FAILED: patch "[PATCH] mm/shmem: preserve allocation errors when swapping in large entries" failed to apply to 6.18-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x f4ce23b1feef16d435aa1188b6c2a637a5373ed5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109200304-failed-6.18-f4ce23b1feef@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From f4ce23b1feef16d435aa1188b6c2a637a5373ed5 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Thu, 29 Oct 2026 11:43:33 +0100
Subject: [PATCH] mm/shmem: preserve allocation errors when swapping in large
 entries

shmem_swapin_folio() adjusts swap to the requested slot within a large
swap entry before allocating a folio. If allocation or charging fails,
the error path compares this adjusted value with the original entry
still stored in the XArray in shmem_confirm_swap().

Unless the index is at the start of the entry, this turns -ENOMEM into
-EEXIST even though nothing changed.

The caller retries -EEXIST, so persistent allocation failures can leave
the task spinning instead of returning the error.

Restore swap to index_entry on allocation failure so the existing error
path checks the original entry. This preserves the allocation error if
the entry is unchanged and still retries if it changed concurrently.

Fixes: 91ab656ece13 ("mm/shmem, swap: tidy up swap entry splitting")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Link: https://patch.msgid.link/20261029-fixes_shmem-v4-12-392fc715d8f6@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index e1b7cf4b5280a..02de6326ef3b3 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2552,6 +2552,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			else
 				error = -ENOMEM;
 			folio = NULL;
+			/* Make sure to validate the original value. */
+			swap = index_entry;
 			goto failed;
 		}
 		if (fault_type) {
-- 
2.53.0