Re: [PATCH] iomap: allow writeback from kswapd again

[Christian Brauner]

On Wed, Nov 04, 2026 at 07:07:37AM +1100, Dave Chinner wrote:
> On Tue, Nov 03, 2026 at 10:38:04AM +0800, Yiqing Wu wrote:
> > Commit ff59f71f1c95 ("iomap: simplify writepages reclaim guard") removed
> > the PF_KSWAPD exemption from the iomap_writepages() guard on the
> > assumption that kswapd can no longer reach filesystem writeback now
> > that pageout() returns PAGE_ACTIVATE for non-shmem, non-anon folios.
> > That assumption only covers the folio reclaim path.  kswapd can still
> > reach filesystem writeback through the inode shrinkers, and syzbot
> > reports exactly that on GFS2:
> > 
> >   WARNING: (current->flags & PF_MEMALLOC) in iomap_writepages
> >   kswapd -> balance_pgdat -> shrink_node -> shrink_slab
> >   -> super_cache_scan -> prune_icache_sb -> evict
> >   -> gfs2_evict_inode -> evict_linked_inode -> gfs2_log_flush
> >   -> gfs2_ordered_write -> filemap_fdatawrite -> do_writepages
> >   -> gfs2_writepages -> iomap_writepages
> 
> Oh, that's awful. Nothing should be calling into iomap_writepages
> from reclaim context.
> 
> i.e. Doing data writeback from the inode cache shrinker is extremely
> suboptimal for many reasons, so I'd consider this a gfs2 bug, not an
> iomap issue. It's even worse when data writeback is being driven by
> an inode cache shrinker. I'm surprised that lockdep hasn't thrown a
> hissy-fit over this (i.e. inode locks taken in writeback appearing
> both above and below reclaim.
> 
> This is the patch series that Jan Kara came up with to deal with
> these issues in inode eviction:
> 
> https://lore.kernel.org/linux-fsdevel/20261009133253.20102-1-jack@xxxxxxx/
> 
> This allows filesystems to defer inode eviction that might block
> (e.g. due to transaction reservations, journal commits, metadata
> read requirements, data writeback requirements, etc) to a context
> where the work is allowed to block and isn't running under memory
> reclaim context or restrictions.
> 
> That patchset is needed for ext4, XFS alreayd does this, and it
> would seem that gfs2 needs it for glock reclaim....

I agree.