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From: John Garry <john.g.garry@oracle.com>
To: axboe@kernel.dk, tytso@mit.edu, dchinner@redhat.com,
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        John Garry <john.g.garry@oracle.com>
Subject: [PATCH v4 09/22] xfs: align args->minlen for forced allocation alignment
Date: Fri,  7 Jun 2024 14:39:06 +0000
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From: Dave Chinner <dchinner@redhat.com>

If args->minlen is not aligned to the constraints of forced
alignment, we may do minlen allocations that are not aligned when we
approach ENOSPC. Avoid this by always aligning args->minlen
appropriately. If alignment of minlen results in a value smaller
than the alignment constraint, fail the allocation immediately.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/libxfs/xfs_bmap.c | 45 +++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 14 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 9131ba8113a6..c9cf138e13c4 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -3278,33 +3278,48 @@ xfs_bmap_longest_free_extent(
 	return 0;
 }
 
-static xfs_extlen_t
+static int
 xfs_bmap_select_minlen(
 	struct xfs_bmalloca	*ap,
 	struct xfs_alloc_arg	*args,
 	xfs_extlen_t		blen)
 {
-
 	/* Adjust best length for extent start alignment. */
 	if (blen > args->alignment)
 		blen -= args->alignment;
 
 	/*
 	 * Since we used XFS_ALLOC_FLAG_TRYLOCK in _longest_free_extent(), it is
-	 * possible that there is enough contiguous free space for this request.
+	 * possible that there is enough contiguous free space for this request
+	 * even if best length is less that the minimum length we need.
+	 *
+	 * If the best length won't satisfy the maximum length we requested,
+	 * then use it as the minimum length so we get as large an allocation
+	 * as possible.
 	 */
 	if (blen < ap->minlen)
-		return ap->minlen;
+		blen = ap->minlen;
+	else if (blen > args->maxlen)
+		blen = args->maxlen;
 
 	/*
-	 * If the best seen length is less than the request length,
-	 * use the best as the minimum, otherwise we've got the maxlen we
-	 * were asked for.
+	 * If we have alignment constraints, round the minlen down to match the
+	 * constraint so that alignment will be attempted. This may reduce the
+	 * allocation to smaller than was requested, so clamp the minimum to
+	 * ap->minlen to allow unaligned allocation to succeed. If we are forced
+	 * to align the allocation, return ENOSPC at this point because we don't
+	 * have enough contiguous free space to guarantee aligned allocation.
 	 */
-	if (blen < args->maxlen)
-		return blen;
-	return args->maxlen;
-
+	if (args->alignment > 1) {
+		blen = rounddown(blen, args->alignment);
+		if (blen < ap->minlen) {
+			if (args->datatype & XFS_ALLOC_FORCEALIGN)
+				return -ENOSPC;
+			blen = ap->minlen;
+		}
+	}
+	args->minlen = blen;
+	return 0;
 }
 
 static int
@@ -3340,8 +3355,7 @@ xfs_bmap_btalloc_select_lengths(
 	if (pag)
 		xfs_perag_rele(pag);
 
-	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
-	return error;
+	return xfs_bmap_select_minlen(ap, args, blen);
 }
 
 /* Update all inode and quota accounting for the allocation we just did. */
@@ -3661,7 +3675,10 @@ xfs_bmap_btalloc_filestreams(
 		goto out_low_space;
 	}
 
-	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
+	error = xfs_bmap_select_minlen(ap, args, blen);
+	if (error)
+		goto out_low_space;
+
 	if (ap->aeof && ap->offset)
 		error = xfs_bmap_btalloc_at_eof(ap, args);
 
-- 
2.31.1