On 09/03/2025 22:03, Dave Chinner wrote:
On Mon, Mar 03, 2025 at 05:11:20PM +0000, John Garry wrote:
When issuing an atomic write by the CoW method, give the block allocator a
hint to align to the extszhint.
This means that we have a better chance to issuing the atomic write via
HW offload next time.
It does mean that the inode extszhint should be set appropriately for the
expected atomic write size.
Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
fs/xfs/libxfs/xfs_bmap.c | 7 ++++++-
fs/xfs/libxfs/xfs_bmap.h | 6 +++++-
fs/xfs/xfs_reflink.c | 8 ++++++--
3 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 0ef19f1469ec..9bfdfb7cdcae 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -3454,6 +3454,12 @@ xfs_bmap_compute_alignments(
align = xfs_get_cowextsz_hint(ap->ip);
else if (ap->datatype & XFS_ALLOC_USERDATA)
align = xfs_get_extsz_hint(ap->ip);
+
+ if (align > 1 && ap->flags & XFS_BMAPI_EXTSZALIGN)
needs () around the & logic.
ok
if (align > 1 && (ap->flags & XFS_BMAPI_EXTSZALIGN))
+ args->alignment = align;
+ else
+ args->alignment = 1;
When is args->alignment not already initialised to 1?
+
if (align) {
if (xfs_bmap_extsize_align(mp, &ap->got, &ap->prev, align, 0,
ap->eof, 0, ap->conv, &ap->offset,
@@ -3782,7 +3788,6 @@ xfs_bmap_btalloc(
.wasdel = ap->wasdel,
.resv = XFS_AG_RESV_NONE,
.datatype = ap->datatype,
- .alignment = 1,
.minalignslop = 0,
};
Oh, you removed the initialisation to 1, so now we have the
possibility of getting args->alignment = 0 anywhere in the
allocation stack?
FWIW, we've been trying to get rid of that case - args->alignment should
always be 1 if no alignment is necessary so we don't ahve to special
case alignment of 0 (meaning no alignemnt) anywhere. This seems
like a step backwards from that perspective...
As I recall, doing this was a suggestion when developing the forcealign
support (as it had similar logic).
Anyway, I can leave the init to 1 in xfs_bmap_btalloc()
xfs_fileoff_t orig_offset;
diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
index 4b721d935994..e6baa81e20d8 100644
--- a/fs/xfs/libxfs/xfs_bmap.h
+++ b/fs/xfs/libxfs/xfs_bmap.h
@@ -87,6 +87,9 @@ struct xfs_bmalloca {
/* Do not update the rmap btree. Used for reconstructing bmbt from rmapbt. */
#define XFS_BMAPI_NORMAP (1u << 10)
+/* Try to align allocations to the extent size hint */
+#define XFS_BMAPI_EXTSZALIGN (1u << 11)
Don't we already do that?
Or is this doing something subtle and non-obvious like overriding
stripe width alignment for large atomic writes?
stripe alignment only comes into play for eof allocation.
args->alignment is used in xfs_alloc_compute_aligned() to actually align
the start bno.
If I don't have this, then we can get this ping-pong affect when
overwriting atomically the same region:
# dd if=/dev/zero of=mnt/file bs=1M count=10 conv=fsync
# xfs_bmap -vp mnt/file
mnt/file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..20479]: 192..20671 0 (192..20671) 20480 000000
# /xfs_io -d -C "pwrite -b 64k -V 1 -A -D 0 64k" mnt/file
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0525 sec (1.190 MiB/sec and 19.0425 ops/sec)
# xfs_bmap -vp mnt/file
mnt/file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..127]: 20672..20799 0 (20672..20799) 128 000000
1: [128..20479]: 320..20671 0 (320..20671) 20352 000000
# /xfs_io -d -C "pwrite -b 64k -V 1 -A -D 0 64k" mnt/file
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0524 sec (1.191 MiB/sec and 19.0581 ops/sec)
# xfs_bmap -vp mnt/file
mnt/file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..20479]: 192..20671 0 (192..20671) 20480 000000
# /xfs_io -d -C "pwrite -b 64k -V 1 -A -D 0 64k" mnt/file
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0524 sec (1.191 MiB/sec and 19.0611 ops/sec)
# xfs_bmap -vp mnt/file
mnt/file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..127]: 20672..20799 0 (20672..20799) 128 000000
1: [128..20479]: 320..20671 0 (320..20671) 20352 000000
We are never getting aligned extents wrt write length, and so have to
fall back to the SW-based atomic write always. That is not what we want.
Thanks,
John
