Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

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On 02/02/2024 18:05, Darrick J. Wong wrote:
On Wed, Jan 24, 2024 at 02:26:43PM +0000, John Garry wrote:
Support providing info on atomic write unit min and max for an inode.

For simplicity, currently we limit the min at the FS block size, but a
lower limit could be supported in future.

The atomic write unit min and max is limited by the guaranteed extent
alignment for the inode.

Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
  fs/xfs/xfs_iops.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
  fs/xfs/xfs_iops.h |  4 ++++
  2 files changed, 49 insertions(+)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index a0d77f5f512e..0890d2f70f4d 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -546,6 +546,44 @@ xfs_stat_blksize(
  	return PAGE_SIZE;
  }
+void xfs_get_atomic_write_attr(

static void?

We use this in the iomap and statx code


+	struct xfs_inode *ip,
+	unsigned int *unit_min,
+	unsigned int *unit_max)

Weird indenting here.

hmmm... I thought that this was the XFS style

Can you show how it should look?


+{
+	xfs_extlen_t		extsz = xfs_get_extsz(ip);
+	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
+	struct block_device	*bdev = target->bt_bdev;
+	unsigned int		awu_min, awu_max, align;
+	struct request_queue	*q = bdev->bd_queue;
+	struct xfs_mount	*mp = ip->i_mount;
+
+	/*
+	 * Convert to multiples of the BLOCKSIZE (as we support a minimum
+	 * atomic write unit of BLOCKSIZE).
+	 */
+	awu_min = queue_atomic_write_unit_min_bytes(q);
+	awu_max = queue_atomic_write_unit_max_bytes(q);
+
+	awu_min &= ~mp->m_blockmask;

Why do you round /down/ the awu_min value here?

This is just to ensure that we returning *unit_min >= BLOCKSIZE

For example, if awu_min, max 1K, 64K from the bdev, we now have 0 and 64K. And below this gives us awu_min, max of 4k, 64k.

Maybe there is a more logical way of doing this.


+	awu_max &= ~mp->m_blockmask;

Actually -- since the atomic write units have to be powers of 2, why is
rounding needed here at all?

Sure, but the bdev can report a awu_min < BLOCKSIZE


+
+	align = XFS_FSB_TO_B(mp, extsz);
+
+	if (!awu_max || !xfs_inode_atomicwrites(ip) || !align ||
+	    !is_power_of_2(align)) {

...and if you take my suggestion to make a common helper to validate the
atomic write unit parameters, this can collapse into:

	alloc_unit_bytes = xfs_inode_alloc_unitsize(ip);
	if (!xfs_inode_has_atomicwrites(ip) ||
	    !bdev_validate_atomic_write(bdev, alloc_unit_bytes))  > 		/* not supported, return zeroes */
		*unit_min = 0;
		*unit_max = 0;
		return;
	}

	*unit_min = max(alloc_unit_bytes, awu_min);
	*unit_max = min(alloc_unit_bytes, awu_max);

Again, we need to ensure that *unit_min >= BLOCKSIZE

Thanks,
John





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