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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 7/6/26 1:59 PM, Kairui Song wrote:
> On Mon, Jul 6, 2026 at 11:25 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> 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@xiaomi.com/
>> [2] https://lore.kernel.org/all/20260517-swap-table-p4-v5-0-88ae43e064c7@tencent.com/
>> Fixes: 1dd44c0af4fa ("mm: shmem: skip swapcache for swapin of synchronous swap device")
>> Reported-by: Ma Chao <machao26@xiaomi.com>
>> Closes: https://lore.kernel.org/all/700a2cbf90a2484f979aac858f08f5d4@xiaomi.com/
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> 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.

I agree. I'd like to keep the bugfix as simple as possible, if the 
reporter's scenario isn't latency-sensitive.