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Subject[PATCH 5.7 229/265] mm, slab: fix sign conversion problem in memcg_uncharge_slab()
Date
From: Waiman Long <longman@redhat.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@redhat.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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
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