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 9:04 PM, Barry Song wrote:
> On Mon, Jul 6, 2026 at 8:08 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> 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.
> 
> On Android, we don't see much shmem; it's much less common
> than anon. So the chance of this concurrency happening should
> be lower than for anon. However, shmem can be shared by
> multiple processes, so could this still happen if process A is
> blocked by process B?

Could you be more specific about how that happens? I think we should fix 
this starvation/livelock issue if you think it could still happen.