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Date:   Mon, 31 Aug 2020 12:03:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     syzbot <syzbot+3622cea378100f45d59f@...kaller.appspotmail.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        tytso@....edu, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: kernel BUG at fs/ext4/inode.c:LINE!

On Fri 28-08-20 12:07:55, Jan Kara wrote:
> On Wed 26-08-20 19:48:16, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    c3d8f220 Merge tag 'kbuild-fixes-v5.9' of git://git.kernel..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15f83cb6900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=bb68b9e8a8cc842f
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3622cea378100f45d59f
> > compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1287ac96900000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11c7ac46900000
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+3622cea378100f45d59f@...kaller.appspotmail.com
> > 
> > ------------[ cut here ]------------
> > kernel BUG at fs/ext4/inode.c:2598!
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 1 PID: 27612 Comm: syz-executor879 Not tainted 5.9.0-rc1-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:mpage_prepare_extent_to_map+0xd34/0xd40 fs/ext4/inode.c:2598
> 
> Doh, so this is:
> 
>                         wait_on_page_writeback(page);
> >>>                     BUG_ON(PageWriteback(page));
> 
> in mpage_prepare_extent_to_map(). So we have PageWriteback() page after we
> have called wait_on_page_writeback() on a locked page. Not sure how this
> could ever happen even less how ext4 could cause this...

I was poking a bit into this and there were actually recent changes into
page bit waiting logic by Linus. Linus, any idea?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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