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SubjectRe: kernel BUG at include/linux/swapops.h:LINE!
On Mon, Jul 20, 2020 at 04:51:44PM -0700, Andrew Morton wrote:
> On Sun, 19 Jul 2020 14:10:19 -0700 syzbot <syzbot+c48f34012b06c4ac67dd@syzkaller.appspotmail.com> wrote:
>
> > syzbot has found a reproducer for the following issue on:
> >
> > HEAD commit: 4c43049f Add linux-next specific files for 20200716
> > git tree: linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12c56087100000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=2c76d72659687242
> > dashboard link: https://syzkaller.appspot.com/bug?extid=c48f34012b06c4ac67dd
> > compiler: gcc (GCC) 10.1.0-syz 20200507
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1344abeb100000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+c48f34012b06c4ac67dd@syzkaller.appspotmail.com
>
> Thanks.
>
> __handle_mm_fault
> ->pmd_migration_entry_wait
> ->migration_entry_to_page
>
> stumbled onto an unlocked page.
>
> I don't immediately see a cause. Perhaps Matthew's "THP prep patches",
> perhaps something else.

That's interesting. I'm currently chasing that signature too. Of course,
almost anything can cause this.

What I do have in my tree is a patch to turn that WARN_ON into a
VM_BUG_ON_PAGE and what I see is not just an unlocked page, but one
that's been freed.

> Is it possible to perform a bisection?

My testing (xfstests with the full THP patch set) takes about 45 minutes
to hit this bug usually. Sometimes two hours. I haven't tried running
it against fewer patches because I thought it was related to having THPs
smaller than PMD size in the page cache.

I don't think it is my patches because they're essentially just a rename.
But of course, I've been wrong before ...

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