linux-kernel - Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev

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Message-ID: <20231201152822.GA404241@google.com>
Date:   Sat, 2 Dec 2023 00:28:22 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Jens Axboe <axboe@...nel.dk>, Dongyun Liu <dongyun.liu3@...il.com>
Cc:     minchan@...nel.org, senozhatsky@...omium.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        lincheng.yang@...nssion.com, jiajun.ling@...nssion.com,
        ldys2014@...mail.com, Dongyun Liu <dongyun.liu@...nssion.com>
Subject: Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate
 bitmap memory in backing_dev_store

On (23/12/01 07:19), Jens Axboe wrote:
> >> IOW, you have a slew of GFP_KERNEL allocations in there, and you
> >> probably just patched the largest one. But the core issue remains.
> >>
> >> The whole handling of backing_dev_store() looks pretty broken.
> >>
> > 
> > Indeed, this patch only solves the biggest problem and does not
> > fundamentally solve it, because there are many processes for holding
> > zram->init_lock before allocation memory in backing_dev_store that
> > need to be fully modified, and I did not consider it thoroughly.
> > Obviously, a larger and better patch is needed to eliminate this risk,
> > but it is currently not necessary.
> 
> You agree that it doesn't fix the issue, it just happens to fix the one
> that you hit. And then you jump to the conclusion that this is all
> that's needed to fix it. Ehm, confused?

Yeah.

zram is very sensitive to memory - zsmalloc pool needs physical pages
(allocated on demand) to keep data written to zram. zram probably simply
should not be used on systems under such heavy memory pressure. Throwing
GPF_ATOMICs at zram isn't going to fix anything.

Jens, you said that zram's backing device handling is broken. Got a minute
to elaborate?