// SPDX-License-Identifier: GPL-2.0-or-later
/* Network filesystem high-level read support.
 *
 * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#include <linux/module.h>
#include <linux/export.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/slab.h>
#include <linux/uio.h>
#include <linux/sched/mm.h>
#include <linux/task_io_accounting_ops.h>
#include "internal.h"

/*
 * Clear the unread part of an I/O request.
 */
static void netfs_clear_unread(struct netfs_io_subrequest *subreq)
{
        struct iov_iter iter;

        iov_iter_xarray(&iter, READ, &subreq->rreq->mapping->i_pages,
                        subreq->start + subreq->transferred,
                        subreq->len   - subreq->transferred);
        iov_iter_zero(iov_iter_count(&iter), &iter);
}

static void netfs_cache_read_terminated(void *priv, ssize_t transferred_or_error,
                                        bool was_async)
{
        struct netfs_io_subrequest *subreq = priv;

        netfs_subreq_terminated(subreq, transferred_or_error, was_async);
}

/*
 * Issue a read against the cache.
 * - Eats the caller's ref on subreq.
 */
static void netfs_read_from_cache(struct netfs_io_request *rreq,
                                  struct netfs_io_subrequest *subreq,
                                  enum netfs_read_from_hole read_hole)
{
        struct netfs_cache_resources *cres = &rreq->cache_resources;
        struct iov_iter iter;

        netfs_stat(&netfs_n_rh_read);
        iov_iter_xarray(&iter, READ, &rreq->mapping->i_pages,
                        subreq->start + subreq->transferred,
                        subreq->len   - subreq->transferred);

        cres->ops->read(cres, subreq->start, &iter, read_hole,
                        netfs_cache_read_terminated, subreq);
}

/*
 * Fill a subrequest region with zeroes.
 */
static void netfs_fill_with_zeroes(struct netfs_io_request *rreq,
                                   struct netfs_io_subrequest *subreq)
{
        netfs_stat(&netfs_n_rh_zero);
        __set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
        netfs_subreq_terminated(subreq, 0, false);
}

/*
 * Ask the netfs to issue a read request to the server for us.
 *
 * The netfs is expected to read from subreq->pos + subreq->transferred to
 * subreq->pos + subreq->len - 1.  It may not backtrack and write data into the
 * buffer prior to the transferred point as it might clobber dirty data
 * obtained from the cache.
 *
 * Alternatively, the netfs is allowed to indicate one of two things:
 *
 * - NETFS_SREQ_SHORT_READ: A short read - it will get called again to try and
 *   make progress.
 *
 * - NETFS_SREQ_CLEAR_TAIL: A short read - the rest of the buffer will be
 *   cleared.
 */
static void netfs_read_from_server(struct netfs_io_request *rreq,
                                   struct netfs_io_subrequest *subreq)
{
        netfs_stat(&netfs_n_rh_download);
        rreq->netfs_ops->issue_read(subreq);
}

/*
 * Release those waiting.
 */
static void netfs_rreq_completed(struct netfs_io_request *rreq, bool was_async)
{
        trace_netfs_rreq(rreq, netfs_rreq_trace_done);
        netfs_clear_subrequests(rreq, was_async);
        netfs_put_request(rreq, was_async, netfs_rreq_trace_put_complete);
}

/*
 * Deal with the completion of writing the data to the cache.  We have to clear
 * the PG_fscache bits on the folios involved and release the caller's ref.
 *
 * May be called in softirq mode and we inherit a ref from the caller.
 */
static void netfs_rreq_unmark_after_write(struct netfs_io_request *rreq,
                                          bool was_async)
{
        struct netfs_io_subrequest *subreq;
        struct folio *folio;
        pgoff_t unlocked = 0;
        bool have_unlocked = false;

        rcu_read_lock();

        list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
                XA_STATE(xas, &rreq->mapping->i_pages, subreq->start / PAGE_SIZE);

                xas_for_each(&xas, folio, (subreq->start + subreq->len - 1) / PAGE_SIZE) {
                        /* We might have multiple writes from the same huge
                         * folio, but we mustn't unlock a folio more than once.
                         */
                        if (have_unlocked && folio_index(folio) <= unlocked)
                                continue;
                        unlocked = folio_index(folio);
                        folio_end_fscache(folio);
                        have_unlocked = true;
                }
        }

        rcu_read_unlock();
        netfs_rreq_completed(rreq, was_async);
}

static void netfs_rreq_copy_terminated(void *priv, ssize_t transferred_or_error,
                                       bool was_async)
{
        struct netfs_io_subrequest *subreq = priv;
        struct netfs_io_request *rreq = subreq->rreq;

        if (IS_ERR_VALUE(transferred_or_error)) {
                netfs_stat(&netfs_n_rh_write_failed);
                trace_netfs_failure(rreq, subreq, transferred_or_error,
                                    netfs_fail_copy_to_cache);
        } else {
                netfs_stat(&netfs_n_rh_write_done);
        }

        trace_netfs_sreq(subreq, netfs_sreq_trace_write_term);

        /* If we decrement nr_copy_ops to 0, the ref belongs to us. */
        if (atomic_dec_and_test(&rreq->nr_copy_ops))
                netfs_rreq_unmark_after_write(rreq, was_async);

        netfs_put_subrequest(subreq, was_async, netfs_sreq_trace_put_terminated);
}

/*
 * Perform any outstanding writes to the cache.  We inherit a ref from the
 * caller.
 */
static void netfs_rreq_do_write_to_cache(struct netfs_io_request *rreq)
{
        struct netfs_cache_resources *cres = &rreq->cache_resources;
        struct netfs_io_subrequest *subreq, *next, *p;
        struct iov_iter iter;
        int ret;

        trace_netfs_rreq(rreq, netfs_rreq_trace_copy);

        /* We don't want terminating writes trying to wake us up whilst we're
         * still going through the list.
         */
        atomic_inc(&rreq->nr_copy_ops);

        list_for_each_entry_safe(subreq, p, &rreq->subrequests, rreq_link) {
                if (!test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags)) {
                        list_del_init(&subreq->rreq_link);
                        netfs_put_subrequest(subreq, false,
                                             netfs_sreq_trace_put_no_copy);
                }
        }

        list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
                /* Amalgamate adjacent writes */
                while (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
                        next = list_next_entry(subreq, rreq_link);
                        if (next->start != subreq->start + subreq->len)
                                break;
                        subreq->len += next->len;
                        list_del_init(&next->rreq_link);
                        netfs_put_subrequest(next, false,
                                             netfs_sreq_trace_put_merged);
                }

                ret = cres->ops->prepare_write(cres, &subreq->start, &subreq->len,
                                               rreq->i_size, true);
                if (ret < 0) {
                        trace_netfs_failure(rreq, subreq, ret, netfs_fail_prepare_write);
                        trace_netfs_sreq(subreq, netfs_sreq_trace_write_skip);
                        continue;
                }

                iov_iter_xarray(&iter, WRITE, &rreq->mapping->i_pages,
                                subreq->start, subreq->len);

                atomic_inc(&rreq->nr_copy_ops);
                netfs_stat(&netfs_n_rh_write);
                netfs_get_subrequest(subreq, netfs_sreq_trace_get_copy_to_cache);
                trace_netfs_sreq(subreq, netfs_sreq_trace_write);
                cres->ops->write(cres, subreq->start, &iter,
                                 netfs_rreq_copy_terminated, subreq);
        }

        /* If we decrement nr_copy_ops to 0, the usage ref belongs to us. */
        if (atomic_dec_and_test(&rreq->nr_copy_ops))
                netfs_rreq_unmark_after_write(rreq, false);
}

static void netfs_rreq_write_to_cache_work(struct work_struct *work)
{
        struct netfs_io_request *rreq =
                container_of(work, struct netfs_io_request, work);

        netfs_rreq_do_write_to_cache(rreq);
}

static void netfs_rreq_write_to_cache(struct netfs_io_request *rreq)
{
        rreq->work.func = netfs_rreq_write_to_cache_work;
        if (!queue_work(system_unbound_wq, &rreq->work))
                BUG();
}

/*
 * Handle a short read.
 */
static void netfs_rreq_short_read(struct netfs_io_request *rreq,
                                  struct netfs_io_subrequest *subreq)
{
        __clear_bit(NETFS_SREQ_SHORT_IO, &subreq->flags);
        __set_bit(NETFS_SREQ_SEEK_DATA_READ, &subreq->flags);

        netfs_stat(&netfs_n_rh_short_read);
        trace_netfs_sreq(subreq, netfs_sreq_trace_resubmit_short);

        netfs_get_subrequest(subreq, netfs_sreq_trace_get_short_read);
        atomic_inc(&rreq->nr_outstanding);
        if (subreq->source == NETFS_READ_FROM_CACHE)
                netfs_read_from_cache(rreq, subreq, NETFS_READ_HOLE_CLEAR);
        else
                netfs_read_from_server(rreq, subreq);
}

/*
 * Resubmit any short or failed operations.  Returns true if we got the rreq
 * ref back.
 */
static bool netfs_rreq_perform_resubmissions(struct netfs_io_request *rreq)
{
        struct netfs_io_subrequest *subreq;

        WARN_ON(in_interrupt());

        trace_netfs_rreq(rreq, netfs_rreq_trace_resubmit);

        /* We don't want terminating submissions trying to wake us up whilst
         * we're still going through the list.
         */
        atomic_inc(&rreq->nr_outstanding);

        __clear_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
        list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
                if (subreq->error) {
                        if (subreq->source != NETFS_READ_FROM_CACHE)
                                break;
                        subreq->source = NETFS_DOWNLOAD_FROM_SERVER;
                        subreq->error = 0;
                        netfs_stat(&netfs_n_rh_download_instead);
                        trace_netfs_sreq(subreq, netfs_sreq_trace_download_instead);
                        netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
                        atomic_inc(&rreq->nr_outstanding);
                        netfs_read_from_server(rreq, subreq);
                } else if (test_bit(NETFS_SREQ_SHORT_IO, &subreq->flags)) {
                        netfs_rreq_short_read(rreq, subreq);
                }
        }

        /* If we decrement nr_outstanding to 0, the usage ref belongs to us. */
        if (atomic_dec_and_test(&rreq->nr_outstanding))
                return true;

        wake_up_var(&rreq->nr_outstanding);
        return false;
}

/*
 * Check to see if the data read is still valid.
 */
static void netfs_rreq_is_still_valid(struct netfs_io_request *rreq)
{
        struct netfs_io_subrequest *subreq;

        if (!rreq->netfs_ops->is_still_valid ||
            rreq->netfs_ops->is_still_valid(rreq))
                return;

        list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
                if (subreq->source == NETFS_READ_FROM_CACHE) {
                        subreq->error = -ESTALE;
                        __set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
                }
        }
}

/*
 * Assess the state of a read request and decide what to do next.
 *
 * Note that we could be in an ordinary kernel thread, on a workqueue or in
 * softirq context at this point.  We inherit a ref from the caller.
 */
static void netfs_rreq_assess(struct netfs_io_request *rreq, bool was_async)
{
        trace_netfs_rreq(rreq, netfs_rreq_trace_assess);

again:
        netfs_rreq_is_still_valid(rreq);

        if (!test_bit(NETFS_RREQ_FAILED, &rreq->flags) &&
            test_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags)) {
                if (netfs_rreq_perform_resubmissions(rreq))
                        goto again;
                return;
        }

        netfs_rreq_unlock_folios(rreq);

        clear_bit_unlock(NETFS_RREQ_IN_PROGRESS, &rreq->flags);
        wake_up_bit(&rreq->flags, NETFS_RREQ_IN_PROGRESS);

        if (test_bit(NETFS_RREQ_COPY_TO_CACHE, &rreq->flags))
                return netfs_rreq_write_to_cache(rreq);

        netfs_rreq_completed(rreq, was_async);
}

static void netfs_rreq_work(struct work_struct *work)
{
        struct netfs_io_request *rreq =
                container_of(work, struct netfs_io_request, work);
        netfs_rreq_assess(rreq, false);
}

/*
 * Handle the completion of all outstanding I/O operations on a read request.
 * We inherit a ref from the caller.
 */
static void netfs_rreq_terminated(struct netfs_io_request *rreq,
                                  bool was_async)
{
        if (test_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags) &&
            was_async) {
                if (!queue_work(system_unbound_wq, &rreq->work))
                        BUG();
        } else {
                netfs_rreq_assess(rreq, was_async);
        }
}

/**
 * netfs_subreq_terminated - Note the termination of an I/O operation.
 * @subreq: The I/O request that has terminated.
 * @transferred_or_error: The amount of data transferred or an error code.
 * @was_async: The termination was asynchronous
 *
 * This tells the read helper that a contributory I/O operation has terminated,
 * one way or another, and that it should integrate the results.
 *
 * The caller indicates in @transferred_or_error the outcome of the operation,
 * supplying a positive value to indicate the number of bytes transferred, 0 to
 * indicate a failure to transfer anything that should be retried or a negative
 * error code.  The helper will look after reissuing I/O operations as
 * appropriate and writing downloaded data to the cache.
 *
 * If @was_async is true, the caller might be running in softirq or interrupt
 * context and we can't sleep.
 */
void netfs_subreq_terminated(struct netfs_io_subrequest *subreq,
                             ssize_t transferred_or_error,
                             bool was_async)
{
        struct netfs_io_request *rreq = subreq->rreq;
        int u;

        _enter("[%u]{%llx,%lx},%zd",
               subreq->debug_index, subreq->start, subreq->flags,
               transferred_or_error);

        switch (subreq->source) {
        case NETFS_READ_FROM_CACHE:
                netfs_stat(&netfs_n_rh_read_done);
                break;
        case NETFS_DOWNLOAD_FROM_SERVER:
                netfs_stat(&netfs_n_rh_download_done);
                break;
        default:
                break;
        }

        if (IS_ERR_VALUE(transferred_or_error)) {
                subreq->error = transferred_or_error;
                trace_netfs_failure(rreq, subreq, transferred_or_error,
                                    netfs_fail_read);
                goto failed;
        }

        if (WARN(transferred_or_error > subreq->len - subreq->transferred,
                 "Subreq overread: R%x[%x] %zd > %zu - %zu",
                 rreq->debug_id, subreq->debug_index,
                 transferred_or_error, subreq->len, subreq->transferred))
                transferred_or_error = subreq->len - subreq->transferred;

        subreq->error = 0;
        subreq->transferred += transferred_or_error;
        if (subreq->transferred < subreq->len)
                goto incomplete;

complete:
        __clear_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags);
        if (test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags))
                set_bit(NETFS_RREQ_COPY_TO_CACHE, &rreq->flags);

out:
        trace_netfs_sreq(subreq, netfs_sreq_trace_terminated);

        /* If we decrement nr_outstanding to 0, the ref belongs to us. */
        u = atomic_dec_return(&rreq->nr_outstanding);
        if (u == 0)
                netfs_rreq_terminated(rreq, was_async);
        else if (u == 1)
                wake_up_var(&rreq->nr_outstanding);

        netfs_put_subrequest(subreq, was_async, netfs_sreq_trace_put_terminated);
        return;

incomplete:
        if (test_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags)) {
                netfs_clear_unread(subreq);
                subreq->transferred = subreq->len;
                goto complete;
        }

        if (transferred_or_error == 0) {
                if (__test_and_set_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags)) {
                        subreq->error = -ENODATA;
                        goto failed;
                }
        } else {
                __clear_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags);
        }

        __set_bit(NETFS_SREQ_SHORT_IO, &subreq->flags);
        set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
        goto out;

failed:
        if (subreq->source == NETFS_READ_FROM_CACHE) {
                netfs_stat(&netfs_n_rh_read_failed);
                set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
        } else {
                netfs_stat(&netfs_n_rh_download_failed);
                set_bit(NETFS_RREQ_FAILED, &rreq->flags);
                rreq->error = subreq->error;
        }
        goto out;
}
EXPORT_SYMBOL(netfs_subreq_terminated);

static enum netfs_io_source netfs_cache_prepare_read(struct netfs_io_subrequest *subreq,
                                                       loff_t i_size)
{
        struct netfs_io_request *rreq = subreq->rreq;
        struct netfs_cache_resources *cres = &rreq->cache_resources;

        if (cres->ops)
                return cres->ops->prepare_read(subreq, i_size);
        if (subreq->start >= rreq->i_size)
                return NETFS_FILL_WITH_ZEROES;
        return NETFS_DOWNLOAD_FROM_SERVER;
}

/*
 * Work out what sort of subrequest the next one will be.
 */
static enum netfs_io_source
netfs_rreq_prepare_read(struct netfs_io_request *rreq,
                        struct netfs_io_subrequest *subreq)
{
        enum netfs_io_source source;

        _enter("%llx-%llx,%llx", subreq->start, subreq->start + subreq->len, rreq->i_size);

        source = netfs_cache_prepare_read(subreq, rreq->i_size);
        if (source == NETFS_INVALID_READ)
                goto out;

        if (source == NETFS_DOWNLOAD_FROM_SERVER) {
                /* Call out to the netfs to let it shrink the request to fit
                 * its own I/O sizes and boundaries.  If it shinks it here, it
                 * will be called again to make simultaneous calls; if it wants
                 * to make serial calls, it can indicate a short read and then
                 * we will call it again.
                 */
                if (subreq->len > rreq->i_size - subreq->start)
                        subreq->len = rreq->i_size - subreq->start;

                if (rreq->netfs_ops->clamp_length &&
                    !rreq->netfs_ops->clamp_length(subreq)) {
                        source = NETFS_INVALID_READ;
                        goto out;
                }
        }

        if (WARN_ON(subreq->len == 0))
                source = NETFS_INVALID_READ;

out:
        subreq->source = source;
        trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
        return source;
}

/*
 * Slice off a piece of a read request and submit an I/O request for it.
 */
static bool netfs_rreq_submit_slice(struct netfs_io_request *rreq,
                                    unsigned int *_debug_index)
{
        struct netfs_io_subrequest *subreq;
        enum netfs_io_source source;

        subreq = netfs_alloc_subrequest(rreq);
        if (!subreq)
                return false;

        subreq->debug_index     = (*_debug_index)++;
        subreq->start           = rreq->start + rreq->submitted;
        subreq->len             = rreq->len   - rreq->submitted;

        _debug("slice %llx,%zx,%zx", subreq->start, subreq->len, rreq->submitted);
        list_add_tail(&subreq->rreq_link, &rreq->subrequests);

        /* Call out to the cache to find out what it can do with the remaining
         * subset.  It tells us in subreq->flags what it decided should be done
         * and adjusts subreq->len down if the subset crosses a cache boundary.
         *
         * Then when we hand the subset, it can choose to take a subset of that
         * (the starts must coincide), in which case, we go around the loop
         * again and ask it to download the next piece.
         */
        source = netfs_rreq_prepare_read(rreq, subreq);
        if (source == NETFS_INVALID_READ)
                goto subreq_failed;

        atomic_inc(&rreq->nr_outstanding);

        rreq->submitted += subreq->len;

        trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
        switch (source) {
        case NETFS_FILL_WITH_ZEROES:
                netfs_fill_with_zeroes(rreq, subreq);
                break;
        case NETFS_DOWNLOAD_FROM_SERVER:
                netfs_read_from_server(rreq, subreq);
                break;
        case NETFS_READ_FROM_CACHE:
                netfs_read_from_cache(rreq, subreq, NETFS_READ_HOLE_IGNORE);
                break;
        default:
                BUG();
        }

        return true;

subreq_failed:
        rreq->error = subreq->error;
        netfs_put_subrequest(subreq, false, netfs_sreq_trace_put_failed);
        return false;
}

/*
 * Begin the process of reading in a chunk of data, where that data may be
 * stitched together from multiple sources, including multiple servers and the
 * local cache.
 */
int netfs_begin_read(struct netfs_io_request *rreq, bool sync)
{
        unsigned int debug_index = 0;
        int ret;

        _enter("R=%x %llx-%llx",
               rreq->debug_id, rreq->start, rreq->start + rreq->len - 1);

        if (rreq->len == 0) {
                pr_err("Zero-sized read [R=%x]\n", rreq->debug_id);
                netfs_put_request(rreq, false, netfs_rreq_trace_put_zero_len);
                return -EIO;
        }

        INIT_WORK(&rreq->work, netfs_rreq_work);

        if (sync)
                netfs_get_request(rreq, netfs_rreq_trace_get_hold);

        /* Chop the read into slices according to what the cache and the netfs
         * want and submit each one.
         */
        atomic_set(&rreq->nr_outstanding, 1);
        do {
                if (!netfs_rreq_submit_slice(rreq, &debug_index))
                        break;

        } while (rreq->submitted < rreq->len);

        if (sync) {
                /* Keep nr_outstanding incremented so that the ref always belongs to
                 * us, and the service code isn't punted off to a random thread pool to
                 * process.
                 */
                for (;;) {
                        wait_var_event(&rreq->nr_outstanding,
                                       atomic_read(&rreq->nr_outstanding) == 1);
                        netfs_rreq_assess(rreq, false);
                        if (!test_bit(NETFS_RREQ_IN_PROGRESS, &rreq->flags))
                                break;
                        cond_resched();
                }

                ret = rreq->error;
                if (ret == 0 && rreq->submitted < rreq->len) {
                        trace_netfs_failure(rreq, NULL, ret, netfs_fail_short_read);
                        ret = -EIO;
                }
                netfs_put_request(rreq, false, netfs_rreq_trace_put_hold);
        } else {
                /* If we decrement nr_outstanding to 0, the ref belongs to us. */
                if (atomic_dec_and_test(&rreq->nr_outstanding))
                        netfs_rreq_assess(rreq, false);
                ret = 0;
        }
        return ret;
}