[PATCH 5.15] xfs: short circuit xfs_growfs_data_private() if delta is zero

[Amir Goldstein <amir73il@gmail.com>]

From: Eric Sandeen <sandeen@redhat.com>

[ Upstream commit 84712492e6dab803bf595fb8494d11098b74a652 ]

Although xfs_growfs_data() doesn't call xfs_growfs_data_private()
if in->newblocks == mp->m_sb.sb_dblocks, xfs_growfs_data_private()
further massages the new block count so that we don't i.e. try
to create a too-small new AG.

This may lead to a delta of "0" in xfs_growfs_data_private(), so
we end up in the shrink case and emit the EXPERIMENTAL warning
even if we're not changing anything at all.

Fix this by returning straightaway if the block delta is zero.

(nb: in older kernels, the result of entering the shrink case
with delta == 0 may actually let an -ENOSPC escape to userspace,
which is confusing for users.)

Fixes: fb2fc1720185 ("xfs: support shrinking unused space in the last AG")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Greg,

This fixes fstests failure in xfs/606.

The upstream fix applies cleanly to 5.15.y and was already backported
to 6.6.y and 6.1.y by Leah:
https://lore.kernel.org/linux-xfs/20240501184112.3799035-24-leah.rumancik@gmail.com/

Wasn't sure if you'd prefer to keep Leah's SOB and Darrick's ACK from
the 6.1.y backport, so did not keep them.

This is tested and running on our production 5.15 kernel for a while.

Thanks,
Amir.

 fs/xfs/xfs_fsops.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
index 5b5b68affe66..2d7467be2a48 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -126,6 +126,10 @@ xfs_growfs_data_private(
 	if (delta < 0 && nagcount < 2)
 		return -EINVAL;
 
+	/* No work to do */
+	if (delta == 0)
+		return 0;
+
 	oagcount = mp->m_sb.sb_agcount;
 
 	/* allocate the new per-ag structures */
-- 
2.47.1