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

[Kairui Song]

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.

>
> > 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.