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.
Fixes: 9c315e4d7d8c ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()").
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
mm/slab.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/slab.h b/mm/slab.h
index 207c83ef6e06..74f7e09a7cfd 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 @@ static __always_inline int memcg_charge_slab(struct page *page,
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;
--
2.18.1
