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

[Barry Song]

On Tue, Jul 7, 2026 at 9:53 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 7/6/26 9:04 PM, Barry Song wrote:
> > On Mon, Jul 6, 2026 at 8:08 PM Baolin Wang
> > <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 7/6/26 1:59 PM, Kairui Song wrote:
> >>> 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.
> >>
> >> 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.

Hi Baolin,

I think your change has fixed the livelock issue, but an unconditional
one-tick sleep could still be problematic, as commit 01626a1823 tried to
address in do_swap_page():

"mm: avoid unconditional one-tick sleep when swapcache_prepare fails"

If possible, I would suggest that your fix also include the change from
commit 01626a1823 to avoid the issue caused by
schedule_timeout_uninterruptible(1): an unconditional one-tick sleep
could cause UI stuttering. At least, this would make the code more
defensive.

Thanks
Barry