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    Subject[PATCH 5.12 187/700] btrfs: dont clear page extent mapped if were not invalidating the full page
    Date
    From: Qu Wenruo <wqu@suse.com>

    [ Upstream commit bcd77455d590eaa0422a5e84ae852007cfce574a ]

    [BUG]
    With current btrfs subpage rw support, the following script can lead to
    fs hang:

    $ mkfs.btrfs -f -s 4k $dev
    $ mount $dev -o nospace_cache $mnt
    $ fsstress -w -n 100 -p 1 -s 1608140256 -v -d $mnt

    The fs will hang at btrfs_start_ordered_extent().

    [CAUSE]
    In above test case, btrfs_invalidate() will be called with the following
    parameters:

    offset = 0 length = 53248 page dirty = 1 subpage dirty bitmap = 0x2000

    Since @offset is 0, btrfs_invalidate() will try to invalidate the full
    page, and finally call clear_page_extent_mapped() which will detach
    subpage structure from the page.

    And since the page no longer has subpage structure, the subpage dirty
    bitmap will be cleared, preventing the dirty range from being written
    back, thus no way to wake up the ordered extent.

    [FIX]
    Just follow other filesystems, only to invalidate the page if the range
    covers the full page.

    There are cases like truncate_setsize() which can call
    btrfs_invalidatepage() with offset == 0 and length != 0 for the last
    page of an inode.

    Although the old code will still try to invalidate the full page, we are
    still safe to just wait for ordered extent to finish.
    So it shouldn't cause extra problems.

    Tested-by: Ritesh Harjani <riteshh@linux.ibm.com> # [ppc64]
    Tested-by: Anand Jain <anand.jain@oracle.com> # [aarch64]
    Signed-off-by: Qu Wenruo <wqu@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/btrfs/inode.c | 14 +++++++++++++-
    1 file changed, 13 insertions(+), 1 deletion(-)

    diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
    index 3e7173c7660d..1b22ded1a799 100644
    --- a/fs/btrfs/inode.c
    +++ b/fs/btrfs/inode.c
    @@ -8386,7 +8386,19 @@ static void btrfs_invalidatepage(struct page *page, unsigned int offset,
    */
    wait_on_page_writeback(page);

    - if (offset) {
    + /*
    + * For subpage case, we have call sites like
    + * btrfs_punch_hole_lock_range() which passes range not aligned to
    + * sectorsize.
    + * If the range doesn't cover the full page, we don't need to and
    + * shouldn't clear page extent mapped, as page->private can still
    + * record subpage dirty bits for other part of the range.
    + *
    + * For cases that can invalidate the full even the range doesn't
    + * cover the full page, like invalidating the last page, we're
    + * still safe to wait for ordered extent to finish.
    + */
    + if (!(offset == 0 && length == PAGE_SIZE)) {
    btrfs_releasepage(page, GFP_NOFS);
    return;
    }
    --
    2.30.2


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