[PATCH 1/5] xfs: only allow minlen allocations when near ENOSPC

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

When we are near ENOSPC and don't have enough free
space for an args->maxlen allocation, xfs_alloc_space_available()
will trim args->maxlen to equal the available space. However, this
function has only checked that there is enough contiguous free space
for an aligned args->minlen allocation to succeed. Hence there is no
guarantee that an args->maxlen allocation will succeed, nor that the
available space will allow for correct alignment of an args->maxlen
allocation.

Further, by trimming args->maxlen arbitrarily, it breaks an
assumption made in xfs_alloc_fix_len() that if the caller wants
aligned allocation, then args->maxlen will be set to an aligned
value. It then skips the tail alignment and so we end up with
extents that aren't aligned to extent size hint boundaries as we
approach ENOSPC.

To avoid this problem, don't reduce args->maxlen by some random,
arbitrary amount. If args->maxlen is too large for the available
space, reduce the allocation to a minlen allocation as we know we
have contiguous free space available for this to succeed and always
be correctly aligned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 9da52e92172a..215265e0f68f 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -2411,14 +2411,23 @@ xfs_alloc_space_available(
 	if (available < (int)max(args->total, alloc_len))
 		return false;
 
+	if (flags & XFS_ALLOC_FLAG_CHECK)
+		return true;
+
 	/*
-	 * Clamp maxlen to the amount of free space available for the actual
-	 * extent allocation.
+	 * If we can't do a maxlen allocation, then we must reduce the size of
+	 * the allocation to match the available free space. We know how big
+	 * the largest contiguous free space we can allocate is, so that's our
+	 * upper bound. However, we don't exaclty know what alignment/size
+	 * constraints have been placed on the allocation, so we can't
+	 * arbitrarily select some new max size. Hence make this a minlen
+	 * allocation as we know that will definitely succeed and match the
+	 * callers alignment constraints.
 	 */
-	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
-		args->maxlen = available;
+	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
+	if (longest < alloc_len) {
+		args->maxlen = args->minlen;
 		ASSERT(args->maxlen > 0);
-		ASSERT(args->maxlen >= args->minlen);
 	}
 
 	return true;
-- 
2.43.0