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

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Message-ID: <3f54542e-235b-8456-f13f-786d21c6c14f@redhat.com>
Date:   Thu, 15 Jun 2023 13:19:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     贺中坤 <hezhongkun.hzk@...edance.com>
Cc:     minchan@...nel.org, senozhatsky@...omium.org, mhocko@...e.com,
        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 15.06.23 13:15, 贺中坤 wrote:
> On Thu, Jun 15, 2023 at 5:27 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> Interesting. When looking at possible ways to achieve that in a clean
>> way, my understanding was that the page might not always be accounted to
>> a memcg (e.g., simply some buffer?). Besides the swap->zram case I was
>> thinking about other fs->zram case, or just a straight read/write to the
>> zram device.
>>
>> The easiest way to see where that goes wrong I think is
>> zram_bvec_write_partial(), where we simply allocate a temporary page.
>>
>> Either I am missing something important, or this only works in some
>> special cases.
>>
>> I came to the conclusion that the only reasonable way is to assign a
>> complete zram device to a single cgroup and have all memory accounted to
>> that cgroup. Does also not solve all use cases (especially the
>> swap->zram case that might be better offjust using zswap?) but at least
>> the memory gets accounted in a more predictable way.
>>
>> Happy to learn more.
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
> 
> Hi david, thanks for your reply. This should be my mistake, I didn't
> describe the
> problem clearly.
> 
> The reason for the problem is not the temporary page allocated at the beginning
> of zram_bvec_write_partial() and released at the end,but the compressed memory
> allocated by zs_malloc() in zram_write_page().
> 
> So, it may not meet the need to  assign a complete zram device to a
> single cgroup.
> we need to charge the compressed memory to the original page's memory cgroup,
> which is not charged so far.
> 

Yes, but my point is that there are cases where the pages you get are 
not charged. zram_bvec_write_partial() is just one such example that 
highlights the issue.

-- 
Cheers,

David / dhildenb