On Fri 28-08-20 12:07:55, Jan Kara wrote:

On Wed 26-08-20 19:48:16, syzbot wrote:

quoted

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@syzkaller.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);

quoted

quoted

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                    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 [off-list ref]
SUSE Labs, CR