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authorHugh Dickins <hughd@google.com>2021-05-05 04:37:57 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2021-05-05 21:27:25 +0300
commit75083aae114c2738af28eef2fb0c2515e818885a (patch)
tree49859a936068728836ba313c40b8f2ef53a1bf16 /mm
parent6d99a4c029c01cd7d075f7f9fa3b8b620e49a9f7 (diff)
downloadlinux-75083aae114c2738af28eef2fb0c2515e818885a.tar.xz
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.anvils Signed-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>
Diffstat (limited to 'mm')
-rw-r--r--mm/vmstat.c15
1 files changed, 15 insertions, 0 deletions
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 1fb91d5a2b60..5437a7861089 100644
--- a/mm/vmstat.c
+++ b/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",