Re: memory cgroup pagecache and inode problem

[Fam Zheng <zhengfeiran@bytedance.com>]

> On Jan 4, 2019, at 18:12, Michal Hocko <mhocko@kernel.org> wrote:
> 
> On Fri 04-01-19 18:02:19, Fam Zheng wrote:
>> 
>> 
>>> On Jan 4, 2019, at 17:04, Michal Hocko <mhocko@kernel.org> wrote:
>>> 
>>> This is a natural side effect of shared memory, I am afraid. Isolated
>>> memory cgroups should limit any shared resources to bare minimum. You
>>> will get "who touches first gets charged" behavior otherwise and that is
>>> not really deterministic.
>> 
>> I don’t quite understand your comment. I think the current behavior
>> for the ext4_inode_cachep slab family is just “who touches first
>> gets charged”, and later users of the same file from a different mem
>> cgroup can benefit from the cache, keep it from being released, but
>> doesn’t get charged.
> 
> Yes, this is exactly what I've said. And that leads to non-deterministic
> behavior because users from other memcgs are keeping charges alive and
> the isolation really doesn't work properly. Think of it as using memory
> on behalf of other party that is supposed to be isolated from you.
> 
> Sure this can work reasonably well if the sharing is not really
> predominated.

OK, I see what you mean. The reality is that the applications want to share files (e.g. docker run -v ...) , and IMO charging accuracy is not the trouble here. The problem is that there are memory usages which are not strictly necessary once a mem cgroup is deleted, such as the biggish struct mem_cgroup and the shadow slabs from which we no longer alloc new objects.

Fam

> -- 
> Michal Hocko
> SUSE Labs