On Thu, Jan 21, 2021 at 09:59:06AM +0100, Christoph Hellwig wrote:

From: Dave Chinner <redacted>

Attempt shared locking for unaligned DIO, but only if the the
underlying extent is already allocated and in written state. On
failure, retry with the existing exclusive locking.

Test case is fio randrw of 512 byte IOs using AIO and an iodepth of
32 IOs.

Vanilla:

  READ: bw=4560KiB/s (4670kB/s), 4560KiB/s-4560KiB/s (4670kB/s-4670kB/s), io=134MiB (140MB), run=30001-30001msec
  WRITE: bw=4567KiB/s (4676kB/s), 4567KiB/s-4567KiB/s (4676kB/s-4676kB/s), io=134MiB (140MB), run=30001-30001msec

Patched:
   READ: bw=37.6MiB/s (39.4MB/s), 37.6MiB/s-37.6MiB/s (39.4MB/s-39.4MB/s), io=1127MiB (1182MB), run=30002-30002msec
  WRITE: bw=37.6MiB/s (39.4MB/s), 37.6MiB/s-37.6MiB/s (39.4MB/s-39.4MB/s), io=1128MiB (1183MB), run=30002-30002msec

That's an improvement from ~18k IOPS to a ~150k IOPS, which is
about the IOPS limit of the VM block device setup I'm testing on.

4kB block IO comparison:

   READ: bw=296MiB/s (310MB/s), 296MiB/s-296MiB/s (310MB/s-310MB/s), io=8868MiB (9299MB), run=30002-30002msec
  WRITE: bw=296MiB/s (310MB/s), 296MiB/s-296MiB/s (310MB/s-310MB/s), io=8878MiB (9309MB), run=30002-30002msec

Which is ~150k IOPS, same as what the test gets for sub-block
AIO+DIO writes with this patch.

Signed-off-by: Dave Chinner <redacted>
[hch: rebased, split unaligned from nowait]
Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks good, minor nit:

+	/*
+	 * For overwrite only I/O, we cannot convert unwritten extents without
+	 * requiring sub-block zeroing.  This can only be done under an
+	 * exclusive IOLOCK, hence return -EAGAIN if this is not a written
+	 * extent to tell the caller to try again.
+	 */
+	if (flags & IOMAP_OVERWRITE_ONLY) {
+		error = -EAGAIN;
+		if (imap.br_state != XFS_EXT_NORM &&
+		    ((offset & mp->m_blockmask) ||
+		     ((offset + length) & mp->m_blockmask)))
+			goto out_unlock;

Why not use the ((offset | length) & mp->blockmask) form of
alignment checking here?

Other than that,

Reviewed-by: Dave Chinner <redacted>

-- 
Dave Chinner
david@fromorbit.com