From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EE467C433ED for ; Wed, 5 May 2021 01:38:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D505061029 for ; Wed, 5 May 2021 01:38:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232081AbhEEBiz (ORCPT ); Tue, 4 May 2021 21:38:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:41838 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231124AbhEEBiy (ORCPT ); Tue, 4 May 2021 21:38:54 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id E6BDD611AB; Wed, 5 May 2021 01:37:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1620178678; bh=zRaMD7NDC8X234DCFeEdf4fFNZL89gffTWjEMit1v4Q=; h=Date:From:To:Subject:In-Reply-To:From; b=s+AwWBVeVKNVhegKmAOx169GWix2rZ54bMR5WOn+LryMtxzBZR4G/tv9FnURvNXVb IRZ0HsmYrYkc52MRzZ/FKTbTa20FznvnMj8nTEFCXjVsrHNb8pwRe6aW3pEPvtEwhk l1rMMCBmY2FazLAb+msweSWvIgai7uaD3eEZxRyk= Date: Tue, 04 May 2021 18:37:57 -0700 From: Andrew Morton To: akpm@linux-foundation.org, guro@fb.com, hannes@cmpxchg.org, hughd@google.com, linux-mm@kvack.org, mhocko@kernel.org, mm-commits@vger.kernel.org, torvalds@linux-foundation.org, vbabka@suse.cz Subject: [patch 098/143] mm: /proc/sys/vm/stat_refresh skip checking known negative stats Message-ID: <20210505013757.SFVvsG-JV%akpm@linux-foundation.org> In-Reply-To: <20210504183219.a3cc46aee4013d77402276c5@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Hugh Dickins Subject: 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 --- mm/vmstat.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) --- a/mm/vmstat.c~mm-proc-sys-vm-stat_refresh-skip-checking-known-negative-stats +++ a/mm/vmstat.c @@ -1858,6 +1858,14 @@ 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", @@ -1874,6 +1882,13 @@ 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", _