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

[Zhaoyu Liu <liuzhaoyu.zackary@bytedance.com>]

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.

> 
> >> So skip page allocation if SWAP_HAS_CACHE was set, just
> >> schedule_timeout_uninterruptible and continue to acquire page
> >> via filemap_get_folio() from swap cache, to speedup
> >> __read_swap_cache_async.
> 
> --
> Best Regards,
> Huang, Ying