rdwr.c

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/* Storage object read/write
 *
 * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells ([email protected])
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/mount.h>
#include <linux/slab.h>
#include <linux/file.h>
#include "internal.h"

/*
 * detect wake up events generated by the unlocking of pages in which we're
 * interested
 * - we use this to detect read completion of backing pages
 * - the caller holds the waitqueue lock
 */
static int cachefiles_read_waiter(wait_queue_t *wait, unsigned mode,
                  int sync, void *_key)
{
    struct cachefiles_one_read *monitor =
        container_of(wait, struct cachefiles_one_read, monitor);
    struct cachefiles_object *object;
    struct wait_bit_key *key = _key;
    struct page *page = wait->private;

    ASSERT(key);

    _enter("{%lu},%u,%d,{%p,%u}",
           monitor->netfs_page->index, mode, sync,
           key->flags, key->bit_nr);

    if (key->flags != &page->flags ||
        key->bit_nr != PG_locked)
        return 0;

    _debug("--- monitor %p %lx ---", page, page->flags);

    if (!PageUptodate(page) && !PageError(page)) {
        /* unlocked, not uptodate and not erronous? */
        _debug("page probably truncated");
    }

    /* remove from the waitqueue */
    list_del(&wait->task_list);

    /* move onto the action list and queue for FS-Cache thread pool */
    ASSERT(monitor->op);

    object = container_of(monitor->op->op.object,
                  struct cachefiles_object, fscache);

    spin_lock(&object->work_lock);
    list_add_tail(&monitor->op_link, &monitor->op->to_do);
    spin_unlock(&object->work_lock);

    fscache_enqueue_retrieval(monitor->op);
    return 0;
}

/*
 * handle a probably truncated page
 * - check to see if the page is still relevant and reissue the read if
 *   possible
 * - return -EIO on error, -ENODATA if the page is gone, -EINPROGRESS if we
 *   must wait again and 0 if successful
 */
static int cachefiles_read_reissue(struct cachefiles_object *object,
                   struct cachefiles_one_read *monitor)
{
    struct address_space *bmapping = object->backer->d_inode->i_mapping;
    struct page *backpage = monitor->back_page, *backpage2;
    int ret;

    kenter("{ino=%lx},{%lx,%lx}",
           object->backer->d_inode->i_ino,
           backpage->index, backpage->flags);

    /* skip if the page was truncated away completely */
    if (backpage->mapping != bmapping) {
        kleave(" = -ENODATA [mapping]");
        return -ENODATA;
    }

    backpage2 = find_get_page(bmapping, backpage->index);
    if (!backpage2) {
        kleave(" = -ENODATA [gone]");
        return -ENODATA;
    }

    if (backpage != backpage2) {
        put_page(backpage2);
        kleave(" = -ENODATA [different]");
        return -ENODATA;
    }

    /* the page is still there and we already have a ref on it, so we don't
     * need a second */
    put_page(backpage2);

    INIT_LIST_HEAD(&monitor->op_link);
    add_page_wait_queue(backpage, &monitor->monitor);

    if (trylock_page(backpage)) {
        ret = -EIO;
        if (PageError(backpage))
            goto unlock_discard;
        ret = 0;
        if (PageUptodate(backpage))
            goto unlock_discard;

        kdebug("reissue read");
        ret = bmapping->a_ops->readpage(NULL, backpage);
        if (ret < 0)
            goto unlock_discard;
    }

    /* but the page may have been read before the monitor was installed, so
     * the monitor may miss the event - so we have to ensure that we do get
     * one in such a case */
    if (trylock_page(backpage)) {
        _debug("jumpstart %p {%lx}", backpage, backpage->flags);
        unlock_page(backpage);
    }

    /* it'll reappear on the todo list */
    kleave(" = -EINPROGRESS");
    return -EINPROGRESS;

unlock_discard:
    unlock_page(backpage);
    spin_lock_irq(&object->work_lock);
    list_del(&monitor->op_link);
    spin_unlock_irq(&object->work_lock);
    kleave(" = %d", ret);
    return ret;
}

/*
 * copy data from backing pages to netfs pages to complete a read operation
 * - driven by FS-Cache's thread pool
 */
static void cachefiles_read_copier(struct fscache_operation *_op)
{
    struct cachefiles_one_read *monitor;
    struct cachefiles_object *object;
    struct fscache_retrieval *op;
    struct pagevec pagevec;
    int error, max;

    op = container_of(_op, struct fscache_retrieval, op);
    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);

    _enter("{ino=%lu}", object->backer->d_inode->i_ino);

    pagevec_init(&pagevec, 0);

    max = 8;
    spin_lock_irq(&object->work_lock);

    while (!list_empty(&op->to_do)) {
        monitor = list_entry(op->to_do.next,
                     struct cachefiles_one_read, op_link);
        list_del(&monitor->op_link);

        spin_unlock_irq(&object->work_lock);

        _debug("- copy {%lu}", monitor->back_page->index);

    recheck:
        if (PageUptodate(monitor->back_page)) {
            copy_highpage(monitor->netfs_page, monitor->back_page);

            pagevec_add(&pagevec, monitor->netfs_page);
            fscache_mark_pages_cached(monitor->op, &pagevec);
            error = 0;
        } else if (!PageError(monitor->back_page)) {
            /* the page has probably been truncated */
            error = cachefiles_read_reissue(object, monitor);
            if (error == -EINPROGRESS)
                goto next;
            goto recheck;
        } else {
            cachefiles_io_error_obj(
                object,
                "Readpage failed on backing file %lx",
                (unsigned long) monitor->back_page->flags);
            error = -EIO;
        }

        page_cache_release(monitor->back_page);

        fscache_end_io(op, monitor->netfs_page, error);
        page_cache_release(monitor->netfs_page);
        fscache_put_retrieval(op);
        kfree(monitor);

    next:
        /* let the thread pool have some air occasionally */
        max--;
        if (max < 0 || need_resched()) {
            if (!list_empty(&op->to_do))
                fscache_enqueue_retrieval(op);
            _leave(" [maxed out]");
            return;
        }

        spin_lock_irq(&object->work_lock);
    }

    spin_unlock_irq(&object->work_lock);
    _leave("");
}

/*
 * read the corresponding page to the given set from the backing file
 * - an uncertain page is simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
                        struct fscache_retrieval *op,
                        struct page *netpage,
                        struct pagevec *pagevec)
{
    struct cachefiles_one_read *monitor;
    struct address_space *bmapping;
    struct page *newpage, *backpage;
    int ret;

    _enter("");

    pagevec_reinit(pagevec);

    _debug("read back %p{%lu,%d}",
           netpage, netpage->index, page_count(netpage));

    monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
    if (!monitor)
        goto nomem;

    monitor->netfs_page = netpage;
    monitor->op = fscache_get_retrieval(op);

    init_waitqueue_func_entry(&monitor->monitor, cachefiles_read_waiter);

    /* attempt to get hold of the backing page */
    bmapping = object->backer->d_inode->i_mapping;
    newpage = NULL;

    for (;;) {
        backpage = find_get_page(bmapping, netpage->index);
        if (backpage)
            goto backing_page_already_present;

        if (!newpage) {
            newpage = page_cache_alloc_cold(bmapping);
            if (!newpage)
                goto nomem_monitor;
        }

        ret = add_to_page_cache(newpage, bmapping,
                    netpage->index, GFP_KERNEL);
        if (ret == 0)
            goto installed_new_backing_page;
        if (ret != -EEXIST)
            goto nomem_page;
    }

    /* we've installed a new backing page, so now we need to add it
     * to the LRU list and start it reading */
installed_new_backing_page:
    _debug("- new %p", newpage);

    backpage = newpage;
    newpage = NULL;

    page_cache_get(backpage);
    pagevec_add(pagevec, backpage);
    __pagevec_lru_add_file(pagevec);

read_backing_page:
    ret = bmapping->a_ops->readpage(NULL, backpage);
    if (ret < 0)
        goto read_error;

    /* set the monitor to transfer the data across */
monitor_backing_page:
    _debug("- monitor add");

    /* install the monitor */
    page_cache_get(monitor->netfs_page);
    page_cache_get(backpage);
    monitor->back_page = backpage;
    monitor->monitor.private = backpage;
    add_page_wait_queue(backpage, &monitor->monitor);
    monitor = NULL;

    /* but the page may have been read before the monitor was installed, so
     * the monitor may miss the event - so we have to ensure that we do get
     * one in such a case */
    if (trylock_page(backpage)) {
        _debug("jumpstart %p {%lx}", backpage, backpage->flags);
        unlock_page(backpage);
    }
    goto success;

    /* if the backing page is already present, it can be in one of
     * three states: read in progress, read failed or read okay */
backing_page_already_present:
    _debug("- present");

    if (newpage) {
        page_cache_release(newpage);
        newpage = NULL;
    }

    if (PageError(backpage))
        goto io_error;

    if (PageUptodate(backpage))
        goto backing_page_already_uptodate;

    if (!trylock_page(backpage))
        goto monitor_backing_page;
    _debug("read %p {%lx}", backpage, backpage->flags);
    goto read_backing_page;

    /* the backing page is already up to date, attach the netfs
     * page to the pagecache and LRU and copy the data across */
backing_page_already_uptodate:
    _debug("- uptodate");

    pagevec_add(pagevec, netpage);
    fscache_mark_pages_cached(op, pagevec);

    copy_highpage(netpage, backpage);
    fscache_end_io(op, netpage, 0);

success:
    _debug("success");
    ret = 0;

out:
    if (backpage)
        page_cache_release(backpage);
    if (monitor) {
        fscache_put_retrieval(monitor->op);
        kfree(monitor);
    }
    _leave(" = %d", ret);
    return ret;

read_error:
    _debug("read error %d", ret);
    if (ret == -ENOMEM)
        goto out;
io_error:
    cachefiles_io_error_obj(object, "Page read error on backing file");
    ret = -ENOBUFS;
    goto out;

nomem_page:
    page_cache_release(newpage);
nomem_monitor:
    fscache_put_retrieval(monitor->op);
    kfree(monitor);
nomem:
    _leave(" = -ENOMEM");
    return -ENOMEM;
}

/*
 * read a page from the cache or allocate a block in which to store it
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - if the page is backed by a block in the cache:
 *   - a read will be started which will call the callback on completion
 *   - 0 will be returned
 * - else if the page is unbacked:
 *   - the metadata will be retained
 *   - -ENODATA will be returned
 */
int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,
                  struct page *page,
                  gfp_t gfp)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;
    struct pagevec pagevec;
    struct inode *inode;
    sector_t block0, block;
    unsigned shift;
    int ret;

    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);
    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    _enter("{%p},{%lx},,,", object, page->index);

    if (!object->backer)
        return -ENOBUFS;

    inode = object->backer->d_inode;
    ASSERT(S_ISREG(inode->i_mode));
    ASSERT(inode->i_mapping->a_ops->bmap);
    ASSERT(inode->i_mapping->a_ops->readpages);

    /* calculate the shift required to use bmap */
    if (inode->i_sb->s_blocksize > PAGE_SIZE)
        return -ENOBUFS;

    shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

    op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
    op->op.flags |= FSCACHE_OP_ASYNC;
    op->op.processor = cachefiles_read_copier;

    pagevec_init(&pagevec, 0);

    /* we assume the absence or presence of the first block is a good
     * enough indication for the page as a whole
     * - TODO: don't use bmap() for this as it is _not_ actually good
     *   enough for this as it doesn't indicate errors, but it's all we've
     *   got for the moment
     */
    block0 = page->index;
    block0 <<= shift;

    block = inode->i_mapping->a_ops->bmap(inode->i_mapping, block0);
    _debug("%llx -> %llx",
           (unsigned long long) block0,
           (unsigned long long) block);

    if (block) {
        /* submit the apparently valid page to the backing fs to be
         * read from disk */
        ret = cachefiles_read_backing_file_one(object, op, page,
                               &pagevec);
    } else if (cachefiles_has_space(cache, 0, 1) == 0) {
        /* there's space in the cache we can use */
        pagevec_add(&pagevec, page);
        fscache_mark_pages_cached(op, &pagevec);
        ret = -ENODATA;
    } else {
        ret = -ENOBUFS;
    }

    _leave(" = %d", ret);
    return ret;
}

/*
 * read the corresponding pages to the given set from the backing file
 * - any uncertain pages are simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file(struct cachefiles_object *object,
                    struct fscache_retrieval *op,
                    struct list_head *list,
                    struct pagevec *mark_pvec)
{
    struct cachefiles_one_read *monitor = NULL;
    struct address_space *bmapping = object->backer->d_inode->i_mapping;
    struct pagevec lru_pvec;
    struct page *newpage = NULL, *netpage, *_n, *backpage = NULL;
    int ret = 0;

    _enter("");

    pagevec_init(&lru_pvec, 0);

    list_for_each_entry_safe(netpage, _n, list, lru) {
        list_del(&netpage->lru);

        _debug("read back %p{%lu,%d}",
               netpage, netpage->index, page_count(netpage));

        if (!monitor) {
            monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
            if (!monitor)
                goto nomem;

            monitor->op = fscache_get_retrieval(op);
            init_waitqueue_func_entry(&monitor->monitor,
                          cachefiles_read_waiter);
        }

        for (;;) {
            backpage = find_get_page(bmapping, netpage->index);
            if (backpage)
                goto backing_page_already_present;

            if (!newpage) {
                newpage = page_cache_alloc_cold(bmapping);
                if (!newpage)
                    goto nomem;
            }

            ret = add_to_page_cache(newpage, bmapping,
                        netpage->index, GFP_KERNEL);
            if (ret == 0)
                goto installed_new_backing_page;
            if (ret != -EEXIST)
                goto nomem;
        }

        /* we've installed a new backing page, so now we need to add it
         * to the LRU list and start it reading */
    installed_new_backing_page:
        _debug("- new %p", newpage);

        backpage = newpage;
        newpage = NULL;

        page_cache_get(backpage);
        if (!pagevec_add(&lru_pvec, backpage))
            __pagevec_lru_add_file(&lru_pvec);

    reread_backing_page:
        ret = bmapping->a_ops->readpage(NULL, backpage);
        if (ret < 0)
            goto read_error;

        /* add the netfs page to the pagecache and LRU, and set the
         * monitor to transfer the data across */
    monitor_backing_page:
        _debug("- monitor add");

        ret = add_to_page_cache(netpage, op->mapping, netpage->index,
                    GFP_KERNEL);
        if (ret < 0) {
            if (ret == -EEXIST) {
                page_cache_release(netpage);
                continue;
            }
            goto nomem;
        }

        page_cache_get(netpage);
        if (!pagevec_add(&lru_pvec, netpage))
            __pagevec_lru_add_file(&lru_pvec);

        /* install a monitor */
        page_cache_get(netpage);
        monitor->netfs_page = netpage;

        page_cache_get(backpage);
        monitor->back_page = backpage;
        monitor->monitor.private = backpage;
        add_page_wait_queue(backpage, &monitor->monitor);
        monitor = NULL;

        /* but the page may have been read before the monitor was
         * installed, so the monitor may miss the event - so we have to
         * ensure that we do get one in such a case */
        if (trylock_page(backpage)) {
            _debug("2unlock %p {%lx}", backpage, backpage->flags);
            unlock_page(backpage);
        }

        page_cache_release(backpage);
        backpage = NULL;

        page_cache_release(netpage);
        netpage = NULL;
        continue;

        /* if the backing page is already present, it can be in one of
         * three states: read in progress, read failed or read okay */
    backing_page_already_present:
        _debug("- present %p", backpage);

        if (PageError(backpage))
            goto io_error;

        if (PageUptodate(backpage))
            goto backing_page_already_uptodate;

        _debug("- not ready %p{%lx}", backpage, backpage->flags);

        if (!trylock_page(backpage))
            goto monitor_backing_page;

        if (PageError(backpage)) {
            _debug("error %lx", backpage->flags);
            unlock_page(backpage);
            goto io_error;
        }

        if (PageUptodate(backpage))
            goto backing_page_already_uptodate_unlock;

        /* we've locked a page that's neither up to date nor erroneous,
         * so we need to attempt to read it again */
        goto reread_backing_page;

        /* the backing page is already up to date, attach the netfs
         * page to the pagecache and LRU and copy the data across */
    backing_page_already_uptodate_unlock:
        _debug("uptodate %lx", backpage->flags);
        unlock_page(backpage);
    backing_page_already_uptodate:
        _debug("- uptodate");

        ret = add_to_page_cache(netpage, op->mapping, netpage->index,
                    GFP_KERNEL);
        if (ret < 0) {
            if (ret == -EEXIST) {
                page_cache_release(netpage);
                continue;
            }
            goto nomem;
        }

        copy_highpage(netpage, backpage);

        page_cache_release(backpage);
        backpage = NULL;

        if (!pagevec_add(mark_pvec, netpage))
            fscache_mark_pages_cached(op, mark_pvec);

        page_cache_get(netpage);
        if (!pagevec_add(&lru_pvec, netpage))
            __pagevec_lru_add_file(&lru_pvec);

        fscache_end_io(op, netpage, 0);
        page_cache_release(netpage);
        netpage = NULL;
        continue;
    }

    netpage = NULL;

    _debug("out");

out:
    /* tidy up */
    pagevec_lru_add_file(&lru_pvec);

    if (newpage)
        page_cache_release(newpage);
    if (netpage)
        page_cache_release(netpage);
    if (backpage)
        page_cache_release(backpage);
    if (monitor) {
        fscache_put_retrieval(op);
        kfree(monitor);
    }

    list_for_each_entry_safe(netpage, _n, list, lru) {
        list_del(&netpage->lru);
        page_cache_release(netpage);
    }

    _leave(" = %d", ret);
    return ret;

nomem:
    _debug("nomem");
    ret = -ENOMEM;
    goto out;

read_error:
    _debug("read error %d", ret);
    if (ret == -ENOMEM)
        goto out;
io_error:
    cachefiles_io_error_obj(object, "Page read error on backing file");
    ret = -ENOBUFS;
    goto out;
}

/*
 * read a list of pages from the cache or allocate blocks in which to store
 * them
 */
int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,
                   struct list_head *pages,
                   unsigned *nr_pages,
                   gfp_t gfp)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;
    struct list_head backpages;
    struct pagevec pagevec;
    struct inode *inode;
    struct page *page, *_n;
    unsigned shift, nrbackpages;
    int ret, ret2, space;

    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);
    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    _enter("{OBJ%x,%d},,%d,,",
           object->fscache.debug_id, atomic_read(&op->op.usage),
           *nr_pages);

    if (!object->backer)
        return -ENOBUFS;

    space = 1;
    if (cachefiles_has_space(cache, 0, *nr_pages) < 0)
        space = 0;

    inode = object->backer->d_inode;
    ASSERT(S_ISREG(inode->i_mode));
    ASSERT(inode->i_mapping->a_ops->bmap);
    ASSERT(inode->i_mapping->a_ops->readpages);

    /* calculate the shift required to use bmap */
    if (inode->i_sb->s_blocksize > PAGE_SIZE)
        return -ENOBUFS;

    shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

    pagevec_init(&pagevec, 0);

    op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
    op->op.flags |= FSCACHE_OP_ASYNC;
    op->op.processor = cachefiles_read_copier;

    INIT_LIST_HEAD(&backpages);
    nrbackpages = 0;

    ret = space ? -ENODATA : -ENOBUFS;
    list_for_each_entry_safe(page, _n, pages, lru) {
        sector_t block0, block;

        /* we assume the absence or presence of the first block is a
         * good enough indication for the page as a whole
         * - TODO: don't use bmap() for this as it is _not_ actually
         *   good enough for this as it doesn't indicate errors, but
         *   it's all we've got for the moment
         */
        block0 = page->index;
        block0 <<= shift;

        block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
                              block0);
        _debug("%llx -> %llx",
               (unsigned long long) block0,
               (unsigned long long) block);

        if (block) {
            /* we have data - add it to the list to give to the
             * backing fs */
            list_move(&page->lru, &backpages);
            (*nr_pages)--;
            nrbackpages++;
        } else if (space && pagevec_add(&pagevec, page) == 0) {
            fscache_mark_pages_cached(op, &pagevec);
            ret = -ENODATA;
        }
    }

    if (pagevec_count(&pagevec) > 0)
        fscache_mark_pages_cached(op, &pagevec);

    if (list_empty(pages))
        ret = 0;

    /* submit the apparently valid pages to the backing fs to be read from
     * disk */
    if (nrbackpages > 0) {
        ret2 = cachefiles_read_backing_file(object, op, &backpages,
                            &pagevec);
        if (ret2 == -ENOMEM || ret2 == -EINTR)
            ret = ret2;
    }

    if (pagevec_count(&pagevec) > 0)
        fscache_mark_pages_cached(op, &pagevec);

    _leave(" = %d [nr=%u%s]",
           ret, *nr_pages, list_empty(pages) ? " empty" : "");
    return ret;
}

/*
 * allocate a block in the cache in which to store a page
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - otherwise:
 *   - the metadata will be retained
 *   - 0 will be returned
 */
int cachefiles_allocate_page(struct fscache_retrieval *op,
                 struct page *page,
                 gfp_t gfp)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;
    struct pagevec pagevec;
    int ret;

    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);
    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    _enter("%p,{%lx},", object, page->index);

    ret = cachefiles_has_space(cache, 0, 1);
    if (ret == 0) {
        pagevec_init(&pagevec, 0);
        pagevec_add(&pagevec, page);
        fscache_mark_pages_cached(op, &pagevec);
    } else {
        ret = -ENOBUFS;
    }

    _leave(" = %d", ret);
    return ret;
}

/*
 * allocate blocks in the cache in which to store a set of pages
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if some buffers couldn't be made available
 * - returns -ENOBUFS if some pages are beyond EOF
 * - otherwise:
 *   - -ENODATA will be returned
 * - metadata will be retained for any page marked
 */
int cachefiles_allocate_pages(struct fscache_retrieval *op,
                  struct list_head *pages,
                  unsigned *nr_pages,
                  gfp_t gfp)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;
    struct pagevec pagevec;
    struct page *page;
    int ret;

    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);
    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    _enter("%p,,,%d,", object, *nr_pages);

    ret = cachefiles_has_space(cache, 0, *nr_pages);
    if (ret == 0) {
        pagevec_init(&pagevec, 0);

        list_for_each_entry(page, pages, lru) {
            if (pagevec_add(&pagevec, page) == 0)
                fscache_mark_pages_cached(op, &pagevec);
        }

        if (pagevec_count(&pagevec) > 0)
            fscache_mark_pages_cached(op, &pagevec);
        ret = -ENODATA;
    } else {
        ret = -ENOBUFS;
    }

    _leave(" = %d", ret);
    return ret;
}

/*
 * request a page be stored in the cache
 * - cache withdrawal is prevented by the caller
 * - this request may be ignored if there's no cache block available, in which
 *   case -ENOBUFS will be returned
 * - if the op is in progress, 0 will be returned
 */
int cachefiles_write_page(struct fscache_storage *op, struct page *page)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;
    mm_segment_t old_fs;
    struct file *file;
    struct path path;
    loff_t pos, eof;
    size_t len;
    void *data;
    int ret;

    ASSERT(op != NULL);
    ASSERT(page != NULL);

    object = container_of(op->op.object,
                  struct cachefiles_object, fscache);

    _enter("%p,%p{%lx},,,", object, page, page->index);

    if (!object->backer) {
        _leave(" = -ENOBUFS");
        return -ENOBUFS;
    }

    ASSERT(S_ISREG(object->backer->d_inode->i_mode));

    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    /* write the page to the backing filesystem and let it store it in its
     * own time */
    path.mnt = cache->mnt;
    path.dentry = object->backer;
    file = dentry_open(&path, O_RDWR | O_LARGEFILE, cache->cache_cred);
    if (IS_ERR(file)) {
        ret = PTR_ERR(file);
    } else {
        ret = -EIO;
        if (file->f_op->write) {
            pos = (loff_t) page->index << PAGE_SHIFT;

            /* we mustn't write more data than we have, so we have
             * to beware of a partial page at EOF */
            eof = object->fscache.store_limit_l;
            len = PAGE_SIZE;
            if (eof & ~PAGE_MASK) {
                ASSERTCMP(pos, <, eof);
                if (eof - pos < PAGE_SIZE) {
                    _debug("cut short %llx to %llx",
                           pos, eof);
                    len = eof - pos;
                    ASSERTCMP(pos + len, ==, eof);
                }
            }

            data = kmap(page);
            old_fs = get_fs();
            set_fs(KERNEL_DS);
            ret = file->f_op->write(
                file, (const void __user *) data, len, &pos);
            set_fs(old_fs);
            kunmap(page);
            if (ret != len)
                ret = -EIO;
        }
        fput(file);
    }

    if (ret < 0) {
        if (ret == -EIO)
            cachefiles_io_error_obj(
                object, "Write page to backing file failed");
        ret = -ENOBUFS;
    }

    _leave(" = %d", ret);
    return ret;
}

/*
 * detach a backing block from a page
 * - cache withdrawal is prevented by the caller
 */
void cachefiles_uncache_page(struct fscache_object *_object, struct page *page)
{
    struct cachefiles_object *object;
    struct cachefiles_cache *cache;

    object = container_of(_object, struct cachefiles_object, fscache);
    cache = container_of(object->fscache.cache,
                 struct cachefiles_cache, cache);

    _enter("%p,{%lu}", object, page->index);

    spin_unlock(&object->fscache.cookie->lock);
}