[PATCH] mm/swap: don't SetPageWorkingset unconditionally during swapin

[Yu Zhao <yuzhao@google.com>]

We are capable of SetPageWorkingset based on refault distances after
commit aae466b0052e ("mm/swap: implement workingset detection for
anonymous LRU").  This is done by workingset_refault(), which is right
above the unconditional SetPageWorkingset deleted by this patch.

The unconditional SetPageWorkingset miscategorizes pages that are read
ahead or never belonged to the working set (e.g., tmpfs pages accessed
only once by fd).  When those pages are swapped in (after they were
swapped out) for the first time, they skew PSI (when using async swap).
When this happens again, depending on their refault distances, they might
skew workingset_restore_anon counter in addition to PSI because their
shadows indicate they were part of the working set.

Historically, SetPageWorkingset was added as part of the PSI series, and
Johannes said:
 "It was meant to mark incoming pages under IO with SetPageWorkingset
  when waiting for them constituted a memory stall.

  On the page cache side, because we HAVE workingset detection, this was
  specific to recently evicted pages that had been active in their
  previous life. On the anon side, the aging algorithm had no
  distinction between workingset and sporadically used pages. Given the
  choice between a) no swapin stalls are pressure and b) all swapin
  stalls are pressure, I went with the latter in order to detect swap
  storms. The false positive case - high rate of swapin without severe
  memory pressure - was relatively unlikely, because we tried to avoid
  swapping until everything was completely on fire in the first place."

Link: https://lkml.kernel.org/r/20201209012400.1771150-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/swap_state.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 1a01235156d1..6ecc84448d75 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -536,7 +536,6 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		workingset_refault(page, shadow);
 
 	/* Caller will initiate read into locked page */
-	SetPageWorkingset(page);
 	lru_cache_add(page);
 	*new_page_allocated = true;
 	return page;
-- 
2.29.2.684.gfbc64c5ab5-goog