mm: /proc/sys/vm/stat_refresh skip checking known negative stats

vmstat_refresh() can occasionally catch nr_zone_write_pending and
nr_writeback when they are transiently negative.  The reason is partly
that the interrupt which decrements them in test_clear_page_writeback()
can come in before __test_set_page_writeback() got to increment them;
but transient negatives are still seen even when that is prevented, and
I am not yet certain why (but see Roman's note below).  Those stats are
not buggy, they have never been seen to drift away from 0 permanently:
so just avoid the annoyance of showing a warning on them.

Similarly avoid showing a warning on nr_free_cma: CMA users have seen
that one reported negative from /proc/sys/vm/stat_refresh too, but it
does drift away permanently: I believe that's because its incrementation
and decrementation are decided by page migratetype, but the migratetype
of a pageblock is not guaranteed to be constant.

Roman Gushchin points out:
 "For performance reasons, vmstat counters are incremented and
  decremented using per-cpu batches. vmstat_refresh() flushes the
  per-cpu batches on all CPUs, to get values as accurate as possible;
  but this method is not atomic, so the resulting value is not always
  precise.

  As a consequence, for those counters whose actual value is close to 0,
  a small negative value may occasionally be reported. If the value is
  small and the state is transient, it is not an indication of an error"

Link: https://lore.kernel.org/linux-mm/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Reported-by: Roman Gushchin <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions