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

[Hugh Dickins <hughd@google.com>]

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
we have not yet resolved why (Roman believes that it is an unavoidable
consequence of the refresh scheduled on each cpu).  But 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.

Use switch statements so we can most easily add or remove cases later.

Link: https://lore.kernel.org/linux-mm/20200714173747.3315771-1-guro@fb.com/
Reported-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 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",