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

["Huang, Ying" <ying.huang@intel.com>]

Nhat Pham <nphamcs@gmail.com> writes:

> 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)?

READ_ONCE() will save us from partial/invalid results.

--
Best Regards,
Huang, Ying