linux-kernel - kernfs memcg accounting

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Date:   Mon, 2 May 2022 22:37:49 +0300
From:   Vasily Averin <vvs@...nvz.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>, kernel@...nvz.org,
        Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>, cgroups@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>
Subject: kernfs memcg accounting

On 4/27/22 17:01, Michal Koutný wrote:
> Hello Vasily.
> 
> On Wed, Apr 27, 2022 at 01:37:50PM +0300, Vasily Averin <vvs@...nvz.org> wrote:
>> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
>> index cfa79715fc1a..2881aeeaa880 100644
>> --- a/fs/kernfs/mount.c
>> +++ b/fs/kernfs/mount.c
>> @@ -391,7 +391,7 @@ void __init kernfs_init(void)
>>  {
>>  	kernfs_node_cache = kmem_cache_create("kernfs_node_cache",
>>  					      sizeof(struct kernfs_node),
>> -					      0, SLAB_PANIC, NULL);
>> +					      0, SLAB_PANIC | SLAB_ACCOUNT, NULL);
> 
> kernfs accounting you say?
> kernfs backs up also cgroups, so the parent-child accounting comes to my
> mind.
> See the temporary switch to parent memcg in mem_cgroup_css_alloc().

Dear Michal,
I did not understand your statement. Could you please explain it in more details?

I see that cgroup_mkdir()->cgroup_create() creates new kernfs node for new
sub-directory, and with my patch account memory of kernfs node to memcg 
of current process.
Do you think it is incorrect and new kernfs node should be accounted
to memcg of parent cgroup, as mem_cgroup_css_alloc()-> mem_cgroup_alloc() does?
Perhaps you mean that in this case kernfs should not be counted at all,
as almost all neighboring allocations do?

Thank you,
	Vasily Averin