linux-kernel - Re: [PATCH 0/4] Track exported dma-buffers with memcg

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Message-ID: <Y7/gIRv9aJfPK7/f@dhcp22.suse.cz>
Date:   Thu, 12 Jan 2023 11:25:37 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     "T.J. Mercier" <tjmercier@...gle.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
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        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
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Subject: Re: [PATCH 0/4] Track exported dma-buffers with memcg

On Thu 12-01-23 07:56:31, Shakeel Butt wrote:
> On Wed, Jan 11, 2023 at 11:56:45PM +0100, Daniel Vetter wrote:
> > 
> [...]
> > I think eventually, at least for other "account gpu stuff in cgroups" use
> > case we do want to actually charge the memory.
> > 
> > The problem is a bit that with gpu allocations reclaim is essentially "we
> > pass the error to userspace and they get to sort the mess out". There are
> > some exceptions (some gpu drivers to have shrinkers) would we need to make
> > sure these shrinkers are tied into the cgroup stuff before we could enable
> > charging for them?
> > 
> 
> No, there is no requirement to have shrinkers or making such memory
> reclaimable before charging it. Though existing shrinkers and the
> possible future shrinkers would need to be converted into memcg aware
> shrinkers.
> 
> Though there will be a need to update user expectations that if they 
> use memcgs with hard limits, they may start seeing memcg OOMs after the
> charging of dmabuf.

Agreed. This wouldn't be the first in kernel memory charged memory that
is not directly reclaimable. With a dedicated counter an excessive
dmabuf usage would be visible in the oom report because we do print
memcg stats.

It is definitely preferable to have a shrinker mechanism but if that is
to be done in a follow up step then this is acceptable. But leaving out
charging from early on sounds like a bad choice to me.
-- 
Michal Hocko
SUSE Labs