Re: [PATCH 03/13] mm: shmem: provide oom badness for shmem files

[Christian König]

Am 09.06.22 um 17:07 schrieb Michal Hocko:
> On Thu 09-06-22 16:29:46, Christian König wrote:
> [...]
> > Is that a show stopper? How should we address this?
> This is a hard problem to deal with and I am not sure this simple
> solution is really a good fit. Not only because of the memcg side of
> things. I have my doubts that sparse files handling is ok as well.

Well I didn't claimed that this would be easy, we juts need to start 
somewhere.

Regarding the sparse file handling, how about using 
file->f_mapping->nrpages as badness for shmem files?

That should give us the real number of pages allocated through this 
shmem file and gracefully handles sparse files.

> I do realize this is a long term problem and there is a demand for some
> solution at least. I am not sure how to deal with shared resources
> myself. The best approximation I can come up with is to limit the scope
> of the damage into a memcg context. One idea I was playing with (but
> never convinced myself it is really a worth) is to allow a new mode of
> the oom victim selection for the global oom event. It would be an opt in
> and the victim would be selected from the biggest leaf memcg (or kill
> the whole memcg if it has group_oom configured.
>
> That would address at least some of the accounting issue because charges
> are better tracked than per process memory consumption. It is a crude
> and ugly hack and it doesn't solve the underlying problem as shared
> resources are not guaranteed to be freed when processes die but maybe it
> would be just slightly better than the existing scheme which is clearly
> lacking behind existing userspace.

Well, what is so bad at the approach of giving each process holding a 
reference to some shared memory it's equal amount of badness even when 
the processes belong to different memory control groups?

If you really think that this would be a hard problem for upstreaming we 
could as well keep the behavior for memcg as it is for now. We would 
just need to adjust the paramters to oom_badness() a bit.

Regards,
Christian.