• Brian Foster's avatar
    xfs: don't skip cow forks w/ delalloc blocks in cowblocks scan · 39937234
    Brian Foster authored
    The cowblocks background scanner currently clears the cowblocks tag
    for inodes without any real allocations in the cow fork. This
    excludes inodes with only delalloc blocks in the cow fork. While we
    might never expect to clear delalloc blocks from the cow fork in the
    background scanner, it is not necessarily correct to clear the
    cowblocks tag from such inodes.
    
    For example, if the background scanner happens to process an inode
    between a buffered write and writeback, the scanner catches the
    inode in a state after delalloc blocks have been allocated to the
    cow fork but before the delalloc blocks have been converted to real
    blocks by writeback. The background scanner then incorrectly clears
    the cowblocks tag, even if part of the aforementioned delalloc
    reservation will not be remapped to the data fork (i.e., extra
    blocks due to the cowextsize hint). This means that any such
    additional blocks in the cow fork might never be reclaimed by the
    background scanner and could persist until the inode itself is
    reclaimed.
    
    To address this problem, only skip and clear inodes without any cow
    fork allocations whatsoever from the background scanner. While we
    generally do not want to cancel delalloc reservations from the
    background scanner, the pagecache dirty check following the
    cowblocks check should prevent that situation. If we do end up with
    delalloc cow fork blocks without a dirty address space mapping, this
    is probably an indication that something has gone wrong and the
    blocks should be reclaimed, as they may never be converted to a real
    allocation.
    Signed-off-by: default avatarBrian Foster <bfoster@redhat.com>
    Reviewed-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: default avatarDave Chinner <david@fromorbit.com>
    39937234
xfs_reflink.h 2.33 KB