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/20200714173747.3315771-1-guro@fb.com/
Link: https://lkml.kernel.org/r/alpine.LSU.2.11.2103012158540.7549@eggly.anvilsSigned-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Roman Gushchin <guro@fb.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -1858,6 +1858,14 @@ int vmstat_refresh(struct ctl_table *table, int write,
 	if (err)
 		return err;
 	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
+		/*
+		 * Skip checking stats known to go negative occasionally.
+		 */
+		switch (i) {
+		case NR_ZONE_WRITE_PENDING:
+		case NR_FREE_CMA_PAGES:
+			continue;
+		}
 		val = atomic_long_read(&vm_zone_stat[i]);
 		if (val < 0) {
 			pr_warn("%s: %s %ld\n",
@@ -1874,6 +1882,13 @@ int vmstat_refresh(struct ctl_table *table, int write,
 	}
 #endif
 	for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
+		/*
+		 * Skip checking stats known to go negative occasionally.
+		 */
+		switch (i) {
+		case NR_WRITEBACK:
+			continue;
+		}
 		val = atomic_long_read(&vm_node_stat[i]);
 		if (val < 0) {
 			pr_warn("%s: %s %ld\n",