Re: [PATCH v3] xfs: avoid deadlock when trigger memory reclaim in ->writepages

From: Yafang Shao <laoar.shao@gmail.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Michal Hocko <mhocko@kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Linux MM <linux-mm@kvack.org>
Subject: Re: [PATCH v3] xfs: avoid deadlock when trigger memory reclaim in ->writepages
Date: Thu, 18 Jun 2020 19:04:49 +0800	[thread overview]
Message-ID: <CALOAHbAtmSzyj-r3ghX311+BLmHHT-Bb0WRmzmAFRV6SuvevGw@mail.gmail.com> (raw)
In-Reply-To: <20200618003427.GZ2040@dread.disaster.area>

On Thu, Jun 18, 2020 at 8:34 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Tue, Jun 16, 2020 at 12:48:06PM +0200, Michal Hocko wrote:
> > On Tue 16-06-20 17:39:33, Yafang Shao wrote:
> > > The history is complicated, but it doesn't matter.
> > > Let's  turn back to the upstream kernel now. As I explained in the commit log,
> > > xfs_vm_writepages
> > >   -> iomap_writepages.
> > >      -> write_cache_pages
> > >         -> lock_page <<<< This page is locked.
> > >         -> writepages ( which is  iomap_do_writepage)
> > >            -> xfs_map_blocks
> > >               -> xfs_convert_blocks
> > >                  -> xfs_bmapi_convert_delalloc
> > >                     -> xfs_trans_alloc
> > >                          -> kmem_zone_zalloc //It should alloc page
> > > with GFP_NOFS
> > >
> > > If GFP_NOFS isn't set in xfs_trans_alloc(), the kmem_zone_zalloc() may
> > > trigger the memory reclaim then it may wait on the page locked in
> > > write_cache_pages() ...
> >
> > This cannot happen because the memory reclaim backs off on locked pages.
>
> ->writepages can hold a bio with multiple PageWriteback pages
> already attached to it. Direct GFP_KERNEL page reclaim can wait on
> them - if that happens the the bio will never be issued and so
> reclaim will deadlock waiting for the writeback state to clear...
>

Thanks for the explanation.

> > > That means the ->writepages should be set with GFP_NOFS to avoid this
> > > recursive filesystem reclaim.
>
> Indeed. We already have parts of the IO submission path under
> PF_MEMALLOC_NOFS so we can do transaction allocation, etc. See
> xfs_prepare_ioend(), which is called from iomap via:
>
> iomap_submit_ioend()
>   ->prepare_ioend()
>     xfs_prepare_ioend()
>
> we can get there from:
>
> iomap_writepage()
>   iomap_do_writepage()
>     iomap_writepage_map()
>       iomap_submit_ioend()
>   iomap_submit_ioend()
>
> and:
>
> iomap_writepages()
>   write_cache_pages()
>     iomap_do_writepage()
>       iomap_writepage_map()
>         iomap_submit_ioend()
>   iomap_submit_ioend()
>
> Which says that we really should be putting both iomap_writepage()
> and iomap_writepages() under PF_MEMALLOC_NOFS context so that
> filesystem callouts don't have to repeatedly enter and exit
> PF_MEMALLOC_NOFS context to avoid memory reclaim recursion...
>

Looks reasonable.
I will update this patch to put both iomap_writepage() and
iomap_writepages() under PF_MEMALLOC_NOFS context and try to remove
the usage of PF_MEMALLOC_NOFS from the filesystem callouts.

-- 
Thanks
Yafang