The patch titled
Subject: mm: /proc/sys/vm/stat_refresh skip checking known negative stats
has been added to the -mm tree. Its filename is
mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats.patch
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------------------------------------------------------
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: 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@xxxxxx/
Link: https://lkml.kernel.org/r/alpine.LSU.2.11.2103012158540.7549@eggly.anvils
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmstat.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- a/mm/vmstat.c~mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats
+++ a/mm/vmstat.c
@@ -1858,6 +1858,14 @@ 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",
@@ -1874,6 +1882,13 @@ 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",
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
mm-restore-node-stat-checking-in-proc-sys-vm-stat_refresh.patch
mm-no-more-einval-from-proc-sys-vm-stat_refresh.patch
mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats.patch
mm-proc-sys-vm-stat_refresh-stop-checking-monotonic-numa-stats.patch
