Re: [PATCH 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats

[Roman Gushchin <guro@fb.com>]

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@fb.com/
> Reported-by: Roman Gushchin <guro@fb.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> 
>  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",