linux-kernel - Re: kernel BUG at include/linux/swapops.h:LINE!

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Date:   Tue, 21 Jul 2020 03:14:49 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     syzbot <syzbot+c48f34012b06c4ac67dd@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: kernel BUG at include/linux/swapops.h:LINE!

On Tue, Jul 21, 2020 at 01:21:47AM +0100, Matthew Wilcox wrote:
> 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@...kaller.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@...kaller.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.

Here's an example crash:

1404 086 (25392): drop_caches: 3
1404 page:00000000c8b7c292 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x1 pfn:0xac20
1404 flags: 0x4000000000000000()
1404 raw: 4000000000000000 fffff7b501775808 fffff7b501ab7008 0000000000000000
1404 raw: 0000000000000001 0000000000000005 00000000ffffff7f 0000000000000000
1404 page dumped because: VM_BUG_ON_PAGE(!PageLocked(p))

(that's generic/086 for what it's worth, but you have to run
through a number of other tests in order to hit it; even starting at
generic/08[0123456] isn't enough to hit it, and it doesn't always hit)

A mapcount of -128 indicates PageBuddy, but I've also seen a mapcount of 0
indicating it's still on the per-cpu freelist.