To activate a page, mark_page_accessed() always holds a reference on it.
It either gets a new reference when adding a page to
lru_pvecs.activate_page or reuses an existing one it previously got when
it added a page to lru_pvecs.lru_add.  So it doesn't call SetPageActive()
on a page that doesn't have any reference left.  Therefore, the race is
impossible these days (I didn't brother to dig into its history).

For other paths, namely reclaim and migration, a reference count is always
held while calling SetPageActive() on a page.

SetPageSlabPfmemalloc() also uses SetPageActive(), but it's irrelevant to
LRU pages.
Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200818184704.3625199-2-yuzhao@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>