Re: BUG: Bad page state in process - page dumped because: page still charged to cgroup

[Vlastimil Babka]

On 7/2/20 6:22 PM, Michal Hocko wrote:
> On Wed 01-07-20 11:45:52, Roman Gushchin wrote:
> [...]
>> >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001
>> From: Roman Gushchin <guro@xxxxxx>
>> Date: Wed, 1 Jul 2020 11:05:32 -0700
>> Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible
>> 
>> Historically the kernel memory accounting was an opt-in feature, which
>> could be enabled for individual cgroups. But now it's not true, and
>> it's on by default both on cgroup v1 and cgroup v2.  And as long as a
>> user has at least one non-root memory cgroup, the kernel memory
>> accounting is on. So in most setups it's either always on (if memory
>> cgroups are in use and kmem accounting is not disabled), either always
>> off (otherwise).
>> 
>> memcg_kmem_enabled() is used in many places to guard the kernel memory
>> accounting code. If memcg_kmem_enabled() can reverse from returning
>> true to returning false (as now), we can't rely on it on release paths
>> and have to check if it was on before.
>> 
>> If we'll make memcg_kmem_enabled() irreversible (always returning true
>> after returning it for the first time), it'll make the general logic
>> more simple and robust. It also will allow to guard some checks which
>> otherwise would stay unguarded.
>> 
>> Signed-off-by: Roman Gushchin <guro@xxxxxx>

Fixes: ? or let Andrew squash it to some patch of your series (it's in mmotm I
think)?

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

But see below:

>> ---
>>  mm/memcontrol.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>> 
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 50ae77f3985e..2d018a51c941 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3582,7 +3582,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
>>  	objcg->memcg = memcg;
>>  	rcu_assign_pointer(memcg->objcg, objcg);
>>  
>> -	static_branch_inc(&memcg_kmem_enabled_key);
>> +	if (!memcg_kmem_enabled())
>> +		static_branch_inc(&memcg_kmem_enabled_key);
> 
> Wouldn't be static_branch_enable() more readable?

Yes, and drop the if(). It will just do nothing and return if already enabled.
Maybe slightly less efficient, but this is not a fast path anyway, and it feels
weird to modify the static key in a branch controlled by the static key itself
(CC peterz in case he wants to add something).

>>  	/*
>>  	 * A memory cgroup is considered kmem-online as soon as it gets
>>  	 * kmemcg_id. Setting the id after enabling static branching will
>> @@ -3643,9 +3644,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg)
>>  	/* css_alloc() failed, offlining didn't happen */
>>  	if (unlikely(memcg->kmem_state == KMEM_ONLINE))
>>  		memcg_offline_kmem(memcg);
>> -
>> -	if (memcg->kmem_state == KMEM_ALLOCATED)
>> -		static_branch_dec(&memcg_kmem_enabled_key);
>>  }
>>  #else
>>  static int memcg_online_kmem(struct mem_cgroup *memcg)
>> -- 
>> 2.26.2
>