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 11:44:01 +0800
From:   贺中坤 <hezhongkun.hzk@...edance.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     minchan@...nel.org, senozhatsky@...omium.org, mhocko@...e.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM to
 the page's memcgroup

>
> Yes. But just to stress again, one issue I see is that if there is a
> page in the BIO that is not charged, you cannot charge the compressed page.

I got it. Thanks.

>
> Assume you have some FS on that zram block device and  you want to make
> sure it gets properly charged to whoever is reading/writing a file on
> that filesystem. (imagine something like a compress shmem)
>
> If a user (or the filesystem?) can trigger a BIO that has an uncharged
> page in it, it would not get charged accordingly.
>
> The "easy" reproducer would have been O_DIRECT write() using the shared
> zeropage, but zram_write_page() is smart enough to optimize for that
> case (page_same_filled()). :)

Ok, I will try it.

>
> Maybe I'm over-thinking this (well, the we do have partial I/O support,
> so something seems to be able to trigger such cases), and it would be
> great if someone with more FS->BIO experience could comment.
>
> I'll note that this is fundamentally different to zswap, because with
> zswap you don't get arbitrary BIOs, you get an anon or shmem page (that
> should be charged).
>

Hi David， I know your concern and I will try to find the uncharged case.

> --
> Cheers,
>
> David / dhildenb
>