From 75083aae114c2738af28eef2fb0c2515e818885a Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 4 May 2021 18:37:57 -0700 Subject: [PATCH] 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 Reported-by: Roman Gushchin Acked-by: Roman Gushchin Cc: Johannes Weiner Cc: Michal Hocko Cc: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmstat.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) 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",