[PATCH 13/16] mm/shmem: preserve allocation errors when swapping in large entries

["David Hildenbrand (Arm)" <david@kernel.org>]

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.

Original fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 91ab656ece13 ("mm/shmem, swap: tidy up swap entry splitting")
Cc: stable@vger.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 9e0034a424908..d1716a8da9075 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2563,6 +2563,8 @@ 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) {

-- 
2.43.0