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    SubjectRe: [PATCH] vfs: Merge sync_supers(), sync_filesystems() and sync_blockdevs()
    On Wed, Apr 22, 2009 at 05:56:21PM +0200, Jan Kara wrote:
    > These three functions are quite similar so merge them to save superblock list
    > traversal code. As a bonus we get livelock avoidance for all these superblock
    > traversals. Also remove the condition that if wait == 0 and sb->s_dirt is
    > not set, then ->sync_fs() is not called. This does not really make much sence
    > since s_dirt is generally used by filesystem to mean that ->write_super() needs
    > to be called. But ->sync_fs() does different things. I even suspect that some
    > filesystems (btrfs?) sets s_dirt just to fool this logic.

    Some more comments after looking at it in more details:

    - the FSSYNC_SUPER case really needs to do a trylock on the mutex,
    otherwise any in-progress sync would block pdflush for a long time.
    And as any real sync should write out the superblock it's not needed
    anyway during that time. (Need to double-check the filesystems,
    - sync_filesystems really should move to fs/sync.c
    - I get more and more inclined to make sync just case of looping over
    the superblocks and do an fsync_super. A plain sync fsync_super
    might be too slow so we can try to do an async one first and then
    a sync one as a second pass
    - that wakeup_pdflush in do_sync looks extremly fishy, we need to do
    all page writeback via sync_inodes_(sb) anyway, and doing this in
    parallel from pdflush just introduced tons of potential race
    - now if sync_filesystems just ends up calling __fsync_super for
    the normal sync path I wonder if there really is a point unifying
    it with the periodic write_super case.

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