[PATCH v2 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats

[Hugh Dickins]

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@xxxxxx/
Reported-by: Roman Gushchin <guro@xxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/vmstat.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- vmstat2/mm/vmstat.c	2021-02-25 11:56:18.000000000 -0800
+++ vmstat3/mm/vmstat.c	2021-02-25 12:42:15.000000000 -0800
@@ -1840,6 +1840,14 @@ int vmstat_refresh(struct ctl_table *tab
 	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",
@@ -1856,6 +1864,13 @@ int vmstat_refresh(struct ctl_table *tab
 	}
 #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",