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    Subject[PATCH 3.16 096/127] xfs: ensure WB_SYNC_ALL writeback handles partial pages correctly
    Date
    3.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: Dave Chinner <dchinner@redhat.com>

    commit 0d085a529b427d97710e6a41f8a4f23e1757cd12 upstream.

    XFS has been having trouble with stray delayed allocation extents
    beyond EOF for a long time. Recent changes to the collapse range
    code has triggered erroneous EBUSY errors on page invalidtion for
    block size smaller than page size filesystems. These
    have been caused by dirty buffers beyond EOF on a partial page which
    do not get written to disk during a sync.

    The issue is that write-ahead in xfs_cluster_write() finds such a
    partial page and handles it by leaving the page dirty but pushing it
    into a writeback state. This used to work just fine, as the
    write_cache_pages() code would then find the dirty partial page in
    the next mapping tree lookup as the dirty tag is still set.

    Unfortunately, when we moved to a mark and sweep approach to
    writeback to fix other writeback sync issues, we broken this. THe
    act of marking the page as under writeback now clears the TOWRITE
    tag in the radix tree, even though the page is still dirty. This
    causes the TOWRITE tag to be cleared, and hence the next lookup on
    the mapping tree does not find the dirty partial page and so doesn't
    try to write it again.

    This same writeback bug was found recently in ext4 and fixed in
    commit 1c8349a ("ext4: fix data integrity sync in ordered mode")
    without communication to the wider filesystem community. We can use
    exactly the same fix here so the TOWRITE flag is not cleared on
    partial page writes.

    cc: stable@vger.kernel.org # dependent on 1c8349a17137b93f0a83f276c764a6df1b9a116e
    Root-cause-found-by: Brian Foster <bfoster@redhat.com>
    Signed-off-by: Dave Chinner <dchinner@redhat.com>
    Reviewed-by: Brian Foster <bfoster@redhat.com>
    Signed-off-by: Dave Chinner <david@fromorbit.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/xfs/xfs_aops.c | 16 ++++++++++++++--
    1 file changed, 14 insertions(+), 2 deletions(-)

    --- a/fs/xfs/xfs_aops.c
    +++ b/fs/xfs/xfs_aops.c
    @@ -434,10 +434,22 @@ xfs_start_page_writeback(
    {
    ASSERT(PageLocked(page));
    ASSERT(!PageWriteback(page));
    - if (clear_dirty)
    +
    + /*
    + * if the page was not fully cleaned, we need to ensure that the higher
    + * layers come back to it correctly. That means we need to keep the page
    + * dirty, and for WB_SYNC_ALL writeback we need to ensure the
    + * PAGECACHE_TAG_TOWRITE index mark is not removed so another attempt to
    + * write this page in this writeback sweep will be made.
    + */
    + if (clear_dirty) {
    clear_page_dirty_for_io(page);
    - set_page_writeback(page);
    + set_page_writeback(page);
    + } else
    + set_page_writeback_keepwrite(page);
    +
    unlock_page(page);
    +
    /* If no buffers on the page are to be written, finish it here */
    if (!buffers)
    end_page_writeback(page);



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