Re: [PATCH v2 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats

[Roman Gushchin]

On Mon, Mar 01, 2021 at 10:03:26PM -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
> 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/
> Reported-by: Roman Gushchin <guro@xxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---

Oh, sorry, it looks like I missed to ack it. Thank you for updating
the commit log!

Acked-by: Roman Gushchin <guro@xxxxxx>