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

[Kairui Song]

On Mon, Jul 6, 2026 at 11:25 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> 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
>

Thanks! That's much more simpler than I expected. Do we need a wakeup
queue like the one in commit 01626a1823024? Perhaps the reporter can
help confirm and test? I personally prefer to keep it simple if shmem
users aren't as sensitive as anon users.