linux-kernel - [PATCH 5.7 229/265] mm, slab: fix sign conversion problem in memcg_uncharge

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Message-Id: <20200629151818.2493727-230-sashal@kernel.org>
Date:   Mon, 29 Jun 2020 11:17:42 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Waiman Long <longman@...hat.com>, Roman Gushchin <guro@...com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 5.7 229/265] mm, slab: fix sign conversion problem in memcg_uncharge_slab()

From: Waiman Long <longman@...hat.com>

commit d7670879c5c4aa443d518fb234a9e5f30931efa3 upstream.

It was found that running the LTP test on a PowerPC system could produce
erroneous values in /proc/meminfo, like:

  MemTotal:       531915072 kB
  MemFree:        507962176 kB
  MemAvailable:   1100020596352 kB

Using bisection, the problem is tracked down to commit 9c315e4d7d8c ("mm:
memcg/slab: cache page number in memcg_(un)charge_slab()").

In memcg_uncharge_slab() with a "int order" argument:

  unsigned int nr_pages = 1 << order;
    :
  mod_lruvec_state(lruvec, cache_vmstat_idx(s), -nr_pages);

The mod_lruvec_state() function will eventually call the
__mod_zone_page_state() which accepts a long argument.  Depending on the
compiler and how inlining is done, "-nr_pages" may be treated as a
negative number or a very large positive number.  Apparently, it was
treated as a large positive number in that PowerPC system leading to
incorrect stat counts.  This problem hasn't been seen in x86-64 yet,
perhaps the gcc compiler there has some slight difference in behavior.

It is fixed by making nr_pages a signed value.  For consistency, a similar
change is applied to memcg_charge_slab() as well.

Link: http://lkml.kernel.org/r/20200620184719.10994-1-longman@redhat.com
Fixes: 9c315e4d7d8c ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()").
Signed-off-by: Waiman Long <longman@...hat.com>
Acked-by: Roman Gushchin <guro@...com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Shakeel Butt <shakeelb@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Vladimir Davydov <vdavydov.dev@...il.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 mm/slab.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 207c83ef6e06d..74f7e09a7cfd1 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -348,7 +348,7 @@ static __always_inline int memcg_charge_slab(struct page *page,
 					     gfp_t gfp, int order,
 					     struct kmem_cache *s)
 {
-	unsigned int nr_pages = 1 << order;
+	int nr_pages = 1 << order;
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 	int ret;
@@ -388,7 +388,7 @@ out:
 static __always_inline void memcg_uncharge_slab(struct page *page, int order,
 						struct kmem_cache *s)
 {
-	unsigned int nr_pages = 1 << order;
+	int nr_pages = 1 << order;
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 
-- 
2.25.1