Re: [PATCH] block: remove plugging at buffered write time

[Wu Fengguang <fengguang.wu@intel.com>]

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:

--- linux-next.orig/fs/sync.c   2012-02-09 15:59:52.000000000 +0800
+++ linux-next/fs/sync.c        2012-02-09 16:01:02.000000000 +0800
@@ -164,9 +164,17 @@ SYSCALL_DEFINE1(syncfs, int, fd)
  */
 int vfs_fsync_range(struct file *file, loff_t start, loff_t end, int datasync)
 {
+       struct blk_plug plug;
+       int ret;
+
        if (!file->f_op || !file->f_op->fsync)
                return -EINVAL;
-       return file->f_op->fsync(file, start, end, datasync);
+
+       blk_start_plug(&plug);
+       ret = file->f_op->fsync(file, start, end, datasync);
+       blk_finish_plug(&plug);
+
+       return ret;
 }
 EXPORT_SYMBOL(vfs_fsync_range);