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

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Date:   Thu, 15 Jun 2023 14:16:51 +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 Thu 15-06-23 19:58:37, 贺中坤 wrote:
> Hi michal,  glad to hear from you.
> 
> > I am not really deeply familiar with zram implementation nor usage but
> > how is the above allocation going to be charged without __GFP_ACCOUNT in
> > the gfp mask?
> 
> Yes，zs_malloc() did not charge compressed memory, even if we add this gfp.
> so we need to implement this function in this patchset. But this flag should be
> used to enable this feature.

Let me check I understand. This patch on its own doesn't really do
anything. You need the zs_malloc support implemented in patch 3 for this
to have any effect. Even with that in place the zs_malloc doesn't follow
the __GFP_ACCOUNT scheme we use for allocation tracking. Correct?

> > Also what exactly is going to happen for the swap backed by the zram
> > device? Your memcg might be hitting the hard limit and therefore
> > swapping out. Wouldn't zs_malloc fail very likely under that condition
> > making the swap effectively unusable?
> 
> This is the key point, as i said above, zs_malloc() did not charge
> compressed memory,
> so zs_malloc will not fail under that condition. if the zram swap is
> large enough, zs_malloc
> never fails unless system OOM.   so memory.max will be invalidated.

I do not think this is answering my question. Or maybe I just
misunderstand. Let me try again. Say you have a memcg under hard limit
pressure so any further charge is going to fail. How can you reasonably
implement zram back swapout if the memory is charged?

-- 
Michal Hocko
SUSE Labs