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 12:40:51 +0800
From:   贺中坤 <hezhongkun.hzk@...edance.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Yu Zhao <yuzhao@...gle.com>, minchan@...nel.org,
        senozhatsky@...omium.org, mhocko@...e.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Fabian Deutsch <fdeutsch@...hat.com>
Subject: Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM to
 the page's memcgroup

> Thanks Fabian for tagging me.
>
> I am not familiar with #1, so I will speak to #2. Zhongkun, There are
> a few parts that I do not understand -- hopefully you can help me out
> here:
>
> (1) If I understand correctly in this patch we set the active memcg
> trying to charge any pages allocated in a zspage to the current memcg,
> yet that zspage will contain multiple compressed object slots, not
> just the one used by this memcg. Aren't we overcharging the memcg?
> Basically the first memcg that happens to allocate the zspage will pay
> for all the objects in this zspage, even after it stops using the
> zspage completely?

It will not overcharge.  As you said below, we are not using
__GFP_ACCOUNT and charging the compressed slots to the memcgs.

>
> (2) Patch 3 seems to be charging the compressed slots to the memcgs,
> yet this patch is trying to charge the entire zspage. Aren't we double
> charging the zspage? I am guessing this isn't happening because (as
> Michal pointed out) we are not using __GFP_ACCOUNT here anyway, so
> this patch may be NOP, and the actual charging is coming from patch 3
> only.

YES， the actual charging is coming from patch 3. This patch just
delivers the BIO page's  memcg to the current task which is not the
consumer.

>
> (3) Zswap recently implemented per-memcg charging of compressed
> objects in a much simpler way. If your main interest is #2 (which is
> what I understand from the commit log), it seems like zswap might be
> providing this already? Why can't you use zswap? Is it the fact that
> zswap requires a backing swapfile?

Thanks for your reply and review. Yes, the zswap requires a backing
swapfile. The I/O path is very complex, sometimes it will throttle the
whole system if some resources are short , so we hope to use zram.

>
> Thanks!
>