linux-kernel - Re: Zram writeback feature unstable with heavy swap utilization

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Date:   Thu, 26 Jul 2018 08:10:41 +0200
From:   Tino Lehnig <tino.lehnig@...tabo.de>
To:     Minchan Kim <minchan@...nel.org>
Cc:     ngupta@...are.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Zram writeback feature unstable with heavy swap utilization -
 BUG: Bad page state in process...

Hi,

On 07/26/2018 04:03 AM, Minchan Kim wrote:
> A thing I could imagine is
> [0bcac06f27d75, skip swapcache for swapin of synchronous device]
> It was merged into v4.15. Could you check it by bisecting?

Thanks, I will check that.

>> My operating system is a minimal install of Debian 9. I took the kernel
>> configuration from the default Debian kernel and built my own kernel with
>> "make oldconfig" leaving all settings at their defaults. The only thing I
>> changed in the configuration was enabling the zram writeback feature.
> 
> You mean you changed host kernel configuration?
> 
>>
>> All my tests were done on bare-metal hardware with Xeon processors and lots
>> of RAM. I encounter the bug quite quickly, but it still takes several GBs of
>> swap usage. Below is my /proc/meminfo with enough KVM instances running (3
>> in my case) to trigger the bug on my test machine.
> 
> Aha.. you did writeback feature into your bare-metal host machine and execute
> kvm with window images as a guest. So, PG_uptodate warning happens on host side,
> not guest? Right?

Yes, I am only talking about the host kernel. Zram swap is set up on the 
host. I just used Windows guests to fill up the host RAM and force it 
into swap.

>> I will also try to reproduce the problem on some different hardware next.

Just to confirm, I was able to reproduce the problem on another machine 
running Ubuntu 18.04 with the Ubuntu stock kernel (4.15) and no 
modifications to the kernel configuration whatsoever. The host had 8 GB 
of RAM, 32 GB of swap with zram and a 32 GB SSD as backing device. I had 
to start only one Windows VM with "-m 32768" to trigger the bug.

-- 
Kind regards,

Tino Lehnig