mm/zswap: add more comments in shrink_memcg_cb()

mm/zswap: add more comments in shrink_memcg_cb()

Patch series "mm/zswap: optimize zswap lru list", v2.

This series is motivated when observe the zswap lru list shrinking, noted
there are some unexpected cases in zswap_writeback_entry().

bpftrace -e 'kr:zswap_writeback_entry {@[(int32)retval]=count()}'

There are some -ENOMEM because when the swap entry is freed to per-cpu
swap pool, it doesn't invalidate/drop zswap entry.  Then the shrinker
encounter these trashy zswap entries, it can't be reclaimed and return
-ENOMEM.

So move the invalidation ahead to when swap entry freed to the per-cpu
swap pool, since there is no any benefit to leave trashy zswap entries on
the zswap tree and lru list.

Another case is -EEXIST, which is seen more in the case of
!zswap_exclusive_loads_enabled, in which case the swapin folio will leave
compressed copy on the tree and lru list.  And it can't be reclaimed until
the folio is removed from swapcache.

Changing to zswap_exclusive_loads_enabled mode will invalidate when folio
swapin, which has its own drawback if that folio is still clean in
swapcache and swapout again, we need to compress it again.  Please see the
commit for details on why we choose exclusive load as the default for
zswap.

Another optimization for -EEXIST is that we add LRU_STOP to support
terminating the shrinking process to avoid evicting warmer region.

Testing using kernel build in tmpfs, one 50GB swapfile and
zswap shrinker_enabled, with memory.max set to 2GB.

                mm-unstable   zswap-optimize
real               63.90s       63.25s
user             1064.05s     1063.40s
sys               292.32s      270.94s

The main optimization is in sys cpu, about 7% improvement.

This patch (of 6):

Add more comments in shrink_memcg_cb() to describe the deref dance which
is implemented to fix race problem between lru writeback and swapoff, and
the reason why we rotate the entry at the beginning.

Also fix the stale comments in zswap_writeback_entry(), and add more
comments to state that we only deref the tree after we get the swapcache
reference.

Link: https://lkml.kernel.org/r/20240201-b4-zswap-invalidate-entry-v2-0-99d4084260a0@bytedance.com
Link: https://lkml.kernel.org/r/20240201-b4-zswap-invalidate-entry-v2-1-99d4084260a0@bytedance.com
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Suggested-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>