Re: [PATCH] memcg: Fix thresholds for 32b architectures.

[Johannes Weiner]

On Tue, Oct 27, 2015 at 11:37:14AM +0100, mhocko@xxxxxxxxxx wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 424cdc141380 ("memcg: convert threshold to bytes") has fixed a
> regression introduced by 3e32cb2e0a12 ("mm: memcontrol: lockless page
> counters") where thresholds were silently converted to use page units
> rather than bytes when interpreting the user input.
> 
> The fix is not complete, though, as properly pointed out by Ben
> Hutchings during stable backport review. The page count is converted
> to bytes but unsigned long is used to hold the value which would
> be obviously not sufficient for 32b systems with more than 4G
> thresholds. The same applies to usage as taken from mem_cgroup_usage
> which might overflow.
> 
> Let's remove this bytes vs. pages internal tracking differences and
> handle thresholds in page units internally. Chage mem_cgroup_usage()
> to return the value in page units and revert 424cdc141380 because this
> should be sufficient for the consistent handling.
> mem_cgroup_read_u64 as the only users of mem_cgroup_usage outside of
> the threshold handling code is converted to give the proper in bytes
> result. It is doing that already for page_counter output so this is
> more consistent as well.
> 
> The value presented to the userspace is still in bytes units.
> 
> Fixes: 424cdc141380 ("memcg: convert threshold to bytes")
> Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> CC: stable@xxxxxxxxxxxxxxx
> Reported-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

> +++ b/mm/memcontrol.c
> @@ -2802,7 +2802,7 @@ static unsigned long tree_stat(struct mem_cgroup *memcg,
>  	return val;
>  }
>  
> -static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> +static inline unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
>  {
>  	u64 val;

Minor nit, but this should probably be unsigned long now.
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