Re: [BUG -next] "memcg: charge before adding to swapcache on swapin" broken

[Shakeel Butt <shakeelb@google.com>]

On Wed, Mar 17, 2021 at 2:11 PM Heiko Carstens <hca@linux.ibm.com> wrote:
>
> On Wed, Mar 17, 2021 at 09:44:21PM +0100, Heiko Carstens wrote:
> > On Wed, Mar 17, 2021 at 08:44:14AM -0700, Shakeel Butt wrote:
> > > > Config below. And the fun thing is that I cannot reproduce it today
> > > > anymore with the elfutils test case - what _seems_ to be different is
> > > > that the test suite runs much faster than yesterday evening. Usually
> > > > an indication that there is no steal time (other guests which steal
> > > > cpu time), which again _could_ indicate a race / lack of locking
> > > > somewhere.
> > > > This is kind of odd, since yesterday evening it was very reliable to
> > > > trigger the bug :/
> > > >
> > >
> > > Thanks for the config. One question regarding swap, is it disk based
> > > swap or zram?
> >
> > Swap device is a real disk.
> >
> > > By guests, do you mean there was another significant workload running
> > > on the machine in parallel to the tests?
> >
> > That I don't know. I didn't check. I still can't reproduce with
> > elfutils anymore, however...
> >
> > > If you don't mind can you try swapping01 as well.
> >
> > ltp's swapping01 test triggers immediately random processes being
> > killed with SIGSEGV. I also tested with linux-next 20210316 and _only_
> > "memcg: charge before adding to swapcache on swapin" being reverted on
> > top, and the problem is away - so it looks like the result of
> > yesterday's bisect is indeed valid.
>
> I have to correct myself, actually the system has both: a real disk
> _and_ zram as swap devices:
>
> # swapon -s
> Filename                                Type            Size    Used    Priority
> /dev/dasdb1                             partition       21635084        0       -2
> /dev/zram0                              partition       1014780 0       100
>
> When I disable /dev/zram with "swapoff /dev/zram0" the problem is away
> as well, even with your patch applied.

Thanks a lot. This was really helpful. I will try with zram on my setup.

Can you also try with just one type of swap at the time for both? I
really appreciate your help.