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

[Huang, Ying]

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?

> 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.

--
Best Regards,
Huang, Ying