[PATCH 6.18.y] mm: shmem: fix potential livelock issue for shmem direct swapin

[Baolin Wang]

When skipping swapcache for synchronous IO swap devices, swapcache_prepare()
is used to prevent parallel swapin from proceeding with the swap cache flag.
However, on PREEMPT kernels this can lead to a livelock, as reported by Chao[1]:

Thread A starts direct swapin of a shmem folio and calls swapcache_prepare()
to set SWAP_HAS_CACHE. It may then be preempted inside workingset_refault().
Meanwhile, a higher priority thread B also attempts direct swapin of the same
shmem swap entry. Since swapcache_prepare() already marks the entry, thread B
repeatedly gets -EEXIST and busy-loops waiting for thread A to finish. But as
thread B runs at higher priority, thread A cannot preempt it, resulting in
starvation and a livelock.

Fix it by yielding the CPU with schedule_timeout_uninterruptible(1) when
swapcache_prepare() fails, following the same approach used in commits
029c4628b2eb ("mm: swap: get rid of livelock in swapin readahead") and
13ddaf26be32 ("mm/swap: fix race when skipping swapcache").

Note that mainline does not have this potential issue, which has already been
resolved by Kairui's swap refactoring work[2].

[1] https://lore.kernel.org/all/700a2cbf90a2484f979aac858f08f5d4@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20260517-swap-table-p4-v5-0-88ae43e064c7@xxxxxxxxxxx/
Fixes: 1dd44c0af4fa ("mm: shmem: skip swapcache for swapin of synchronous swap device")
Reported-by: Ma Chao <machao26@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/700a2cbf90a2484f979aac858f08f5d4@xxxxxxxxxx/
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Hi Chao, could you try this patch to check if it fixes your issue? Thanks.
---
 mm/shmem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 94c5b0d78ac3..d4cb57b3b0ef 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2066,6 +2066,8 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
 	if (swapcache_prepare(entry, nr_pages)) {
 		folio_put(new);
 		new = ERR_PTR(-EEXIST);
+		/* Relax a bit to prevent rapid repeated page faults */
+		schedule_timeout_uninterruptible(1);
 		/* Try smaller folio to avoid cache conflict */
 		goto fallback;
 	}
-- 
2.47.3