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    Subject[PATCH 8/8] vfs: Avoid unnecessary WB_SYNC_NONE writeback during sys_sync and reorder sync passes
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    wakeup_flusher_threads(0) will queue work doing complete writeback for each
    flusher thread. Thus there is not much point in submitting another work doing
    full inode WB_SYNC_NONE writeback by writeback_inodes_sb().

    After this change it does not make sense to call nonblocking ->sync_fs and
    block device flush before calling sync_inodes_sb() because
    wakeup_flusher_threads() is completely asynchronous and thus these functions
    would be called in parallel with inode writeback running which will effectively
    void any work they do. So we move sync_inodes_sb() call before these two
    functions.

    Signed-off-by: Jan Kara <jack@suse.cz>
    ---
    fs/sync.c | 19 +++++++++----------
    1 files changed, 9 insertions(+), 10 deletions(-)

    diff --git a/fs/sync.c b/fs/sync.c
    index 6d177d1..eb8722d 100644
    --- a/fs/sync.c
    +++ b/fs/sync.c
    @@ -73,12 +73,6 @@ static void sync_inodes_one_sb(struct super_block *sb, void *arg)
    sync_inodes_sb(sb);
    }

    -static void writeback_inodes_one_sb(struct super_block *sb, void *arg)
    -{
    - if (!(sb->s_flags & MS_RDONLY))
    - writeback_inodes_sb(sb);
    -}
    -
    static void sync_fs_one_sb(struct super_block *sb, void *arg)
    {
    if (!(sb->s_flags & MS_RDONLY) && sb->s_op->sync_fs)
    @@ -96,17 +90,22 @@ static void fdatawait_one_bdev(struct block_device *bdev, void *arg)
    }

    /*
    - * sync everything. Start out by waking pdflush, because that writes back
    - * all queues in parallel.
    + * Sync everything. We start by waking flusher threads so that most of
    + * writeback runs on all devices in parallel. Then we sync all inodes reliably
    + * which effectively also waits for all flusher threads to finish doing
    + * writeback. At this point all data is on disk so metadata should be stable
    + * and we tell filesystems to sync their metadata via ->sync_fs() calls.
    + * Finally, we writeout all block devices because some filesystems (e.g. ext2)
    + * just write metadata (such as inodes or bitmaps) to block device page cache
    + * and do not sync it on their own in ->sync_fs().
    */
    SYSCALL_DEFINE0(sync)
    {
    int nowait = 0, wait = 1;

    wakeup_flusher_threads(0, WB_REASON_SYNC);
    - iterate_supers(writeback_inodes_one_sb, NULL);
    - iterate_supers(sync_fs_one_sb, &nowait);
    iterate_supers(sync_inodes_one_sb, NULL);
    + iterate_supers(sync_fs_one_sb, &nowait);
    iterate_supers(sync_fs_one_sb, &wait);
    iterate_bdevs(fdatawrite_one_bdev, NULL);
    iterate_bdevs(fdatawait_one_bdev, NULL);
    --
    1.7.1


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