On Thu, Feb 25, 2021 at 03:14:03PM -0800, Hugh Dickins wrote:
> 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.
I'm OK with the code, but I can't fully agree with the commit log. I don't think
there is any mystery around negative values. Let me copy-paste the explanation
from my original patch:
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.
* warnings about negative NR_FREE_CMA_PAGES
Actually, the same is almost true for ANY other counter. What differs CMA, dirty
and write pending counters is that they can reach 0 value under normal conditions.
Other counters are usually not reaching values small enough to see negative values
on a reasonable sized machine.
Does it makes sense?
>
> Link: https://lore.kernel.org/linux-mm/20200714173747.3315771-1-guro@xxxxxx/
> Reported-by: Roman Gushchin <guro@xxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>
> 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",
