linux-kernel - Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed again

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Message-ID: <be05a470-bb31-47ef-b786-557c347de429@redhat.com>
Date: Wed, 20 Mar 2024 13:09:07 -0400
From: Waiman Long <longman@...hat.com>
To: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
 Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Vladimir Davydov <vdavydov.dev@...il.com>, cgroups@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed
 again


On 3/20/24 06:03, Pavel Tikhomirov wrote:
> In memory_max_write() we first set memcg->memory.max and only then
> try to enforce it in loop. What if while we are in loop someone else
> have changed memcg->memory.max but we are still trying to enforce
> the old value? I believe this can lead to nasty consequence like getting
> an oom on perfectly fine cgroup within it's limits or excess reclaim.

Concurrent write to the same cgroup control file is not possible as the 
underlying kernfs_open_file structure has a mutex that serialize access 
to the file. Concurrent write to different cgroup control files is 
possible, though.

Cheers,
Longman

>
> We also have exactly the same thing in memory_high_write().
>
> So let's stop enforcing old limits if we already have a new ones.
>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> ---
>   mm/memcontrol.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 61932c9215e7..81b303728491 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6769,6 +6769,9 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>   		unsigned long nr_pages = page_counter_read(&memcg->memory);
>   		unsigned long reclaimed;
>   
> +		if (memcg->memory.high != high)
> +			break;
> +
>   		if (nr_pages <= high)
>   			break;
>   
> @@ -6817,6 +6820,9 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>   	for (;;) {
>   		unsigned long nr_pages = page_counter_read(&memcg->memory);
>   
> +		if (memcg->memory.max != max)
> +			break;
> +
>   		if (nr_pages <= max)
>   			break;
>