Re: kernel BUG in ext4_writepages

[Tadeusz Struk]

On 5/20/22 02:50, Jan Kara wrote:
> On Thu 19-05-22 16:14:17, Tadeusz Struk wrote:
> > On 5/19/22 05:23, Jan Kara wrote:
> > > Hi!
> > >
> > > On Tue 10-05-22 15:28:38, Tadeusz Struk wrote:
> > > > Syzbot found another BUG in ext4_writepages [1].
> > > > This time it complains about inode with inline data.
> > > > C reproducer can be found here [2]
> > > > I was able to trigger it on 5.18.0-rc6
> > > >
> > > > [1] https://syzkaller.appspot.com/bug?id=a1e89d09bbbcbd5c4cb45db230ee28c822953984
> > > > [2] https://syzkaller.appspot.com/text?tag=ReproC&x=129da6caf00000
> > >
> > > Thanks for report. This should be fixed by:
> > >
> > > https://lore.kernel.org/all/20220516012752.17241-1-yebin10@xxxxxxxxxx/
> >
> >
> > In case of the syzbot bug there is something messed up with PAGE DIRTY flags
> > and the way syzbot sets up the write. This is what triggers the crash:
>
> Can you tell me where exactly we hit the bug? I've now noticed that this is
> on 5.10 kernel and on vanilla 5.10 there's no BUG_ON on line 2753.

We are hiting this bug:
https://elixir.bootlin.com/linux/latest/source/fs/ext4/inode.c#L2707
Syzbot found it in v5.10, but I recreated it on 5.18-rc7, that's why
the line number mismatch. But this is the same bug.
On 5.10 it's in line 2739:
https://elixir.bootlin.com/linux/v5.10.117/source/fs/ext4/inode.c#L2739

> > $ ftrace -f ./repro
> > ...
> > [pid  2395] open("./bus", O_RDWR|O_CREAT|O_SYNC|O_NOATIME, 000 <unfinished ...>
> > [pid  2395] <... open resumed> )        = 6
> > ...
> > [pid  2395] write(6, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 22 <unfinished ...>
> > ...
> > [pid  2395] <... write resumed> )       = 22
> >
> > One way I could fix it was to clear the PAGECACHE_TAG_DIRTY on the mapping in
> > ext4_try_to_write_inline_data() after the page has been updated:
> >
> > diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> > index 9c076262770d..e4bbb53fa26f 100644
> > --- a/fs/ext4/inline.c
> > +++ b/fs/ext4/inline.c
> > @@ -715,6 +715,7 @@ int ext4_try_to_write_inline_data(struct address_space *mapping,
> >   			put_page(page);
> >   			goto out_up_read;
> >   		}
> > +		__xa_clear_mark(&mapping->i_pages, 0, PAGECACHE_TAG_DIRTY);
> >   	}
> >   	ret = 1;
> >
> > Please let me know it if makes sense any I will send a proper patch.
>
> No, this looks really wrong... We need to better understand what's going
> on.

So I was afraid. I'm trying to diverge the ext4_writepages() to go to the
out_writepages path before we hit this BOG_ON().
Any hints will be much appreciated.

--
Thanks,
Tadeusz