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    SubjectRe: [PATCH] block: remove plugging at buffered write time
    On Fri, Feb 10, 2012 at 09:52:18AM +0800, Wu Fengguang wrote:
    > On Thu, Feb 09, 2012 at 01:30:27PM -0500, Chris Mason wrote:
    > > On Thu, Feb 09, 2012 at 01:06:35PM -0500, Christoph Hellwig wrote:
    > > > On Thu, Feb 09, 2012 at 04:02:24PM +0800, Wu Fengguang wrote:
    > > > > On Thu, Feb 09, 2012 at 10:27:19AM +1100, Dave Chinner wrote:
    > > > > > On Wed, Feb 08, 2012 at 07:01:44PM +0800, Wu Fengguang wrote:
    > > > > > > Buffered write(2) is not directly tied to IO, so it's not suitable to
    > > > > > > handle plug in generic_file_aio_write().
    > > > > >
    > > > > > But generic_sync_write() does issue IO for O_SYNC writes, so unless
    > > > > > there is plugging at a lower layer in the writeback code then it
    > > > > > appears to me that plugging is still necessary (at least inside the
    > > > > > sync branch)....
    > > > >
    > > > > Good catch! It looks that generic_write_sync() eventually calls into
    > > > > vfs_fsync_range() which further calls ->fsync(). We may add plugging
    > > > > around it:
    > > >
    > > >
    > > > NAK, please keep the plugging down in the fs, or the libraries used but
    > > > not common VFS code.
    > >
    > > Please, what Christoph said. At least for btrfs plugging here is wrong.
    >
    > OK, I get the point: the fs knows best when to unplug. Since any
    > higher level plug nesting will turn such low level efforts into no-op,
    > it's highly undesirable to do it in the high level.

    It's actually wrong to do plugging around vfs_fsync_range().

    Because these call paths

    write() with O_SYNC
    generic_write_sync()
    vfs_fsync_range()
    ->fsync()
    generic_file_fsync()

    fsync()
    do_fsync()
    vfs_fsync()
    vfs_fsync_range()

    pass arbitrary @size arguments, which may be much larger than the
    preferable I/O size, or may cross extent/device boundaries.

    generic_file_fsync() starts with a filemap_write_and_wait_range()
    call, which already has proper plugging somewhere underneath. Then
    followed by metadata writes, which has plugging inside
    fsync_buffers_list(). At last, sync_inode_metadata() calls into
    ->write_inode() which may or may not care plugging.

    The other fs specific ->fsync() do similar steps, varying in the
    metadata and fs specific housekeeping part.

    I'll just drop this code. Shall the fs specific metadata I/O be
    plugged accordingly? I'm afraid this is beyond my knowledge base...

    Thanks,
    Fengguang


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