[PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[roy.qing.li]

From: Li RongQing <roy.qing.li@gmail.com>

The margin may be set to the difference value between memory limit and
memory count firstly. which maybe returned wrongly if memsw.count excess
memsw.limit, because try_charge forces charging __GFP_NOFAIL allocations,
which may result in memsw.limit excess. If we are below memory.limit
and there's nothing to reclaim to reduce memsw.usage, might end up
looping in try_charge forever.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.com>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 00981d2..12aaadd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1090,6 +1090,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 		limit = READ_ONCE(memcg->memsw.limit);
 		if (count <= limit)
 			margin = min(margin, limit - count);
+		else
+			margin = 0;
 	}
 
 	return margin;
-- 
2.1.4

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[PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w]

From: Li RongQing <roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>

The margin may be set to the difference value between memory limit and
memory count firstly. which maybe returned wrongly if memsw.count excess
memsw.limit, because try_charge forces charging __GFP_NOFAIL allocations,
which may result in memsw.limit excess. If we are below memory.limit
and there's nothing to reclaim to reduce memsw.usage, might end up
looping in try_charge forever.

Signed-off-by: Li RongQing <roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Acked-by: Vladimir Davydov <vdavydov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
Cc: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 00981d2..12aaadd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1090,6 +1090,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 		limit = READ_ONCE(memcg->memsw.limit);
 		if (count <= limit)
 			margin = min(margin, limit - count);
+		else
+			margin = 0;
 	}
 
 	return margin;
-- 
2.1.4

Re: [PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[Michal Hocko]

On Tue 24-05-16 13:37:46, roy.qing.li@gmail.com wrote:
> From: Li RongQing <roy.qing.li@gmail.com>
> 
> The margin may be set to the difference value between memory limit and
> memory count firstly. which maybe returned wrongly if memsw.count excess
> memsw.limit, because try_charge forces charging __GFP_NOFAIL allocations,
> which may result in memsw.limit excess. If we are below memory.limit
> and there's nothing to reclaim to reduce memsw.usage, might end up
> looping in try_charge forever.

This is quite hard for me to grasp. What would you say about the
following:
"
mem_cgroup_margin might return memory.limit - memory_count when
the memsw.limit is in excess. This
doesn't happen usually because we do not allow excess on hard limits and
memory.limit <= memsw.limit but __GFP_NOFAIL charges can force the charge
and cause the excess when no memory is really swapable (swap is full or
no anonymous memory is left).
"

> 
> Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
> Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
> Cc: Michal Hocko <mhocko@suse.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 00981d2..12aaadd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1090,6 +1090,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
>  		limit = READ_ONCE(memcg->memsw.limit);
>  		if (count <= limit)
>  			margin = min(margin, limit - count);
> +		else
> +			margin = 0;
>  	}
>  
>  	return margin;
> -- 
> 2.1.4

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[Michal Hocko]

On Tue 24-05-16 13:37:46, roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org wrote:
> From: Li RongQing <roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> 
> The margin may be set to the difference value between memory limit and
> memory count firstly. which maybe returned wrongly if memsw.count excess
> memsw.limit, because try_charge forces charging __GFP_NOFAIL allocations,
> which may result in memsw.limit excess. If we are below memory.limit
> and there's nothing to reclaim to reduce memsw.usage, might end up
> looping in try_charge forever.

This is quite hard for me to grasp. What would you say about the
following:
"
mem_cgroup_margin might return memory.limit - memory_count when
the memsw.limit is in excess. This
doesn't happen usually because we do not allow excess on hard limits and
memory.limit <= memsw.limit but __GFP_NOFAIL charges can force the charge
and cause the excess when no memory is really swapable (swap is full or
no anonymous memory is left).
"

> 
> Signed-off-by: Li RongQing <roy.qing.li-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Acked-by: Vladimir Davydov <vdavydov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
> Cc: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

> ---
>  mm/memcontrol.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 00981d2..12aaadd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1090,6 +1090,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
>  		limit = READ_ONCE(memcg->memsw.limit);
>  		if (count <= limit)
>  			margin = min(margin, limit - count);
> +		else
> +			margin = 0;
>  	}
>  
>  	return margin;
> -- 
> 2.1.4

-- 
Michal Hocko
SUSE Labs

Re: [PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[Li RongQing]

On Wed, May 25, 2016 at 11:51 PM, Michal Hocko <mhocko@kernel.org> wrote:
> This is quite hard for me to grasp. What would you say about the
> following:
> "
> mem_cgroup_margin might return memory.limit - memory_count when
> the memsw.limit is in excess. This
> doesn't happen usually because we do not allow excess on hard limits and
> memory.limit <= memsw.limit but __GFP_NOFAIL charges can force the charge
> and cause the excess when no memory is really swapable (swap is full or
> no anonymous memory is left).


Sorry for my poor English, thanks for your description, hope it can be
into the log

-Roy

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