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

[Christian König]

Am 13.06.22 um 09:45 schrieb Michal Hocko:
> On Sat 11-06-22 10:06:18, Christian König wrote:
> > Am 10.06.22 um 16:16 schrieb Michal Hocko:
> [...]
> > I could of course add something to struct page to track which memcg (or
> > process) it was charged against, but extending struct page is most likely a
> > no-go.
> Struct page already maintains is memcg. The one which has charged it and
> it will stay constatnt throughout of the allocation lifetime (cgroup v1
> has a concept of the charge migration but this hasn't been adopted in
> v2).
>
> We have a concept of active_memcg which allows to charge against a
> different memcg than the allocating context. From your example above I
> do not think this is really usable for the described usecase as the X is
> not aware where the request comes from?

Well X/Wayland is aware, but not the underlying kernel drivers.

When X/Wayland would want to forward this information to the kernel we 
would need to extend the existing UAPI quite a bit. And that of course 
doesn't help us at all with existing desktops.

> > Alternative I could try to track the "owner" of a buffer (e.g. a shmem
> > file), but then it can happen that one processes creates the object and
> > another one is writing to it and actually allocating the memory.
> If you can enforce that the owner is really responsible for the
> allocation then all should be fine. That would require MAP_POPULATE like
> semantic and I suspect this is not really feasible with the existing
> userspace. It would be certainly hard to enforce for bad players.

I've tried this today and the result was: "BUG: Bad rss-counter state 
mm:000000008751d9ff type:MM_FILEPAGES val:-571286".

The problem is once more that files are not informed when the process 
clones. So what happened is that somebody called fork() with an 
mm_struct I've accounted my pages to. The result is just that we messed 
up the rss_stats and  the the "BUG..." above.

The key difference between normal allocated pages and the resources here 
is just that we are not bound to an mm_struct in any way.

I could just potentially add a dummy VMA to the mm_struct, but to be 
honest I think that this would just be an absolutely hack.

So I'm running out of ideas how to fix this, except for adding this per 
file oom badness like I proposed.

Regards,
Christian.