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

[Dave Chinner <david@fromorbit.com>]

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...

> > 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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com