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    Subject[PATCH 3/3] tmpfs: fix spurious ENOSPC when racing with unswap
    Testing the shmem_swaplist replacements for igrab() revealed another bug:
    writes to /dev/loop0 on a tmpfs file which fills its filesystem were
    sometimes failing with "Buffer I/O error"s.

    These came from ENOSPC failures of shmem_getpage(), when racing with
    swapoff: the same could happen when racing with another shmem_getpage(),
    pulling the page in from swap in between our find_lock_page() and our
    taking the info->lock (though not in the single-threaded loop case).

    This is unacceptable, and surprising that I've not noticed it before:
    it dates back many years, but (presumably) was made a lot easier to
    reproduce in 2.6.36, which sited a page preallocation in the race window.

    Fix it by rechecking the page cache before settling on an ENOSPC error.

    Signed-off-by: Hugh Dickins <hughd@google.com>
    Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
    Cc: stable@kernel.org
    ---

    mm/shmem.c | 32 ++++++++++++++++++++++----------
    1 file changed, 22 insertions(+), 10 deletions(-)

    --- tmpfs2/mm/shmem.c 2011-05-07 17:39:00.656959448 -0700
    +++ tmpfs3/mm/shmem.c 2011-05-07 17:40:00.665256101 -0700
    @@ -1407,20 +1407,14 @@ repeat:
    if (sbinfo->max_blocks) {
    if (percpu_counter_compare(&sbinfo->used_blocks,
    sbinfo->max_blocks) >= 0 ||
    - shmem_acct_block(info->flags)) {
    - spin_unlock(&info->lock);
    - error = -ENOSPC;
    - goto failed;
    - }
    + shmem_acct_block(info->flags))
    + goto nospace;
    percpu_counter_inc(&sbinfo->used_blocks);
    spin_lock(&inode->i_lock);
    inode->i_blocks += BLOCKS_PER_PAGE;
    spin_unlock(&inode->i_lock);
    - } else if (shmem_acct_block(info->flags)) {
    - spin_unlock(&info->lock);
    - error = -ENOSPC;
    - goto failed;
    - }
    + } else if (shmem_acct_block(info->flags))
    + goto nospace;

    if (!filepage) {
    int ret;
    @@ -1500,6 +1494,24 @@ done:
    error = 0;
    goto out;

    +nospace:
    + /*
    + * Perhaps the page was brought in from swap between find_lock_page
    + * and taking info->lock? We allow for that at add_to_page_cache_lru,
    + * but must also avoid reporting a spurious ENOSPC while working on a
    + * full tmpfs. (When filepage has been passed in to shmem_getpage, it
    + * is already in page cache, which prevents this race from occurring.)
    + */
    + if (!filepage) {
    + struct page *page = find_get_page(mapping, idx);
    + if (page) {
    + spin_unlock(&info->lock);
    + page_cache_release(page);
    + goto repeat;
    + }
    + }
    + spin_unlock(&info->lock);
    + error = -ENOSPC;
    failed:
    if (*pagep != filepage) {
    unlock_page(filepage);

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