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    Subject[PATCH 4.6 198/203] tmpfs: fix regression hang in fallocate undo
    Date
    4.6-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Hugh Dickins <hughd@google.com>

    commit 7f556567036cb7f89aabe2f0954b08566b4efb53 upstream.

    The well-spotted fallocate undo fix is good in most cases, but not when
    fallocate failed on the very first page. index 0 then passes lend -1
    to shmem_undo_range(), and that has two bad effects: (a) that it will
    undo every fallocation throughout the file, unrestricted by the current
    range; but more importantly (b) it can cause the undo to hang, because
    lend -1 is treated as truncation, which makes it keep on retrying until
    every page has gone, but those already fully instantiated will never go
    away. Big thank you to xfstests generic/269 which demonstrates this.

    Fixes: b9b4bb26af01 ("tmpfs: don't undo fallocate past its last page")
    Signed-off-by: Hugh Dickins <hughd@google.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/shmem.c | 8 +++++---
    1 file changed, 5 insertions(+), 3 deletions(-)

    --- a/mm/shmem.c
    +++ b/mm/shmem.c
    @@ -2236,9 +2236,11 @@ static long shmem_fallocate(struct file
    NULL);
    if (error) {
    /* Remove the !PageUptodate pages we added */
    - shmem_undo_range(inode,
    - (loff_t)start << PAGE_SHIFT,
    - ((loff_t)index << PAGE_SHIFT) - 1, true);
    + if (index > start) {
    + shmem_undo_range(inode,
    + (loff_t)start << PAGE_SHIFT,
    + ((loff_t)index << PAGE_SHIFT) - 1, true);
    + }
    goto undone;
    }


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