Re: [PATCH v2] mm/swap: fix race when skipping swapcache

[Huang, Ying]

Kairui Song <ryncsn@xxxxxxxxx> writes:

> On Thu, Feb 8, 2024 at 2:36 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Kairui Song <ryncsn@xxxxxxxxx> writes:
>>
>> > On Thu, Feb 8, 2024 at 2:31 AM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> >>
>> >> On Wed, Feb 07, 2024 at 12:06:15PM +0800, Kairui Song wrote:
>>
>> [snip]
>>
>> >> >
>> >> > So I think the thing is, it's getting complex because this patch
>> >> > wanted to make it simple and just reuse the swap cache flags.
>> >>
>> >> I agree that a simple fix would be the important at this point.
>> >>
>> >> Considering your description, here's my understanding of the other idea:
>> >> Other method, such as increasing the swap count, haven't proven effective
>> >> in your tests. The approach risk forcing racers to rely on the swap cache
>> >> again and the potential performance loss in race scenario.
>> >>
>> >> While I understand that simplicity is important, and performance loss
>> >> in this case may be infrequent, I believe swap_count approach could be a
>> >> suitable solution. What do you think?
>> >
>> > Hi Minchan
>> >
>> > Yes, my main concern was about simplicity and performance.
>> >
>> > Increasing swap_count here will also race with another process from
>> > releasing swap_count to 0 (swapcache was able to sync callers in other
>> > call paths but we skipped swapcache here).
>>
>> What is the consequence of the race condition?
>
> Hi Ying,
>
> It will increase the swap count of an already freed entry, this race
> with multiple swap free/alloc logic that checks if count ==
> SWAP_HAS_CACHE or sets count to zero, or repeated free of an entry,
> all result in random corruption of the swap map. This happens a lot
> during stress testing.

You are right!  Thanks for explanation.

--
Best Regards,
Huang, Ying

>>
>> > So the right step is: 1. Lock the cluster/swap lock; 2. Check if still
>> > have swap_count == 1, bail out if not; 3. Set it to 2;
>> > __swap_duplicate can be modified to support this, it's similar to
>> > existing logics for SWAP_HAS_CACHE.
>> >
>> > And swap freeing path will do more things, swapcache clean up needs to
>> > be handled even in the bypassing path since the racer may add it to
>> > swapcache.
>> >
>> > Reusing SWAP_HAS_CACHE seems to make it much simpler and avoided many
>> > overhead, so I used that way in this patch, the only issue is
>> > potentially repeated page faults now.
>> >
>> > I'm currently trying to add a SWAP_MAP_LOCK (or SWAP_MAP_SYNC, I'm bad
>> > at naming it) special value, so any racer can just spin on it to avoid
>> > all the problems, how do you think about this?
>>
>> Let's try some simpler method firstly.
>
> Another simpler idea is, add a schedule() or
> schedule_timeout_uninterruptible(1) in the swapcache_prepare failure
> path before goto out (just like __read_swap_cache_async). I think this
> should ensure in almost all cases, PTE is ready after it returns, also
> yields more CPU.