Re: [PATCH] mm, slab: Fix sign conversion problem in memcg_uncharge_slab()

[Roman Gushchin <guro@fb.com>]

On Sat, Jun 20, 2020 at 02:47:19PM -0400, Waiman Long wrote:
> 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@redhat.com>

Good catch!

Interesting that I haven't seen it on x86-64, but it's reproducible on Power.

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!

> ---
>  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
>