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>
