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From:   Roman Gushchin <guro@fb.com>
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        Michal Hocko <mhocko@kernel.org>, <linux-mm@kvack.org>,
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        Roman Gushchin <guro@fb.com>, Hugh Dickins <hughd@google.com>
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Subject: [PATCH v2] mm: vmstat: fix /proc/sys/vm/stat_refresh generating false warnings
Date:   Tue, 14 Jul 2020 10:39:20 -0700
Message-ID: <20200714173920.3319063-1-guro@fb.com>
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I've noticed a number of warnings like "vmstat_refresh: nr_free_cma
-5" or "vmstat_refresh: nr_zone_write_pending -11" on our production
hosts. The numbers of these warnings were relatively low and stable,
so it didn't look like we are systematically leaking the counters.
The corresponding vmstat counters also looked sane.

These warnings are generated by the vmstat_refresh() function, which
assumes that atomic zone and numa counters can't go below zero.
However, on a SMP machine it's not quite right: due to per-cpu
caching it can in theory be as low as -(zone threshold) * NR_CPUs.

For instance, let's say all cma pages are in use and NR_FREE_CMA_PAGES
reached 0. Then we've reclaimed a small number of cma pages on each
CPU except CPU0, so that most percpu NR_FREE_CMA_PAGES counters are
slightly positive (the atomic counter is still 0). Then somebody on
CPU0 consumes all these pages. The number of pages can easily exceed
the threshold and a negative value will be committed to the atomic
counter.

To fix the problem and avoid generating false warnings, let's just
relax the condition and warn only if the value is less than minus
the maximum theoretically possible drift value, which is 125 *
number of online CPUs. It will still allow to catch systematic leaks,
but will not generate bogus warnings.

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Hugh Dickins <hughd@google.com>
---
 Documentation/admin-guide/sysctl/vm.rst |  4 ++--
 mm/vmstat.c                             | 30 ++++++++++++++++---------
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admi=
n-guide/sysctl/vm.rst
index 4b9d2e8e9142..95fb80d0c606 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -822,8 +822,8 @@ e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
=20
 As a side-effect, it also checks for negative totals (elsewhere reported
 as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
-(At time of writing, a few stats are known sometimes to be found negativ=
e,
-with no ill effects: errors and warnings on these stats are suppressed.)
+(On a SMP machine some stats can temporarily become negative, with no il=
l
+effects: errors and warnings on these stats are suppressed.)
=20
=20
 numa_stat
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a21140373edb..8f0ef8aaf8ee 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -169,6 +169,8 @@ EXPORT_SYMBOL(vm_node_stat);
=20
 #ifdef CONFIG_SMP
=20
+#define MAX_THRESHOLD 125
+
 int calculate_pressure_threshold(struct zone *zone)
 {
 	int threshold;
@@ -186,11 +188,9 @@ int calculate_pressure_threshold(struct zone *zone)
 	threshold =3D max(1, (int)(watermark_distance / num_online_cpus()));
=20
 	/*
-	 * Maximum threshold is 125
+	 * Threshold is capped by MAX_THRESHOLD
 	 */
-	threshold =3D min(125, threshold);
-
-	return threshold;
+	return min(MAX_THRESHOLD, threshold);
 }
=20
 int calculate_normal_threshold(struct zone *zone)
@@ -610,6 +610,9 @@ void dec_node_page_state(struct page *page, enum node=
_stat_item item)
 }
 EXPORT_SYMBOL(dec_node_page_state);
 #else
+
+#define MAX_THRESHOLD 0
+
 /*
  * Use interrupt disable to serialize counter updates
  */
@@ -1810,7 +1813,7 @@ static void refresh_vm_stats(struct work_struct *wo=
rk)
 int vmstat_refresh(struct ctl_table *table, int write,
 		   void *buffer, size_t *lenp, loff_t *ppos)
 {
-	long val;
+	long val, max_drift;
 	int err;
 	int i;
=20
@@ -1821,17 +1824,22 @@ int vmstat_refresh(struct ctl_table *table, int w=
rite,
 	 * pages, immediately after running a test.  /proc/sys/vm/stat_refresh,
 	 * which can equally be echo'ed to or cat'ted from (by root),
 	 * can be used to update the stats just before reading them.
-	 *
-	 * Oh, and since global_zone_page_state() etc. are so careful to hide
-	 * transiently negative values, report an error here if any of
-	 * the stats is negative, so we know to go looking for imbalance.
 	 */
 	err =3D schedule_on_each_cpu(refresh_vm_stats);
 	if (err)
 		return err;
+
+	/*
+	 * Since global_zone_page_state() etc. are so careful to hide
+	 * transiently negative values, report an error here if any of
+	 * the stats is negative and are less than the maximum drift value,
+	 * so we know to go looking for imbalance.
+	 */
+	max_drift =3D num_online_cpus() * MAX_THRESHOLD;
+
 	for (i =3D 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
 		val =3D atomic_long_read(&vm_zone_stat[i]);
-		if (val < 0) {
+		if (val < -max_drift) {
 			pr_warn("%s: %s %ld\n",
 				__func__, zone_stat_name(i), val);
 			err =3D -EINVAL;
@@ -1840,7 +1848,7 @@ int vmstat_refresh(struct ctl_table *table, int wri=
te,
 #ifdef CONFIG_NUMA
 	for (i =3D 0; i < NR_VM_NUMA_STAT_ITEMS; i++) {
 		val =3D atomic_long_read(&vm_numa_stat[i]);
-		if (val < 0) {
+		if (val < -max_drift) {
 			pr_warn("%s: %s %ld\n",
 				__func__, numa_stat_name(i), val);
 			err =3D -EINVAL;
--=20
2.26.2