linux-kernel - Re: nilfs: WARNING: CPU: 2 PID: 1510 at include/linux/backing-dev.h:269 __folio_mark

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Date:   Sun, 13 Mar 2022 16:28:18 +0900
From:   Ryusuke Konishi <konishi.ryusuke@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-nilfs <linux-nilfs@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: nilfs: WARNING: CPU: 2 PID: 1510 at include/linux/backing-dev.h:269
 __folio_mark_dirty+0x31d/0x3b0

On Sun, Mar 13, 2022 at 3:23 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Sun, 13 Mar 2022 15:09:27 +0900 Ryusuke Konishi <konishi.ryusuke@...il.com> wrote:
>
> > Hi Matthew, and Andrew,
> >
> > On Sat, Mar 12, 2022 at 7:56 AM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Fri, Mar 11, 2022 at 08:43:57PM +0100, David Hildenbrand wrote:
> > > > Hi,
> > > >
> > > > playing with swapfiles on random file systems, I stumbled over the
> > > > following nilfs issue (and reproduced it on latest greatest
> > > > linux/master -- v5.17-rc7+). I did not try finding out when this
> > > > was introduced and I did not run into this issue on other file
> > > > systems I tried.
> > >
> > > It's a known bug in NILFS, and I think yours is the fifth report
> > > of it dating back eight months.
> >
> > The root cause of this issue is that NILFS uses two page caches
> > per inode, one for data blocks and another for b-tree node blocks.
> >
> > Even though __folio_end_writeback(), __folio_start_writeback(), and
> > __folio_mark_dirty() acquire lock for mapping->i_pages,
> > inode_to_wb(inode) inside them performs lockdep test for the former one
> > (i.e. inode->i_mapping->i_pages.xa_lock).
> >
> > So, mark_buffer_dirty(), end_page_writeback(), and set_page_writeback()
> > for pages in the latter NILFS specific page cache hit the LOCKDEP warning.
> >
> > I tried to find a way to  resolve this, but have no good idea so far.
>
> If things are set up appropriately, inode_to_wb() should be able to
> test inode->i_mapping->host->i_mapping->i_pages.xa_lock and get the
> desired result.
>
> At least, that's the case with blockdevs.  I don't know if nilfs2 sets
> things up that way.

Yes, that is the problem of the current implementation of nilfs2.
nilfs2 sets the page cache for data to inode->i_mapping
but the page cache for b-tree nodes is not set to inode->i_mapping
because both are kept in one nilfs_inode together.

To satisfy relation inode->i_mapping->host == inode for both page caches,
I need to change nilfs_inode to assign inode struct for both.

Well, I'll consider if such modification is possible as a solution.

Regards,
Ryusuke Konishi