Re: [PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

Re: [PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

[linmiaohe]

Michal Hocko <mhocko@suse.com> wrote:
> On Thu 17-09-20 06:59:00, Miaohe Lin wrote:
>> Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather 
>> than counter"), the mem_cgroup_unmark_under_oom() is added and the 
>> comment of the mem_cgroup_oom_unlock() is moved here. But this comment 
>> make no sense here because mem_cgroup_oom_lock() does not operate on under_oom field.
>
>OK, so I've looked into this more deeply and I finally remember why we have this comment here. The point is that under_oom shouldn't underflow and that we have to explicitly check for > 0 because a new child memcg could have been added between mem_cgroup_mark_under_oom and mem_cgroup_unmark_under_oom.
>
>So the comment makes sense although it is not as helpful as it could be.
>I think that changing it to the following will be more usefule
>
>	/*
>	 * Be careful about under_oom underflows becase a child memcg
>	 * could have neem added after mem_cgroup_mark_under_oom

Should it be s/neem/been/ ?

>	 */

Many thanks for detailed explanation. Will fix it in v2. Thanks again.

Re: [PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

[Michal Hocko]

On Wed 30-09-20 01:34:25, linmiaohe wrote:
> Michal Hocko <mhocko@suse.com> wrote:
> > On Thu 17-09-20 06:59:00, Miaohe Lin wrote:
> >> Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather 
> >> than counter"), the mem_cgroup_unmark_under_oom() is added and the 
> >> comment of the mem_cgroup_oom_unlock() is moved here. But this comment 
> >> make no sense here because mem_cgroup_oom_lock() does not operate on under_oom field.
> >
> >OK, so I've looked into this more deeply and I finally remember why we have this comment here. The point is that under_oom shouldn't underflow and that we have to explicitly check for > 0 because a new child memcg could have been added between mem_cgroup_mark_under_oom and mem_cgroup_unmark_under_oom.
> >
> >So the comment makes sense although it is not as helpful as it could be.
> >I think that changing it to the following will be more usefule
> >
> >	/*
> >	 * Be careful about under_oom underflows becase a child memcg
> >	 * could have neem added after mem_cgroup_mark_under_oom
> 
> Should it be s/neem/been/ ?

yep, fat fingers...

> 
> >	 */
> 
> Many thanks for detailed explanation. Will fix it in v2. Thanks again.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

[Michal Hocko]

On Thu 17-09-20 06:59:00, Miaohe Lin wrote:
> Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather than
> counter"), the mem_cgroup_unmark_under_oom() is added and the comment of
> the mem_cgroup_oom_unlock() is moved here. But this comment make no sense
> here because mem_cgroup_oom_lock() does not operate on under_oom field.

OK, so I've looked into this more deeply and I finally remember why we
have this comment here. The point is that under_oom shouldn't underflow
and that we have to explicitly check for > 0 because a new child memcg
could have been added between mem_cgroup_mark_under_oom and
mem_cgroup_unmark_under_oom.

So the comment makes sense although it is not as helpful as it could be.
I think that changing it to the following will be more usefule

	/*
	 * Be careful about under_oom underflows becase a child memcg
	 * could have neem added after mem_cgroup_mark_under_oom
	 */
> 
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/memcontrol.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index cd5f83de9a6f..e44f5afaf78b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1848,10 +1848,6 @@ static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
>  {
>  	struct mem_cgroup *iter;
>  
> -	/*
> -	 * When a new child is created while the hierarchy is under oom,
> -	 * mem_cgroup_oom_lock() may not be called. Watch for underflow.
> -	 */
>  	spin_lock(&memcg_oom_lock);
>  	for_each_mem_cgroup_tree(iter, memcg)
>  		if (iter->under_oom > 0)
> -- 
> 2.19.1

-- 
Michal Hocko
SUSE Labs

[PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

[Miaohe Lin]

Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather than
counter"), the mem_cgroup_unmark_under_oom() is added and the comment of
the mem_cgroup_oom_unlock() is moved here. But this comment make no sense
here because mem_cgroup_oom_lock() does not operate on under_oom field.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/memcontrol.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cd5f83de9a6f..e44f5afaf78b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1848,10 +1848,6 @@ static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *iter;
 
-	/*
-	 * When a new child is created while the hierarchy is under oom,
-	 * mem_cgroup_oom_lock() may not be called. Watch for underflow.
-	 */
 	spin_lock(&memcg_oom_lock);
 	for_each_mem_cgroup_tree(iter, memcg)
 		if (iter->under_oom > 0)
-- 
2.19.1

[PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under_oom()

[Miaohe Lin]

Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather than
counter"), the mem_cgroup_unmark_under_oom() is added and the comment of
the mem_cgroup_oom_unlock() is moved here. But this comment make no sense
here because mem_cgroup_oom_lock() does not operate on under_oom field.

Signed-off-by: Miaohe Lin <linmiaohe-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
---
 mm/memcontrol.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cd5f83de9a6f..e44f5afaf78b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1848,10 +1848,6 @@ static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *iter;
 
-	/*
-	 * When a new child is created while the hierarchy is under oom,
-	 * mem_cgroup_oom_lock() may not be called. Watch for underflow.
-	 */
 	spin_lock(&memcg_oom_lock);
 	for_each_mem_cgroup_tree(iter, memcg)
 		if (iter->under_oom > 0)
-- 
2.19.1