// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (C) 2018 HUAWEI, Inc.
 *             https://www.huawei.com/
 * Created by Gao Xiang <gaoxiang25@huawei.com>
 */
#include "zdata.h"
#include "compress.h"
#include <linux/prefetch.h>

#include <trace/events/erofs.h>

/*
 * a compressed_pages[] placeholder in order to avoid
 * being filled with file pages for in-place decompression.
 */
#define PAGE_UNALLOCATED     ((void *)0x5F0E4B1D)

/* how to allocate cached pages for a pcluster */
enum z_erofs_cache_alloctype {
        DONTALLOC,      /* don't allocate any cached pages */
        DELAYEDALLOC,   /* delayed allocation (at the time of submitting io) */
};

/*
 * tagged pointer with 1-bit tag for all compressed pages
 * tag 0 - the page is just found with an extra page reference
 */
typedef tagptr1_t compressed_page_t;

#define tag_compressed_page_justfound(page) \
        tagptr_fold(compressed_page_t, page, 1)

static struct workqueue_struct *z_erofs_workqueue __read_mostly;
static struct kmem_cache *pcluster_cachep __read_mostly;

void z_erofs_exit_zip_subsystem(void)
{
        destroy_workqueue(z_erofs_workqueue);
        kmem_cache_destroy(pcluster_cachep);
}

static inline int z_erofs_init_workqueue(void)
{
        const unsigned int onlinecpus = num_possible_cpus();

        /*
         * no need to spawn too many threads, limiting threads could minimum
         * scheduling overhead, perhaps per-CPU threads should be better?
         */
        z_erofs_workqueue = alloc_workqueue("erofs_unzipd",
                                            WQ_UNBOUND | WQ_HIGHPRI,
                                            onlinecpus + onlinecpus / 4);
        return z_erofs_workqueue ? 0 : -ENOMEM;
}

static void z_erofs_pcluster_init_once(void *ptr)
{
        struct z_erofs_pcluster *pcl = ptr;
        struct z_erofs_collection *cl = z_erofs_primarycollection(pcl);
        unsigned int i;

        mutex_init(&cl->lock);
        cl->nr_pages = 0;
        cl->vcnt = 0;
        for (i = 0; i < Z_EROFS_CLUSTER_MAX_PAGES; ++i)
                pcl->compressed_pages[i] = NULL;
}

int __init z_erofs_init_zip_subsystem(void)
{
        pcluster_cachep = kmem_cache_create("erofs_compress",
                                            Z_EROFS_WORKGROUP_SIZE, 0,
                                            SLAB_RECLAIM_ACCOUNT,
                                            z_erofs_pcluster_init_once);
        if (pcluster_cachep) {
                if (!z_erofs_init_workqueue())
                        return 0;

                kmem_cache_destroy(pcluster_cachep);
        }
        return -ENOMEM;
}

enum z_erofs_collectmode {
        COLLECT_SECONDARY,
        COLLECT_PRIMARY,
        /*
         * The current collection was the tail of an exist chain, in addition
         * that the previous processed chained collections are all decided to
         * be hooked up to it.
         * A new chain will be created for the remaining collections which are
         * not processed yet, therefore different from COLLECT_PRIMARY_FOLLOWED,
         * the next collection cannot reuse the whole page safely in
         * the following scenario:
         *  ________________________________________________________________
         * |      tail (partial) page     |       head (partial) page       |
         * |   (belongs to the next cl)   |   (belongs to the current cl)   |
         * |_______PRIMARY_FOLLOWED_______|________PRIMARY_HOOKED___________|
         */
        COLLECT_PRIMARY_HOOKED,
        COLLECT_PRIMARY_FOLLOWED_NOINPLACE,
        /*
         * The current collection has been linked with the owned chain, and
         * could also be linked with the remaining collections, which means
         * if the processing page is the tail page of the collection, thus
         * the current collection can safely use the whole page (since
         * the previous collection is under control) for in-place I/O, as
         * illustrated below:
         *  ________________________________________________________________
         * |  tail (partial) page |          head (partial) page           |
         * |  (of the current cl) |      (of the previous collection)      |
         * |  PRIMARY_FOLLOWED or |                                        |
         * |_____PRIMARY_HOOKED___|____________PRIMARY_FOLLOWED____________|
         *
         * [  (*) the above page can be used as inplace I/O.               ]
         */
        COLLECT_PRIMARY_FOLLOWED,
};

struct z_erofs_collector {
        struct z_erofs_pagevec_ctor vector;

        struct z_erofs_pcluster *pcl, *tailpcl;
        struct z_erofs_collection *cl;
        struct page **compressedpages;
        z_erofs_next_pcluster_t owned_head;

        enum z_erofs_collectmode mode;
};

struct z_erofs_decompress_frontend {
        struct inode *const inode;

        struct z_erofs_collector clt;
        struct erofs_map_blocks map;

        bool readahead;
        /* used for applying cache strategy on the fly */
        bool backmost;
        erofs_off_t headoffset;
};

#define COLLECTOR_INIT() { \
        .owned_head = Z_EROFS_PCLUSTER_TAIL, \
        .mode = COLLECT_PRIMARY_FOLLOWED }

#define DECOMPRESS_FRONTEND_INIT(__i) { \
        .inode = __i, .clt = COLLECTOR_INIT(), \
        .backmost = true, }

static struct page *z_pagemap_global[Z_EROFS_VMAP_GLOBAL_PAGES];
static DEFINE_MUTEX(z_pagemap_global_lock);

static void preload_compressed_pages(struct z_erofs_collector *clt,
                                     struct address_space *mc,
                                     enum z_erofs_cache_alloctype type)
{
        const struct z_erofs_pcluster *pcl = clt->pcl;
        const unsigned int clusterpages = BIT(pcl->clusterbits);
        struct page **pages = clt->compressedpages;
        pgoff_t index = pcl->obj.index + (pages - pcl->compressed_pages);
        bool standalone = true;

        if (clt->mode < COLLECT_PRIMARY_FOLLOWED)
                return;

        for (; pages < pcl->compressed_pages + clusterpages; ++pages) {
                struct page *page;
                compressed_page_t t;

                /* the compressed page was loaded before */
                if (READ_ONCE(*pages))
                        continue;

                page = find_get_page(mc, index);

                if (page) {
                        t = tag_compressed_page_justfound(page);
                } else if (type == DELAYEDALLOC) {
                        t = tagptr_init(compressed_page_t, PAGE_UNALLOCATED);
                } else {        /* DONTALLOC */
                        if (standalone)
                                clt->compressedpages = pages;
                        standalone = false;
                        continue;
                }

                if (!cmpxchg_relaxed(pages, NULL, tagptr_cast_ptr(t)))
                        continue;

                if (page)
                        put_page(page);
        }

        if (standalone)         /* downgrade to PRIMARY_FOLLOWED_NOINPLACE */
                clt->mode = COLLECT_PRIMARY_FOLLOWED_NOINPLACE;
}

/* called by erofs_shrinker to get rid of all compressed_pages */
int erofs_try_to_free_all_cached_pages(struct erofs_sb_info *sbi,
                                       struct erofs_workgroup *grp)
{
        struct z_erofs_pcluster *const pcl =
                container_of(grp, struct z_erofs_pcluster, obj);
        struct address_space *const mapping = MNGD_MAPPING(sbi);
        const unsigned int clusterpages = BIT(pcl->clusterbits);
        int i;

        /*
         * refcount of workgroup is now freezed as 1,
         * therefore no need to worry about available decompression users.
         */
        for (i = 0; i < clusterpages; ++i) {
                struct page *page = pcl->compressed_pages[i];

                if (!page)
                        continue;

                /* block other users from reclaiming or migrating the page */
                if (!trylock_page(page))
                        return -EBUSY;

                if (page->mapping != mapping)
                        continue;

                /* barrier is implied in the following 'unlock_page' */
                WRITE_ONCE(pcl->compressed_pages[i], NULL);
                set_page_private(page, 0);
                ClearPagePrivate(page);

                unlock_page(page);
                put_page(page);
        }
        return 0;
}

int erofs_try_to_free_cached_page(struct address_space *mapping,
                                  struct page *page)
{
        struct z_erofs_pcluster *const pcl = (void *)page_private(page);
        const unsigned int clusterpages = BIT(pcl->clusterbits);
        int ret = 0;    /* 0 - busy */

        if (erofs_workgroup_try_to_freeze(&pcl->obj, 1)) {
                unsigned int i;

                for (i = 0; i < clusterpages; ++i) {
                        if (pcl->compressed_pages[i] == page) {
                                WRITE_ONCE(pcl->compressed_pages[i], NULL);
                                ret = 1;
                                break;
                        }
                }
                erofs_workgroup_unfreeze(&pcl->obj, 1);

                if (ret) {
                        ClearPagePrivate(page);
                        put_page(page);
                }
        }
        return ret;
}

/* page_type must be Z_EROFS_PAGE_TYPE_EXCLUSIVE */
static inline bool z_erofs_try_inplace_io(struct z_erofs_collector *clt,
                                          struct page *page)
{
        struct z_erofs_pcluster *const pcl = clt->pcl;
        const unsigned int clusterpages = BIT(pcl->clusterbits);

        while (clt->compressedpages < pcl->compressed_pages + clusterpages) {
                if (!cmpxchg(clt->compressedpages++, NULL, page))
                        return true;
        }
        return false;
}

/* callers must be with collection lock held */
static int z_erofs_attach_page(struct z_erofs_collector *clt,
                               struct page *page,
                               enum z_erofs_page_type type)
{
        int ret;
        bool occupied;

        /* give priority for inplaceio */
        if (clt->mode >= COLLECT_PRIMARY &&
            type == Z_EROFS_PAGE_TYPE_EXCLUSIVE &&
            z_erofs_try_inplace_io(clt, page))
                return 0;

        ret = z_erofs_pagevec_enqueue(&clt->vector,
                                      page, type, &occupied);
        clt->cl->vcnt += (unsigned int)ret;

        return ret ? 0 : -EAGAIN;
}

static enum z_erofs_collectmode
try_to_claim_pcluster(struct z_erofs_pcluster *pcl,
                      z_erofs_next_pcluster_t *owned_head)
{
        /* let's claim these following types of pclusters */
retry:
        if (pcl->next == Z_EROFS_PCLUSTER_NIL) {
                /* type 1, nil pcluster */
                if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_NIL,
                            *owned_head) != Z_EROFS_PCLUSTER_NIL)
                        goto retry;

                *owned_head = &pcl->next;
                /* lucky, I am the followee :) */
                return COLLECT_PRIMARY_FOLLOWED;
        } else if (pcl->next == Z_EROFS_PCLUSTER_TAIL) {
                /*
                 * type 2, link to the end of a existing open chain,
                 * be careful that its submission itself is governed
                 * by the original owned chain.
                 */
                if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_TAIL,
                            *owned_head) != Z_EROFS_PCLUSTER_TAIL)
                        goto retry;
                *owned_head = Z_EROFS_PCLUSTER_TAIL;
                return COLLECT_PRIMARY_HOOKED;
        }
        return COLLECT_PRIMARY; /* :( better luck next time */
}

static int z_erofs_lookup_collection(struct z_erofs_collector *clt,
                                     struct inode *inode,
                                     struct erofs_map_blocks *map)
{
        struct z_erofs_pcluster *pcl = clt->pcl;
        struct z_erofs_collection *cl;
        unsigned int length;

        /* to avoid unexpected loop formed by corrupted images */
        if (clt->owned_head == &pcl->next || pcl == clt->tailpcl) {
                DBG_BUGON(1);
                return -EFSCORRUPTED;
        }

        cl = z_erofs_primarycollection(pcl);
        if (cl->pageofs != (map->m_la & ~PAGE_MASK)) {
                DBG_BUGON(1);
                return -EFSCORRUPTED;
        }

        length = READ_ONCE(pcl->length);
        if (length & Z_EROFS_PCLUSTER_FULL_LENGTH) {
                if ((map->m_llen << Z_EROFS_PCLUSTER_LENGTH_BIT) > length) {
                        DBG_BUGON(1);
                        return -EFSCORRUPTED;
                }
        } else {
                unsigned int llen = map->m_llen << Z_EROFS_PCLUSTER_LENGTH_BIT;

                if (map->m_flags & EROFS_MAP_FULL_MAPPED)
                        llen |= Z_EROFS_PCLUSTER_FULL_LENGTH;

                while (llen > length &&
                       length != cmpxchg_relaxed(&pcl->length, length, llen)) {
                        cpu_relax();
                        length = READ_ONCE(pcl->length);
                }
        }
        mutex_lock(&cl->lock);
        /* used to check tail merging loop due to corrupted images */
        if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
                clt->tailpcl = pcl;
        clt->mode = try_to_claim_pcluster(pcl, &clt->owned_head);
        /* clean tailpcl if the current owned_head is Z_EROFS_PCLUSTER_TAIL */
        if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
                clt->tailpcl = NULL;
        clt->cl = cl;
        return 0;
}

static int z_erofs_register_collection(struct z_erofs_collector *clt,
                                       struct inode *inode,
                                       struct erofs_map_blocks *map)
{
        struct z_erofs_pcluster *pcl;
        struct z_erofs_collection *cl;
        struct erofs_workgroup *grp;
        int err;

        /* no available workgroup, let's allocate one */
        pcl = kmem_cache_alloc(pcluster_cachep, GFP_NOFS);
        if (!pcl)
                return -ENOMEM;

        atomic_set(&pcl->obj.refcount, 1);
        pcl->obj.index = map->m_pa >> PAGE_SHIFT;

        pcl->length = (map->m_llen << Z_EROFS_PCLUSTER_LENGTH_BIT) |
                (map->m_flags & EROFS_MAP_FULL_MAPPED ?
                        Z_EROFS_PCLUSTER_FULL_LENGTH : 0);

        if (map->m_flags & EROFS_MAP_ZIPPED)
                pcl->algorithmformat = Z_EROFS_COMPRESSION_LZ4;
        else
                pcl->algorithmformat = Z_EROFS_COMPRESSION_SHIFTED;

        pcl->clusterbits = EROFS_I(inode)->z_physical_clusterbits[0];
        pcl->clusterbits -= PAGE_SHIFT;

        /* new pclusters should be claimed as type 1, primary and followed */
        pcl->next = clt->owned_head;
        clt->mode = COLLECT_PRIMARY_FOLLOWED;

        cl = z_erofs_primarycollection(pcl);

        /* must be cleaned before freeing to slab */
        DBG_BUGON(cl->nr_pages);
        DBG_BUGON(cl->vcnt);

        cl->pageofs = map->m_la & ~PAGE_MASK;

        /*
         * lock all primary followed works before visible to others
         * and mutex_trylock *never* fails for a new pcluster.
         */
        DBG_BUGON(!mutex_trylock(&cl->lock));

        grp = erofs_insert_workgroup(inode->i_sb, &pcl->obj);
        if (IS_ERR(grp)) {
                err = PTR_ERR(grp);
                goto err_out;
        }

        if (grp != &pcl->obj) {
                clt->pcl = container_of(grp, struct z_erofs_pcluster, obj);
                err = -EEXIST;
                goto err_out;
        }
        /* used to check tail merging loop due to corrupted images */
        if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
                clt->tailpcl = pcl;
        clt->owned_head = &pcl->next;
        clt->pcl = pcl;
        clt->cl = cl;
        return 0;

err_out:
        mutex_unlock(&cl->lock);
        kmem_cache_free(pcluster_cachep, pcl);
        return err;
}

static int z_erofs_collector_begin(struct z_erofs_collector *clt,
                                   struct inode *inode,
                                   struct erofs_map_blocks *map)
{
        struct erofs_workgroup *grp;
        int ret;

        DBG_BUGON(clt->cl);

        /* must be Z_EROFS_PCLUSTER_TAIL or pointed to previous collection */
        DBG_BUGON(clt->owned_head == Z_EROFS_PCLUSTER_NIL);
        DBG_BUGON(clt->owned_head == Z_EROFS_PCLUSTER_TAIL_CLOSED);

        if (!PAGE_ALIGNED(map->m_pa)) {
                DBG_BUGON(1);
                return -EINVAL;
        }

        grp = erofs_find_workgroup(inode->i_sb, map->m_pa >> PAGE_SHIFT);
        if (grp) {
                clt->pcl = container_of(grp, struct z_erofs_pcluster, obj);
        } else {
                ret = z_erofs_register_collection(clt, inode, map);

                if (!ret)
                        goto out;
                if (ret != -EEXIST)
                        return ret;
        }

        ret = z_erofs_lookup_collection(clt, inode, map);
        if (ret) {
                erofs_workgroup_put(&clt->pcl->obj);
                return ret;
        }

out:
        z_erofs_pagevec_ctor_init(&clt->vector, Z_EROFS_NR_INLINE_PAGEVECS,
                                  clt->cl->pagevec, clt->cl->vcnt);

        clt->compressedpages = clt->pcl->compressed_pages;
        if (clt->mode <= COLLECT_PRIMARY) /* cannot do in-place I/O */
                clt->compressedpages += Z_EROFS_CLUSTER_MAX_PAGES;
        return 0;
}

/*
 * keep in mind that no referenced pclusters will be freed
 * only after a RCU grace period.
 */
static void z_erofs_rcu_callback(struct rcu_head *head)
{
        struct z_erofs_collection *const cl =
                container_of(head, struct z_erofs_collection, rcu);

        kmem_cache_free(pcluster_cachep,
                        container_of(cl, struct z_erofs_pcluster,
                                     primary_collection));
}

void erofs_workgroup_free_rcu(struct erofs_workgroup *grp)
{
        struct z_erofs_pcluster *const pcl =
                container_of(grp, struct z_erofs_pcluster, obj);
        struct z_erofs_collection *const cl = z_erofs_primarycollection(pcl);

        call_rcu(&cl->rcu, z_erofs_rcu_callback);
}

static void z_erofs_collection_put(struct z_erofs_collection *cl)
{
        struct z_erofs_pcluster *const pcl =
                container_of(cl, struct z_erofs_pcluster, primary_collection);

        erofs_workgroup_put(&pcl->obj);
}

static bool z_erofs_collector_end(struct z_erofs_collector *clt)
{
        struct z_erofs_collection *cl = clt->cl;

        if (!cl)
                return false;

        z_erofs_pagevec_ctor_exit(&clt->vector, false);
        mutex_unlock(&cl->lock);

        /*
         * if all pending pages are added, don't hold its reference
         * any longer if the pcluster isn't hosted by ourselves.
         */
        if (clt->mode < COLLECT_PRIMARY_FOLLOWED_NOINPLACE)
                z_erofs_collection_put(cl);

        clt->cl = NULL;
        return true;
}

static bool should_alloc_managed_pages(struct z_erofs_decompress_frontend *fe,
                                       unsigned int cachestrategy,
                                       erofs_off_t la)
{
        if (cachestrategy <= EROFS_ZIP_CACHE_DISABLED)
                return false;

        if (fe->backmost)
                return true;

        return cachestrategy >= EROFS_ZIP_CACHE_READAROUND &&
                la < fe->headoffset;
}

static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
                                struct page *page)
{
        struct inode *const inode = fe->inode;
        struct erofs_sb_info *const sbi = EROFS_I_SB(inode);
        struct erofs_map_blocks *const map = &fe->map;
        struct z_erofs_collector *const clt = &fe->clt;
        const loff_t offset = page_offset(page);
        bool tight = true;

        enum z_erofs_cache_alloctype cache_strategy;
        enum z_erofs_page_type page_type;
        unsigned int cur, end, spiltted, index;
        int err = 0;

        /* register locked file pages as online pages in pack */
        z_erofs_onlinepage_init(page);

        spiltted = 0;
        end = PAGE_SIZE;
repeat:
        cur = end - 1;

        /* lucky, within the range of the current map_blocks */
        if (offset + cur >= map->m_la &&
            offset + cur < map->m_la + map->m_llen) {
                /* didn't get a valid collection previously (very rare) */
                if (!clt->cl)
                        goto restart_now;
                goto hitted;
        }

        /* go ahead the next map_blocks */
        erofs_dbg("%s: [out-of-range] pos %llu", __func__, offset + cur);

        if (z_erofs_collector_end(clt))
                fe->backmost = false;

        map->m_la = offset + cur;
        map->m_llen = 0;
        err = z_erofs_map_blocks_iter(inode, map, 0);
        if (err)
                goto err_out;

restart_now:
        if (!(map->m_flags & EROFS_MAP_MAPPED))
                goto hitted;

        err = z_erofs_collector_begin(clt, inode, map);
        if (err)
                goto err_out;

        /* preload all compressed pages (maybe downgrade role if necessary) */
        if (should_alloc_managed_pages(fe, sbi->ctx.cache_strategy, map->m_la))
                cache_strategy = DELAYEDALLOC;
        else
                cache_strategy = DONTALLOC;

        preload_compressed_pages(clt, MNGD_MAPPING(sbi), cache_strategy);

hitted:
        /*
         * Ensure the current partial page belongs to this submit chain rather
         * than other concurrent submit chains or the noio(bypass) chain since
         * those chains are handled asynchronously thus the page cannot be used
         * for inplace I/O or pagevec (should be processed in strict order.)
         */
        tight &= (clt->mode >= COLLECT_PRIMARY_HOOKED &&
                  clt->mode != COLLECT_PRIMARY_FOLLOWED_NOINPLACE);

        cur = end - min_t(unsigned int, offset + end - map->m_la, end);
        if (!(map->m_flags & EROFS_MAP_MAPPED)) {
                zero_user_segment(page, cur, end);
                goto next_part;
        }

        /* let's derive page type */
        page_type = cur ? Z_EROFS_VLE_PAGE_TYPE_HEAD :
                (!spiltted ? Z_EROFS_PAGE_TYPE_EXCLUSIVE :
                        (tight ? Z_EROFS_PAGE_TYPE_EXCLUSIVE :
                                Z_EROFS_VLE_PAGE_TYPE_TAIL_SHARED));

        if (cur)
                tight &= (clt->mode >= COLLECT_PRIMARY_FOLLOWED);

retry:
        err = z_erofs_attach_page(clt, page, page_type);
        /* should allocate an additional staging page for pagevec */
        if (err == -EAGAIN) {
                struct page *const newpage =
                                alloc_page(GFP_NOFS | __GFP_NOFAIL);

                newpage->mapping = Z_EROFS_MAPPING_STAGING;
                err = z_erofs_attach_page(clt, newpage,
                                          Z_EROFS_PAGE_TYPE_EXCLUSIVE);
                if (!err)
                        goto retry;
        }

        if (err)
                goto err_out;

        index = page->index - (map->m_la >> PAGE_SHIFT);

        z_erofs_onlinepage_fixup(page, index, true);

        /* bump up the number of spiltted parts of a page */
        ++spiltted;
        /* also update nr_pages */
        clt->cl->nr_pages = max_t(pgoff_t, clt->cl->nr_pages, index + 1);
next_part:
        /* can be used for verification */
        map->m_llen = offset + cur - map->m_la;

        end = cur;
        if (end > 0)
                goto repeat;

out:
        z_erofs_onlinepage_endio(page);

        erofs_dbg("%s, finish page: %pK spiltted: %u map->m_llen %llu",
                  __func__, page, spiltted, map->m_llen);
        return err;

        /* if some error occurred while processing this page */
err_out:
        SetPageError(page);
        goto out;
}

static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
                                       bool sync, int bios)
{
        /* wake up the caller thread for sync decompression */
        if (sync) {
                unsigned long flags;

                spin_lock_irqsave(&io->u.wait.lock, flags);
                if (!atomic_add_return(bios, &io->pending_bios))
                        wake_up_locked(&io->u.wait);
                spin_unlock_irqrestore(&io->u.wait.lock, flags);
                return;
        }

        if (!atomic_add_return(bios, &io->pending_bios))
                queue_work(z_erofs_workqueue, &io->u.work);
}

static void z_erofs_decompressqueue_endio(struct bio *bio)
{
        tagptr1_t t = tagptr_init(tagptr1_t, bio->bi_private);
        struct z_erofs_decompressqueue *q = tagptr_unfold_ptr(t);
        blk_status_t err = bio->bi_status;
        struct bio_vec *bvec;
        struct bvec_iter_all iter_all;

        bio_for_each_segment_all(bvec, bio, iter_all) {
                struct page *page = bvec->bv_page;

                DBG_BUGON(PageUptodate(page));
                DBG_BUGON(!page->mapping);

                if (err)
                        SetPageError(page);

                if (erofs_page_is_managed(EROFS_SB(q->sb), page)) {
                        if (!err)
                                SetPageUptodate(page);
                        unlock_page(page);
                }
        }
        z_erofs_decompress_kickoff(q, tagptr_unfold_tags(t), -1);
        bio_put(bio);
}

static int z_erofs_decompress_pcluster(struct super_block *sb,
                                       struct z_erofs_pcluster *pcl,
                                       struct list_head *pagepool)
{
        struct erofs_sb_info *const sbi = EROFS_SB(sb);
        const unsigned int clusterpages = BIT(pcl->clusterbits);
        struct z_erofs_pagevec_ctor ctor;
        unsigned int i, outputsize, llen, nr_pages;
        struct page *pages_onstack[Z_EROFS_VMAP_ONSTACK_PAGES];
        struct page **pages, **compressed_pages, *page;

        enum z_erofs_page_type page_type;
        bool overlapped, partial;
        struct z_erofs_collection *cl;
        int err;

        might_sleep();
        cl = z_erofs_primarycollection(pcl);
        DBG_BUGON(!READ_ONCE(cl->nr_pages));

        mutex_lock(&cl->lock);
        nr_pages = cl->nr_pages;

        if (nr_pages <= Z_EROFS_VMAP_ONSTACK_PAGES) {
                pages = pages_onstack;
        } else if (nr_pages <= Z_EROFS_VMAP_GLOBAL_PAGES &&
                   mutex_trylock(&z_pagemap_global_lock)) {
                pages = z_pagemap_global;
        } else {
                gfp_t gfp_flags = GFP_KERNEL;

                if (nr_pages > Z_EROFS_VMAP_GLOBAL_PAGES)
                        gfp_flags |= __GFP_NOFAIL;

                pages = kvmalloc_array(nr_pages, sizeof(struct page *),
                                       gfp_flags);

                /* fallback to global pagemap for the lowmem scenario */
                if (!pages) {
                        mutex_lock(&z_pagemap_global_lock);
                        pages = z_pagemap_global;
                }
        }

        for (i = 0; i < nr_pages; ++i)
                pages[i] = NULL;

        err = 0;
        z_erofs_pagevec_ctor_init(&ctor, Z_EROFS_NR_INLINE_PAGEVECS,
                                  cl->pagevec, 0);

        for (i = 0; i < cl->vcnt; ++i) {
                unsigned int pagenr;

                page = z_erofs_pagevec_dequeue(&ctor, &page_type);

                /* all pages in pagevec ought to be valid */
                DBG_BUGON(!page);
                DBG_BUGON(!page->mapping);

                if (z_erofs_put_stagingpage(pagepool, page))
                        continue;

                if (page_type == Z_EROFS_VLE_PAGE_TYPE_HEAD)
                        pagenr = 0;
                else
                        pagenr = z_erofs_onlinepage_index(page);

                DBG_BUGON(pagenr >= nr_pages);

                /*
                 * currently EROFS doesn't support multiref(dedup),
                 * so here erroring out one multiref page.
                 */
                if (pages[pagenr]) {
                        DBG_BUGON(1);
                        SetPageError(pages[pagenr]);
                        z_erofs_onlinepage_endio(pages[pagenr]);
                        err = -EFSCORRUPTED;
                }
                pages[pagenr] = page;
        }
        z_erofs_pagevec_ctor_exit(&ctor, true);

        overlapped = false;
        compressed_pages = pcl->compressed_pages;

        for (i = 0; i < clusterpages; ++i) {
                unsigned int pagenr;

                page = compressed_pages[i];

                /* all compressed pages ought to be valid */
                DBG_BUGON(!page);
                DBG_BUGON(!page->mapping);

                if (!z_erofs_page_is_staging(page)) {
                        if (erofs_page_is_managed(sbi, page)) {
                                if (!PageUptodate(page))
                                        err = -EIO;
                                continue;
                        }

                        /*
                         * only if non-head page can be selected
                         * for inplace decompression
                         */
                        pagenr = z_erofs_onlinepage_index(page);

                        DBG_BUGON(pagenr >= nr_pages);
                        if (pages[pagenr]) {
                                DBG_BUGON(1);
                                SetPageError(pages[pagenr]);
                                z_erofs_onlinepage_endio(pages[pagenr]);
                                err = -EFSCORRUPTED;
                        }
                        pages[pagenr] = page;

                        overlapped = true;
                }

                /* PG_error needs checking for inplaced and staging pages */
                if (PageError(page)) {
                        DBG_BUGON(PageUptodate(page));
                        err = -EIO;
                }
        }

        if (err)
                goto out;

        llen = pcl->length >> Z_EROFS_PCLUSTER_LENGTH_BIT;
        if (nr_pages << PAGE_SHIFT >= cl->pageofs + llen) {
                outputsize = llen;
                partial = !(pcl->length & Z_EROFS_PCLUSTER_FULL_LENGTH);
        } else {
                outputsize = (nr_pages << PAGE_SHIFT) - cl->pageofs;
                partial = true;
        }

        err = z_erofs_decompress(&(struct z_erofs_decompress_req) {
                                        .sb = sb,
                                        .in = compressed_pages,
                                        .out = pages,
                                        .pageofs_out = cl->pageofs,
                                        .inputsize = PAGE_SIZE,
                                        .outputsize = outputsize,
                                        .alg = pcl->algorithmformat,
                                        .inplace_io = overlapped,
                                        .partial_decoding = partial
                                 }, pagepool);

out:
        /* must handle all compressed pages before endding pages */
        for (i = 0; i < clusterpages; ++i) {
                page = compressed_pages[i];

                if (erofs_page_is_managed(sbi, page))
                        continue;

                /* recycle all individual staging pages */
                (void)z_erofs_put_stagingpage(pagepool, page);

                WRITE_ONCE(compressed_pages[i], NULL);
        }

        for (i = 0; i < nr_pages; ++i) {
                page = pages[i];
                if (!page)
                        continue;

                DBG_BUGON(!page->mapping);

                /* recycle all individual staging pages */
                if (z_erofs_put_stagingpage(pagepool, page))
                        continue;

                if (err < 0)
                        SetPageError(page);

                z_erofs_onlinepage_endio(page);
        }

        if (pages == z_pagemap_global)
                mutex_unlock(&z_pagemap_global_lock);
        else if (pages != pages_onstack)
                kvfree(pages);

        cl->nr_pages = 0;
        cl->vcnt = 0;

        /* all cl locks MUST be taken before the following line */
        WRITE_ONCE(pcl->next, Z_EROFS_PCLUSTER_NIL);

        /* all cl locks SHOULD be released right now */
        mutex_unlock(&cl->lock);

        z_erofs_collection_put(cl);
        return err;
}

static void z_erofs_decompress_queue(const struct z_erofs_decompressqueue *io,
                                     struct list_head *pagepool)
{
        z_erofs_next_pcluster_t owned = io->head;

        while (owned != Z_EROFS_PCLUSTER_TAIL_CLOSED) {
                struct z_erofs_pcluster *pcl;

                /* no possible that 'owned' equals Z_EROFS_WORK_TPTR_TAIL */
                DBG_BUGON(owned == Z_EROFS_PCLUSTER_TAIL);

                /* no possible that 'owned' equals NULL */
                DBG_BUGON(owned == Z_EROFS_PCLUSTER_NIL);

                pcl = container_of(owned, struct z_erofs_pcluster, next);
                owned = READ_ONCE(pcl->next);

                z_erofs_decompress_pcluster(io->sb, pcl, pagepool);
        }
}

static void z_erofs_decompressqueue_work(struct work_struct *work)
{
        struct z_erofs_decompressqueue *bgq =
                container_of(work, struct z_erofs_decompressqueue, u.work);
        LIST_HEAD(pagepool);

        DBG_BUGON(bgq->head == Z_EROFS_PCLUSTER_TAIL_CLOSED);
        z_erofs_decompress_queue(bgq, &pagepool);

        put_pages_list(&pagepool);
        kvfree(bgq);
}

static struct page *pickup_page_for_submission(struct z_erofs_pcluster *pcl,
                                               unsigned int nr,
                                               struct list_head *pagepool,
                                               struct address_space *mc,
                                               gfp_t gfp)
{
        const pgoff_t index = pcl->obj.index;
        bool tocache = false;

        struct address_space *mapping;
        struct page *oldpage, *page;

        compressed_page_t t;
        int justfound;

repeat:
        page = READ_ONCE(pcl->compressed_pages[nr]);
        oldpage = page;

        if (!page)
                goto out_allocpage;

        /*
         * the cached page has not been allocated and
         * an placeholder is out there, prepare it now.
         */

        if (page == PAGE_UNALLOCATED) {
                tocache = true;
                goto out_allocpage;
        }

        /* process the target tagged pointer */
        t = tagptr_init(compressed_page_t, page);
        justfound = tagptr_unfold_tags(t);
        page = tagptr_unfold_ptr(t);

        mapping = READ_ONCE(page->mapping);

        /*
         * unmanaged (file) pages are all locked solidly,
         * therefore it is impossible for `mapping' to be NULL.
         */
        if (mapping && mapping != mc)
                /* ought to be unmanaged pages */
                goto out;

        lock_page(page);

        /* only true if page reclaim goes wrong, should never happen */
        DBG_BUGON(justfound && PagePrivate(page));

        /* the page is still in manage cache */
        if (page->mapping == mc) {
                WRITE_ONCE(pcl->compressed_pages[nr], page);

                ClearPageError(page);
                if (!PagePrivate(page)) {
                        /*
                         * impossible to be !PagePrivate(page) for
                         * the current restriction as well if
                         * the page is already in compressed_pages[].
                         */
                        DBG_BUGON(!justfound);

                        justfound = 0;
                        set_page_private(page, (unsigned long)pcl);
                        SetPagePrivate(page);
                }

                /* no need to submit io if it is already up-to-date */
                if (PageUptodate(page)) {
                        unlock_page(page);
                        page = NULL;
                }
                goto out;
        }

        /*
         * the managed page has been truncated, it's unsafe to
         * reuse this one, let's allocate a new cache-managed page.
         */
        DBG_BUGON(page->mapping);
        DBG_BUGON(!justfound);

        tocache = true;
        unlock_page(page);
        put_page(page);
out_allocpage:
        page = erofs_allocpage(pagepool, gfp | __GFP_NOFAIL);
        if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
                /* non-LRU / non-movable temporary page is needed */
                page->mapping = Z_EROFS_MAPPING_STAGING;
                tocache = false;
        }

        if (oldpage != cmpxchg(&pcl->compressed_pages[nr], oldpage, page)) {
                if (tocache) {
                        /* since it added to managed cache successfully */
                        unlock_page(page);
                        put_page(page);
                } else {
                        list_add(&page->lru, pagepool);
                }
                cond_resched();
                goto repeat;
        }

        if (tocache) {
                set_page_private(page, (unsigned long)pcl);
                SetPagePrivate(page);
        }
out:    /* the only exit (for tracing and debugging) */
        return page;
}

static struct z_erofs_decompressqueue *
jobqueue_init(struct super_block *sb,
              struct z_erofs_decompressqueue *fgq, bool *fg)
{
        struct z_erofs_decompressqueue *q;

        if (fg && !*fg) {
                q = kvzalloc(sizeof(*q), GFP_KERNEL | __GFP_NOWARN);
                if (!q) {
                        *fg = true;
                        goto fg_out;
                }
                INIT_WORK(&q->u.work, z_erofs_decompressqueue_work);
        } else {
fg_out:
                q = fgq;
                init_waitqueue_head(&fgq->u.wait);
                atomic_set(&fgq->pending_bios, 0);
        }
        q->sb = sb;
        q->head = Z_EROFS_PCLUSTER_TAIL_CLOSED;
        return q;
}

/* define decompression jobqueue types */
enum {
        JQ_BYPASS,
        JQ_SUBMIT,
        NR_JOBQUEUES,
};

static void *jobqueueset_init(struct super_block *sb,
                              struct z_erofs_decompressqueue *q[],
                              struct z_erofs_decompressqueue *fgq, bool *fg)
{
        /*
         * if managed cache is enabled, bypass jobqueue is needed,
         * no need to read from device for all pclusters in this queue.
         */
        q[JQ_BYPASS] = jobqueue_init(sb, fgq + JQ_BYPASS, NULL);
        q[JQ_SUBMIT] = jobqueue_init(sb, fgq + JQ_SUBMIT, fg);

        return tagptr_cast_ptr(tagptr_fold(tagptr1_t, q[JQ_SUBMIT], *fg));
}

static void move_to_bypass_jobqueue(struct z_erofs_pcluster *pcl,
                                    z_erofs_next_pcluster_t qtail[],
                                    z_erofs_next_pcluster_t owned_head)
{
        z_erofs_next_pcluster_t *const submit_qtail = qtail[JQ_SUBMIT];
        z_erofs_next_pcluster_t *const bypass_qtail = qtail[JQ_BYPASS];

        DBG_BUGON(owned_head == Z_EROFS_PCLUSTER_TAIL_CLOSED);
        if (owned_head == Z_EROFS_PCLUSTER_TAIL)
                owned_head = Z_EROFS_PCLUSTER_TAIL_CLOSED;

        WRITE_ONCE(pcl->next, Z_EROFS_PCLUSTER_TAIL_CLOSED);

        WRITE_ONCE(*submit_qtail, owned_head);
        WRITE_ONCE(*bypass_qtail, &pcl->next);

        qtail[JQ_BYPASS] = &pcl->next;
}

static void z_erofs_submit_queue(struct super_block *sb,
                                 struct z_erofs_decompress_frontend *f,
                                 struct list_head *pagepool,
                                 struct z_erofs_decompressqueue *fgq,
                                 bool *force_fg)
{
        struct erofs_sb_info *const sbi = EROFS_SB(sb);
        z_erofs_next_pcluster_t qtail[NR_JOBQUEUES];
        struct z_erofs_decompressqueue *q[NR_JOBQUEUES];
        void *bi_private;
        z_erofs_next_pcluster_t owned_head = f->clt.owned_head;
        /* since bio will be NULL, no need to initialize last_index */
        pgoff_t last_index;
        unsigned int nr_bios = 0;
        struct bio *bio = NULL;

        bi_private = jobqueueset_init(sb, q, fgq, force_fg);
        qtail[JQ_BYPASS] = &q[JQ_BYPASS]->head;
        qtail[JQ_SUBMIT] = &q[JQ_SUBMIT]->head;

        /* by default, all need io submission */
        q[JQ_SUBMIT]->head = owned_head;

        do {
                struct z_erofs_pcluster *pcl;
                pgoff_t cur, end;
                unsigned int i = 0;
                bool bypass = true;

                /* no possible 'owned_head' equals the following */
                DBG_BUGON(owned_head == Z_EROFS_PCLUSTER_TAIL_CLOSED);
                DBG_BUGON(owned_head == Z_EROFS_PCLUSTER_NIL);

                pcl = container_of(owned_head, struct z_erofs_pcluster, next);

                cur = pcl->obj.index;
                end = cur + BIT(pcl->clusterbits);

                /* close the main owned chain at first */
                owned_head = cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_TAIL,
                                     Z_EROFS_PCLUSTER_TAIL_CLOSED);

                do {
                        struct page *page;

                        page = pickup_page_for_submission(pcl, i++, pagepool,
                                                          MNGD_MAPPING(sbi),
                                                          GFP_NOFS);
                        if (!page)
                                continue;

                        if (bio && cur != last_index + 1) {
submit_bio_retry:
                                submit_bio(bio);
                                bio = NULL;
                        }

                        if (!bio) {
                                bio = bio_alloc(GFP_NOIO, BIO_MAX_PAGES);

                                bio->bi_end_io = z_erofs_decompressqueue_endio;
                                bio_set_dev(bio, sb->s_bdev);
                                bio->bi_iter.bi_sector = (sector_t)cur <<
                                        LOG_SECTORS_PER_BLOCK;
                                bio->bi_private = bi_private;
                                bio->bi_opf = REQ_OP_READ;
                                if (f->readahead)
                                        bio->bi_opf |= REQ_RAHEAD;
                                ++nr_bios;
                        }

                        if (bio_add_page(bio, page, PAGE_SIZE, 0) < PAGE_SIZE)
                                goto submit_bio_retry;

                        last_index = cur;
                        bypass = false;
                } while (++cur < end);

                if (!bypass)
                        qtail[JQ_SUBMIT] = &pcl->next;
                else
                        move_to_bypass_jobqueue(pcl, qtail, owned_head);
        } while (owned_head != Z_EROFS_PCLUSTER_TAIL);

        if (bio)
                submit_bio(bio);

        /*
         * although background is preferred, no one is pending for submission.
         * don't issue workqueue for decompression but drop it directly instead.
         */
        if (!*force_fg && !nr_bios) {
                kvfree(q[JQ_SUBMIT]);
                return;
        }
        z_erofs_decompress_kickoff(q[JQ_SUBMIT], *force_fg, nr_bios);
}

static void z_erofs_runqueue(struct super_block *sb,
                             struct z_erofs_decompress_frontend *f,
                             struct list_head *pagepool, bool force_fg)
{
        struct z_erofs_decompressqueue io[NR_JOBQUEUES];

        if (f->clt.owned_head == Z_EROFS_PCLUSTER_TAIL)
                return;
        z_erofs_submit_queue(sb, f, pagepool, io, &force_fg);

        /* handle bypass queue (no i/o pclusters) immediately */
        z_erofs_decompress_queue(&io[JQ_BYPASS], pagepool);

        if (!force_fg)
                return;

        /* wait until all bios are completed */
        io_wait_event(io[JQ_SUBMIT].u.wait,
                      !atomic_read(&io[JQ_SUBMIT].pending_bios));

        /* handle synchronous decompress queue in the caller context */
        z_erofs_decompress_queue(&io[JQ_SUBMIT], pagepool);
}

static int z_erofs_readpage(struct file *file, struct page *page)
{
        struct inode *const inode = page->mapping->host;
        struct z_erofs_decompress_frontend f = DECOMPRESS_FRONTEND_INIT(inode);
        int err;
        LIST_HEAD(pagepool);

        trace_erofs_readpage(page, false);

        f.headoffset = (erofs_off_t)page->index << PAGE_SHIFT;

        err = z_erofs_do_read_page(&f, page);
        (void)z_erofs_collector_end(&f.clt);

        /* if some compressed cluster ready, need submit them anyway */
        z_erofs_runqueue(inode->i_sb, &f, &pagepool, true);

        if (err)
                erofs_err(inode->i_sb, "failed to read, err [%d]", err);

        if (f.map.mpage)
                put_page(f.map.mpage);

        /* clean up the remaining free pages */
        put_pages_list(&pagepool);
        return err;
}

static void z_erofs_readahead(struct readahead_control *rac)
{
        struct inode *const inode = rac->mapping->host;
        struct erofs_sb_info *const sbi = EROFS_I_SB(inode);

        unsigned int nr_pages = readahead_count(rac);
        bool sync = (nr_pages <= sbi->ctx.max_sync_decompress_pages);
        struct z_erofs_decompress_frontend f = DECOMPRESS_FRONTEND_INIT(inode);
        struct page *page, *head = NULL;
        LIST_HEAD(pagepool);

        trace_erofs_readpages(inode, readahead_index(rac), nr_pages, false);

        f.readahead = true;
        f.headoffset = readahead_pos(rac);

        while ((page = readahead_page(rac))) {
                prefetchw(&page->flags);

                /*
                 * A pure asynchronous readahead is indicated if
                 * a PG_readahead marked page is hitted at first.
                 * Let's also do asynchronous decompression for this case.
                 */
                sync &= !(PageReadahead(page) && !head);

                set_page_private(page, (unsigned long)head);
                head = page;
        }

        while (head) {
                struct page *page = head;
                int err;

                /* traversal in reverse order */
                head = (void *)page_private(page);

                err = z_erofs_do_read_page(&f, page);
                if (err)
                        erofs_err(inode->i_sb,
                                  "readahead error at page %lu @ nid %llu",
                                  page->index, EROFS_I(inode)->nid);
                put_page(page);
        }

        (void)z_erofs_collector_end(&f.clt);

        z_erofs_runqueue(inode->i_sb, &f, &pagepool, sync);

        if (f.map.mpage)
                put_page(f.map.mpage);

        /* clean up the remaining free pages */
        put_pages_list(&pagepool);
}

const struct address_space_operations z_erofs_aops = {
        .readpage = z_erofs_readpage,
        .readahead = z_erofs_readahead,
};