Re: Zram writeback feature unstable with heavy swap utilization - BUG: Bad page state in process...

[Tino Lehnig]

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