linux-kernel - Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM to the page's memcgroup

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Date:   Fri, 16 Jun 2023 08:40:26 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     贺中坤 <hezhongkun.hzk@...edance.com>
Cc:     minchan@...nel.org, senozhatsky@...omium.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM
 to the page's memcgroup

On Fri 16-06-23 11:31:18, 贺中坤 wrote:
> On Thu, Jun 15, 2023 at 10:21 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 15-06-23 22:13:09, 贺中坤 wrote:
> > > > This is not really answering my question though. memcg under hard limit
> > > > is not really my concern. This is a simpler case. I am not saying it
> > > > doesn't need to get addresses but it is the memcg hard limited case that
> > > > is much more interested. Because your charges are going to fail very
> > > > likely and that would mean that swapout would fail AFAIU. If my
> > > > understanding is wrong then it would really help to describe that case
> > > > much more in the changelog.
> > > >
> > >
> > > OK, Got it. In many cases I have tested, it  will not fail because we did
> > > not charge the page directly，but the objects(like  slab,compressed page),
> > > for the zspage may be shared by any memcg.
> >
> > That sounds like a broken design to me.
> > --
> > Michal Hocko
> > SUSE Labs
> 
> I will try more cases in different compression ratios and make sure
> that swapout will not fail.

I do not think different compression methods will cut it. You
essentially need some form of memory reserves - in memcg world
pre-charged pool of memory readily available for the swapout. Another
way would be to allow the charge to bypass the limit with a guarantee
that this will be a temporal breach.

-- 
Michal Hocko
SUSE Labs