode_info *clli;

                                        clli = list_entry(sai->sai_agls.next,
                                                          struct ll_inode_info, lli_agl_list);
                                        list_del_init(&clli->lli_agl_list);
                                        spin_unlock(&lli->lli_agl_lock);

                                        ll_agl_trigger(&clli->lli_vfs_inode,
                                                       sai);

                                        spin_lock(&lli->lli_agl_lock);
                                }
                                spin_unlock(&lli->lli_agl_lock);
                        } while (sa_sent_full(sai) &&
                                 thread_is_running(sa_thread));

                        sa_statahead(parent, name, namelen);
                }

                pos = le64_to_cpu(dp->ldp_hash_end);
                ll_release_page(dir, page,
                                le32_to_cpu(dp->ldp_flags) & LDF_COLLIDE);

                if (sa_low_hit(sai)) {
                        rc = -EFAULT;
                        atomic_inc(&sbi->ll_sa_wrong);
                        CDEBUG(D_READA, "Statahead for dir "DFID" hit ratio too low: hit/miss %llu/%llu, sent/replied %llu/%llu, stopping statahead thread: pid %d\n",
                               PFID(&lli->lli_fid), sai->sai_hit,
                               sai->sai_miss, sai->sai_sent,
                               sai->sai_replied, current_pid());
                        break;
                }
        }
        ll_finish_md_op_data(op_data);

        if (rc < 0) {
                spin_lock(&lli->lli_sa_lock);
                thread_set_flags(sa_thread, SVC_STOPPING);
                lli->lli_sa_enabled = 0;
                spin_unlock(&lli->lli_sa_lock);
        }

        /*
         * statahead is finished, but statahead entries need to be cached, wait
         * for file release to stop me.
         */
        while (thread_is_running(sa_thread)) {
                l_wait_event(sa_thread->t_ctl_waitq,
                             sa_has_callback(sai) ||
                             !agl_list_empty(sai) ||
                             !thread_is_running(sa_thread),
                             &lwi);

                sa_handle_callback(sai);
        }
out:
        if (sai->sai_agl_valid) {
                spin_lock(&lli->lli_agl_lock);
                thread_set_flags(agl_thread, SVC_STOPPING);
                spin_unlock(&lli->lli_agl_lock);
                wake_up(&agl_thread->t_ctl_waitq);

                CDEBUG(D_READA, "stop agl thread: sai %p pid %u\n",
                       sai, (unsigned int)agl_thread->t_pid);
                l_wait_event(agl_thread->t_ctl_waitq,
                             thread_is_stopped(agl_thread),
                             &lwi);
        } else {
                /* Set agl_thread flags anyway. */
                thread_set_flags(agl_thread, SVC_STOPPED);
        }

        /*
         * wait for inflight statahead RPCs to finish, and then we can free sai
         * safely because statahead RPC will access sai data
         */
        while (sai->sai_sent != sai->sai_replied) {
                /* in case we're not woken up, timeout wait */
                lwi = LWI_TIMEOUT(msecs_to_jiffies(MSEC_PER_SEC >> 3),
                                  NULL, NULL);
                l_wait_event(sa_thread->t_ctl_waitq,
                             sai->sai_sent == sai->sai_replied, &lwi);
        }

        /* release resources held by statahead RPCs */
        sa_handle_callback(sai);

        spin_lock(&lli->lli_sa_lock);
        thread_set_flags(sa_thread, SVC_STOPPED);
        spin_unlock(&lli->lli_sa_lock);

        CDEBUG(D_READA, "statahead thread stopped: sai %p, parent %pd\n",
               sai, parent);

        wake_up(&sai->sai_waitq);
        wake_up(&sa_thread->t_ctl_waitq);
        ll_sai_put(sai);

        return rc;
}

/* authorize opened dir handle @key to statahead */
void ll_authorize_statahead(struct inode *dir, void *key)
{
        struct ll_inode_info *lli = ll_i2info(dir);

        spin_lock(&lli->lli_sa_lock);
        if (!lli->lli_opendir_key && !lli->lli_sai) {
                /*
                 * if lli_sai is not NULL, it means previous statahead is not
                 * finished yet, we'd better not start a new statahead for now.
                 */
                LASSERT(!lli->lli_opendir_pid);
                lli->lli_opendir_key = key;
                lli->lli_opendir_pid = current_pid();
                lli->lli_sa_enabled = 1;
        }
        spin_unlock(&lli->lli_sa_lock);
}

/*
 * deauthorize opened dir handle @key to statahead, but statahead thread may
 * still be running, notify it to quit.
 */
void ll_deauthorize_statahead(struct inode *dir, void *key)
{
        struct ll_inode_info *lli = ll_i2info(dir);
        struct ll_statahead_info *sai;

        LASSERT(lli->lli_opendir_key == key);
        LASSERT(lli->lli_opendir_pid);

        CDEBUG(D_READA, "deauthorize statahead for "DFID"\n",
               PFID(&lli->lli_fid));

        spin_lock(&lli->lli_sa_lock);
        lli->lli_opendir_key = NULL;
        lli->lli_opendir_pid = 0;
        lli->lli_sa_enabled = 0;
        sai = lli->lli_sai;
        if (sai && thread_is_running(&sai->sai_thread)) {
                /*
                 * statahead thread may not quit yet because it needs to cache
                 * entries, now it's time to tell it to quit.
                 */
                thread_set_flags(&sai->sai_thread, SVC_STOPPING);
                wake_up(&sai->sai_thread.t_ctl_waitq);
        }
        spin_unlock(&lli->lli_sa_lock);
}

enum {
        /**
         * not first dirent, or is "."
         */
        LS_NOT_FIRST_DE = 0,
        /**
         * the first non-hidden dirent
         */
        LS_FIRST_DE,
        /**
         * the first hidden dirent, that is "."
         */
        LS_FIRST_DOT_DE
};

/* file is first dirent under @dir */
static int is_first_dirent(struct inode *dir, struct dentry *dentry)
{
        const struct qstr  *target = &dentry->d_name;
        struct md_op_data *op_data;
        struct page       *page;
        __u64            pos    = 0;
        int                dot_de;
        int rc = LS_NOT_FIRST_DE;

        op_data = ll_prep_md_op_data(NULL, dir, dir, NULL, 0, 0,
                                     LUSTRE_OPC_ANY, dir);
        if (IS_ERR(op_data))
                return PTR_ERR(op_data);
        /**
         * FIXME choose the start offset of the readdir
         */
        op_data->op_max_pages = ll_i2sbi(dir)->ll_md_brw_pages;

        page = ll_get_dir_page(dir, op_data, pos);

        while (1) {
                struct lu_dirpage *dp;
                struct lu_dirent  *ent;

                if (IS_ERR(page)) {
                        struct ll_inode_info *lli = ll_i2info(dir);

                        rc = PTR_ERR(page);
                        CERROR("%s: error reading dir "DFID" at %llu: opendir_pid = %u : rc = %d\n",
                               ll_get_fsname(dir->i_sb, NULL, 0),
                               PFID(ll_inode2fid(dir)), pos,
                               lli->lli_opendir_pid, rc);
                        break;
                }

                dp = page_address(page);
                for (ent = lu_dirent_start(dp); ent;
                     ent = lu_dirent_next(ent)) {
                        __u64 hash;
                        int namelen;
                        char *name;

                        hash = le64_to_cpu(ent->lde_hash);
                        /* The ll_get_dir_page() can return any page containing
                         * the given hash which may be not the start hash.
                         */
                        if (unlikely(hash < pos))
                                continue;

                        namelen = le16_to_cpu(ent->lde_namelen);
                        if (unlikely(namelen == 0))
                                /*
                                 * skip dummy record.
                                 */
                                continue;

                        name = ent->lde_name;
                        if (name[0] == '.') {
                                if (namelen == 1)
                                        /*
                                         * skip "."
                                         */
                                        continue;
                                else if (name[1] == '.' && namelen == 2)
                                        /*
                                         * skip ".."
                                         */
                                        continue;
                                else
                                        dot_de = 1;
                        } else {
                                dot_de = 0;
                        }

                        if (dot_de && target->name[0] != '.') {
                                CDEBUG(D_READA, "%.*s skip hidden file %.*s\n",
                                       target->len, target->name,
                                       namelen, name);
                                continue;
                        }

                        if (target->len != namelen ||
                            memcmp(target->name, name, namelen) != 0)
                                rc = LS_NOT_FIRST_DE;
                        else if (!dot_de)
                                rc = LS_FIRST_DE;
                        else
                                rc = LS_FIRST_DOT_DE;

                        ll_release_page(dir, page, false);
                        goto out;
                }
                pos = le64_to_cpu(dp->ldp_hash_end);
                if (pos == MDS_DIR_END_OFF) {
                        /*
                         * End of directory reached.
                         */
                        ll_release_page(dir, page, false);
                        goto out;
                } else {
                        /*
                         * chain is exhausted
                         * Normal case: continue to the next page.
                         */
                        ll_release_page(dir, page,
                                        le32_to_cpu(dp->ldp_flags) &
                                        LDF_COLLIDE);
                        page = ll_get_dir_page(dir, op_data, pos);
                }
        }
out:
        ll_finish_md_op_data(op_data);
        return rc;
}

/**
 * revalidate @dentryp from statahead cache
 *
 * \param[in]  dir      parent directory
 * \param[in]  sai      sai structure
 * \param[out] dentryp  pointer to dentry which will be revalidated
 * \param[in]  unplug   unplug statahead window only (normally for negative
 *                      dentry)
 * \retval              1 on success, dentry is saved in @dentryp
 * \retval              0 if revalidation failed (no proper lock on client)
 * \retval              negative number upon error
 */
static int revalidate_statahead_dentry(struct inode *dir,
                                       struct ll_statahead_info *sai,
                                       struct dentry **dentryp,
                                       bool unplug)
{
        struct ll_inode_info *lli = ll_i2info(dir);
        struct sa_entry *entry = NULL;
        struct l_wait_info lwi = { 0 };
        struct ll_dentry_data *ldd;
        int rc = 0;

        if ((*dentryp)->d_name.name[0] == '.') {
                if (sai->sai_ls_all ||
                    sai->sai_miss_hidden >= sai->sai_skip_hidden) {
                        /*
                         * Hidden dentry is the first one, or statahead
                         * thread does not skip so many hidden dentries
                         * before "sai_ls_all" enabled as below.
                         */
                } else {
                        if (!sai->sai_ls_all)
                                /*
                                 * It maybe because hidden dentry is not
                                 * the first one, "sai_ls_all" was not
                                 * set, then "ls -al" missed. Enable
                                 * "sai_ls_all" for such case.
                                 */
                                sai->sai_ls_all = 1;

                        /*
                         * Such "getattr" has been skipped before
                         * "sai_ls_all" enabled as above.
                         */
                        sai->sai_miss_hidden++;
                        return -EAGAIN;
                }
        }

        if (unplug) {
                rc = 1;
                goto out_unplug;
        }

        entry = sa_get(sai, &(*dentryp)->d_name);
        if (!entry) {
                rc = -EAGAIN;
                goto out_unplug;
        }

        /* if statahead is busy in readdir, help it do post-work */
        if (!sa_ready(entry) && sai->sai_in_readpage)
                sa_handle_callback(sai);

        if (!sa_ready(entry)) {
                spin_lock(&lli->lli_sa_lock);
                sai->sai_index_wait = entry->se_index;
                spin_unlock(&lli->lli_sa_lock);
                lwi = LWI_TIMEOUT_INTR(cfs_time_seconds(30), NULL,
                                       LWI_ON_SIGNAL_NOOP, NULL);
                rc = l_wait_event(sai->sai_waitq, sa_ready(entry), &lwi);
                if (rc < 0) {
                        /*
                         * entry may not be ready, so it may be used by inflight
                         * statahead RPC, don't free it.
                         */
                        entry = NULL;
                        rc = -EAGAIN;
                        goto out_unplug;
                }
        }

        if (entry->se_state == SA_ENTRY_SUCC && entry->se_inode) {
                struct inode *inode = entry->se_inode;
                struct lookup_intent it = { .it_op = IT_GETATTR,
                                            .it_lock_handle = entry->se_handle };
                __u64 bits;

                rc = md_revalidate_lock(ll_i2mdexp(dir), &it,
                                        ll_inode2fid(inode), &bits);
                if (rc == 1) {
                        if (!(*dentryp)->d_inode) {
                                struct dentry *alias;

                                alias = ll_splice_alias(inode, *dentryp);
                                if (IS_ERR(alias)) {
                                        ll_intent_release(&it);
                                        rc = PTR_ERR(alias);
                                        goto out_unplug;
                                }
                                *dentryp = alias;
                                /**
                                 * statahead prepared this inode, transfer inode
                                 * refcount from sa_entry to dentry
                                 */
                                entry->se_inode = NULL;
                        } else if ((*dentryp)->d_inode != inode) {
                                /* revalidate, but inode is recreated */
                                CDEBUG(D_READA,
                                       "%s: stale dentry %pd inode "DFID", statahead inode "DFID"\n",
                                       ll_get_fsname((*dentryp)->d_inode->i_sb,
                                                     NULL, 0),
                                       *dentryp,
                                       PFID(ll_inode2fid((*dentryp)->d_inode)),
                                       PFID(ll_inode2fid(inode)));
                                ll_intent_release(&it);
                                rc = -ESTALE;
                                goto out_unplug;
                        }

                        if ((bits & MDS_INODELOCK_LOOKUP) &&
                            d_lustre_invalid(*dentryp))
                                d_lustre_revalidate(*dentryp);
                        ll_intent_release(&it);
                }
        }
out_unplug:
        /*
         * statahead cached sa_entry can be used only once, and will be killed
         * right after use, so if lookup/revalidate accessed statahead cache,
         * set dentry ldd_sa_generation to parent lli_sa_generation, later if we
         * stat this file again, we know we've done statahead before, see
         * dentry_may_statahead().
         */
        ldd = ll_d2d(*dentryp);
        ldd->lld_sa_generation = lli->lli_sa_generation;
        sa_put(sai, entry);
        return rc;
}

/**
 * start statahead thread
 *
 * \param[in] dir       parent directory
 * \param[in] dentry    dentry that triggers statahead, normally the first
 *                      dirent under @dir
 * \retval              -EAGAIN on success, because when this function is
 *                      called, it's already in lookup call, so client should
 *                      do it itself instead of waiting for statahead thread
 *                      to do it asynchronously.
 * \retval              negative number upon error
 */
static int start_statahead_thread(struct inode *dir, struct dentry *dentry)
{
        struct ll_inode_info *lli = ll_i2info(dir);
        struct ll_statahead_info *sai = NULL;
        struct l_wait_info lwi = { 0 };
        struct ptlrpc_thread *thread;
        struct task_struct *task;
        struct dentry *parent = dentry->d_parent;
        int rc;

        /* I am the "lli_opendir_pid" owner, only me can set "lli_sai". */
        rc = is_first_dirent(dir, dentry);
        if (rc == LS_NOT_FIRST_DE) {
                /* It is not "ls -{a}l" operation, no need statahead for it. */
                rc = -EFAULT;
                goto out;
        }

        sai = ll_sai_alloc(parent);
        if (!sai) {
                rc = -ENOMEM;
                goto out;
        }

        sai->sai_ls_all = (rc == LS_FIRST_DOT_DE);
        /*
         * if current lli_opendir_key was deauthorized, or dir re-opened by
         * another process, don't start statahead, otherwise the newly spawned
         * statahead thread won't be notified to quit.
         */
        spin_lock(&lli->lli_sa_lock);
        if (unlikely(lli->lli_sai || lli->lli_opendir_key ||
                     lli->lli_opendir_pid != current->pid)) {
                spin_unlock(&lli->lli_sa_lock);
                rc = -EPERM;
                goto out;
        }
        lli->lli_sai = sai;
        spin_unlock(&lli->lli_sa_lock);

        atomic_inc(&ll_i2sbi(parent->d_inode)->ll_sa_running);

        CDEBUG(D_READA, "start statahead thread: [pid %d] [parent %pd]\n",
               current_pid(), parent);

        task = kthread_run(ll_statahead_thread, parent, "ll_sa_%u",
                           lli->lli_opendir_pid);
        thread = &sai->sai_thread;
        if (IS_ERR(task)) {
                rc = PTR_ERR(task);
                CERROR("can't start ll_sa thread, rc : %d\n", rc);
                goto out;
        }

        l_wait_event(thread->t_ctl_waitq,
                     thread_is_running(thread) || thread_is_stopped(thread),
                     &lwi);
        ll_sai_put(sai);

        /*
         * We don't stat-ahead for the first dirent since we are already in
         * lookup.
         */
        return -EAGAIN;

out:
        /*
         * once we start statahead thread failed, disable statahead so
         * that subsequent stat won't waste time to try it.
         */
        spin_lock(&lli->lli_sa_lock);
        lli->lli_sa_enabled = 0;
        lli->lli_sai = NULL;
        spin_unlock(&lli->lli_sa_lock);
        if (sai)
                ll_sai_free(sai);
        return rc;
}

/**
 * statahead entry function, this is called when client getattr on a file, it
 * will start statahead thread if this is the first dir entry, else revalidate
 * dentry from statahead cache.
 *
 * \param[in]  dir      parent directory
 * \param[out] dentryp  dentry to getattr
 * \param[in]  unplug   unplug statahead window only (normally for negative
 *                      dentry)
 * \retval              1 on success
 * \retval              0 revalidation from statahead cache failed, caller needs
 *                      to getattr from server directly
 * \retval              negative number on error, caller often ignores this and
 *                      then getattr from server
 */
int ll_statahead(struct inode *dir, struct dentry **dentryp, bool unplug)
{
        struct ll_statahead_info *sai;

        sai = ll_sai_get(dir);
        if (sai) {
                int rc;

                rc = revalidate_statahead_dentry(dir, sai, dentryp, unplug);
                CDEBUG(D_READA, "revalidate statahead %pd: %d.\n",
                       *dentryp, rc);
                ll_sai_put(sai);
                return rc;
        }
        return start_statahead_thread(dir, *dentryp);
}