linux-kernel - Re: [REGRESSION] Re: [PATCH 6.1 033/219] memcg: drop kmem.limit_in

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Date:   Fri, 22 Sep 2023 16:00:30 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, patches@...ts.linux.dev,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
        mathieu.tortuyaux@...il.com
Subject: Re: [REGRESSION] Re: [PATCH 6.1 033/219] memcg: drop
 kmem.limit_in_bytes

On Wed, Sep 20, 2023 at 03:47:37PM +0200, Michal Hocko wrote:
> On Wed 20-09-23 15:25:23, Jeremi Piotrowski wrote:
> > On 9/20/2023 1:07 PM, Michal Hocko wrote:
> [...]
> > > I mean, normally I would be just fine reverting this API change because
> > > it is disruptive but the only way to have the file available and not
> > > break somebody is to revert 58056f77502f ("memcg, kmem: further
> > > deprecate kmem.limit_in_bytes") as well. Or to ignore any value written
> > > there but that sounds rather dubious. Although one could argue this
> > > would mimic nokmem kernel option.
> > > 
> > 
> > I just want to make sure we don't introduce yet another new behavior in this legacy
> > system. I have not seen breakage due to 58056f77502f. Mimicing nokmem sounds good but
> > does this mean "don't enforce limits" (that should be fine) or "ignore writes to the limit"
> > (=don't event store the written limit). The latter might have unintended consequences.
> 
> Yes it would mean that the limit is never enforced. Bad as it is the
> thing is that the hard limit on kernel memory is broken by design and
> unfixable.  This causes all sorts of unexpected kernel allocation
> failures that this is simply unsafe to use.
> 
> All that being said I can see the following options
> 1) keep the current upstream status and not export the file
> 2) revert both 58056f77502f and 86327e8eb94 and make it clear
>    that kmem.limit_in_bytes is unsupported so failures or misbehavior
>    as a result of the limit being hit are likely not going to be
>    investigated or fixed.
> 3) reverting like in 2) but never inforce the limit (so basically nokmem
>    semantic)

Since it's a part of cgroup v1 interface, which is in a frozen state as a whole,
and there is no significant (performance, code complexity) benefit of
additionally deprecating kmem.limit_in_bytes, I vote for 2).
1) is also an option.

Thanks!