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

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Date: Thu, 21 Mar 2024 13:15:51 +0800
From: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To: Waiman Long <longman@...hat.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 21/03/2024 01:09, Waiman Long wrote:
> 
> 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.

Thanks for pointing this out, now I see it, in kernfs_fop_write_iter() 
we take of->mutex before ops->write() -> cgroup_file_write(). That means 
patch is not needed.

> 
> 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;
> 

-- 
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.