Re: [PATCH v2] mm: swap: prejudgement swap_has_cache to avoid page allocation

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Apr 9, 2024 at 7:57 AM Zhaoyu Liu
<liuzhaoyu.zackary@bytedance.com> wrote:
>
> On Tue, Apr 09, 2024 at 09:07:29AM +0800, Huang, Ying wrote:
> > Andrew Morton <akpm@linux-foundation.org> writes:
> >
> > > On Mon, 8 Apr 2024 20:14:39 +0800 Zhaoyu Liu <liuzhaoyu.zackary@bytedance.com> wrote:
> > >
> > >> Based on qemu arm64 - latest kernel + 100M memory + 1024M swapfile.
> > >> Create 1G anon mmap and set it to shared, and has two processes
> > >> randomly access the shared memory. When they are racing on swap cache,
> > >> on average, each "alloc_pages_mpol + swapcache_prepare + folio_put"
> > >> took about 1475 us.
> > >
> > > And what effect does this patch have upon the measured time?  ANd upon
> > > overall runtime?
> >
> > And the patch will cause increased lock contention, please test with
> > more processes and perhaps HDD swap device too.
>
> Hi Ying,
>
> Thank you for your suggestion.
> It may indeed cause some lock contention, as mentioned by Kairui before.
>
> If so, is it recommended?
> ---
>   unsigned char swap_map, mapcount, hascache;
>   ...
>   /* Return raw data of the si->swap_map[offset] */
>   swap_map = __swap_map(si, entry);
>   mapcount = swap_map & ~SWAP_HAS_CACHE;
>   if (!mapcount && swap_slot_cache_enabled)
>   ...
>   hascache = swap_map & SWAP_HAS_CACHE;
>   /* Could judge that it's being added to swap cache with high probability */
>   if (mapcount && hascache)
>     goto skip_alloc;
>   ...
> ---
> In doing so, there is no additional use of locks.
>

Hmm so is this a lockless check now? Ummmm... Could someone with more
expertise in the Linux kernel memory model double check that this is
even a valid state we're observing here? Looks like we're performing
an unguarded, unsynchronized, non-atomic read with the possibility of
concurrent write - is there a chance we might see partial/invalid
results?

Could you also test with zswap enabled (and perhaps with zswap
shrinker enabled)?