Re: [PATCH v1 5/8] iomap: add iomap_writeback_dirty_folio()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 09, 2025 at 08:59:53PM -0700, Christoph Hellwig wrote:
> On Mon, Jun 09, 2025 at 10:14:44AM -0700, Darrick J. Wong wrote:
> > > Where "folio laundering" means calling ->launder_folio, right?
> > 
> > What does fuse use folio laundering for, anyway?  It looks to me like
> > the primary users are invalidate_inode_pages*.  Either the caller cares
> > about flushing dirty data and has called filemap_write_and_wait_range;
> > or it doesn't and wants to tear down the pagecache ahead of some other
> > operation that's going to change the file contents and doesn't care.
> > 
> > I suppose it could be useful as a last-chance operation on a dirty folio
> > that was dirtied after a filemap_write_and_wait_range but before
> > invalidate_inode_pages*?  Though for xfs we just return EBUSY and let
> > the caller try again (or not).  Is there a subtlety to fuse here that I
> > don't know about?
> 
> My memory might be betraying me, but I think willy once launched an
> attempt to see if we can kill launder_folio.  Adding him, and the
> mm and nfs lists to check if I have a point :)

I ... got distracted with everything else.

Looking at the original addition of ->launder_page (e3db7691e9f3), I
don't understand why we need it.  invalidate_inode_pages2() isn't
supposed to invalidate dirty pages, so I don't understand why nfs
found it necessary to do writeback from ->releasepage() instead
of just returning false like iomap does.

There's now a new question of what the hell btrfs is up to with
->launder_folio, which they just added recently.