Re: [PATCH 3/7] iomap: optional zero range dirty folio processing

[Brian Foster <bfoster@redhat.com>]

On Mon, Jun 09, 2025 at 09:27:07PM -0700, Christoph Hellwig wrote:
> On Mon, Jun 09, 2025 at 09:04:20AM -0700, Darrick J. Wong wrote:
> > > +	if (iter->fbatch) {
> > > +		struct folio *folio = folio_batch_next(iter->fbatch);
> > > +
> > > +		if (folio) {
> > > +			folio_get(folio);
> > > +			folio_lock(folio);
> > 
> > Hrm.  So each folio that is added to the batch isn't locked, nor does
> > the batch (or iomap) hold a refcount on the folio until we get here.
> 
> find_get_entry references a folio, and filemap_get_folios_dirty
> preserves that reference.  It will be released in folio_batch_release.
> So this just add an extra reference for local operation so that the
> rest of iomap doesn't need to know about that magic.
> 

Exactly.

> > Do
> > we have to re-check that folio->{mapping,index} match what iomap is
> > trying to process?  Or can we assume that nobody has removed the folio
> > from the mapping?
> 
> That's a good point, though  as without having the folio locked it
> could get truncated, so I think we'll have to redo the truncate
> check here.
> 
> 

Hm Ok thanks, I'll take a closer look at that..

Brian