mm: vmscan: make global slab shrink lockless

The shrinker_rwsem is a global read-write lock in shrinkers subsystem,
which protects most operations such as slab shrink, registration and
unregistration of shrinkers, etc.  This can easily cause problems in the
following cases.

1) When the memory pressure is high and there are many
   filesystems mounted or unmounted at the same time,
   slab shrink will be affected (down_read_trylock()
   failed).

   Such as the real workload mentioned by Kirill Tkhai:

   ```
   One of the real workloads from my experience is start
   of an overcommitted node containing many starting
   containers after node crash (or many resuming containers
   after reboot for kernel update). In these cases memory
   pressure is huge, and the node goes round in long reclaim.
   ```

2) If a shrinker is blocked (such as the case mentioned
   in [1]) and a writer comes in (such as mount a fs),
   then this writer will be blocked and cause all
   subsequent shrinker-related operations to be blocked.

Even if there is no competitor when shrinking slab, there may still be a
problem.  If we have a long shrinker list and we do not reclaim enough
memory with each shrinker, then the down_read_trylock() may be called with
high frequency.  Because of the poor multicore scalability of atomic
operations, this can lead to a significant drop in IPC (instructions per
cycle).

So many times in history ([2],[3],[4],[5]), some people wanted to replace
shrinker_rwsem trylock with SRCU in the slab shrink, but all these patches
were abandoned because SRCU was not unconditionally enabled.

But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"), the SRCU
is unconditionally enabled.  So it's time to use SRCU to protect readers
who previously held shrinker_rwsem.

This commit uses SRCU to make global slab shrink lockless,
the memcg slab shrink is handled in the subsequent patch.

[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/
[2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/
[3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/
[4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
[5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/

Link: https://lkml.kernel.org/r/20230313112819.38938-3-zhengqi.arch@bytedance.com


Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <tkhai@ya.ru>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>