zsmalloc: allow only one active pool compaction context

zsmalloc pool can be compacted concurrently by many contexts,
e.g.

 cc1 handle_mm_fault()
      do_anonymous_page()
       __alloc_pages_slowpath()
        try_to_free_pages()
         do_try_to_free_pages(
          lru_gen_shrink_node()
           shrink_slab()
            do_shrink_slab()
             zs_shrinker_scan()
              zs_compact()

Pool compaction is currently (basically) single-threaded as
it is performed under pool->lock. Having multiple compaction
threads results in unnecessary contention, as each thread
competes for pool->lock. This, in turn, affects all zsmalloc
operations such as zs_malloc(), zs_map_object(), zs_free(), etc.

Introduce the pool->compaction_in_progress atomic variable,
which ensures that only one compaction context can run at a
time. This reduces overall pool->lock contention in (corner)
cases when many contexts attempt to shrink zspool simultaneously.

Link: https://lkml.kernel.org/r/20230418074639.1903197-1-senozhatsky@chromium.org


Fixes: c0547d0b ("zsmalloc: consolidate zs_pool's migrate_lock and size_class's locks")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>