linux-kernel - Re: [syzbot] [mm?] general protection fault in hpage_collapse_scan

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Date: Tue, 9 Apr 2024 17:32:55 -0700
From: "Zach O'Keefe" <zokeefe@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: syzbot <syzbot+57adb2a4b9d206521bc2@...kaller.appspotmail.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] general protection fault in hpage_collapse_scan_file

On Tue, Apr 9, 2024 at 4:46 PM Andrew Morton <akpm@...ux-foundationorg> wrote:
>
> On Tue, 09 Apr 2024 03:16:20 -0700 syzbot <syzbot+57adb2a4b9d206521bc2@...kaller.appspotmail.com> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    8568bb2ccc27 Add linux-next specific files for 20240405
> > git tree:       linux-next
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=152f4805180000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=48ca5acf8d2eb3bc
> > dashboard link: https://syzkaller.appspot.com/bug?extid=57adb2a4b9d206521bc2
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1268258d180000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1256598d180000
>
> Help.  From a quick look this seems to be claiming that collapse_file()
> got to
>
>         VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>
> with folio==NULL, but the code look solid regarding this.
>
> Given that we have a reproducer, can we expect the bot to perform a
> bisection for us?
>

I often don't see a successful automatic bisect, even with
reproducers. Hit or miss. I will take a closer look tomorrow -- the
reproducer doesn't look to be doing anything crazy.

Thanks,
Zach