Re: [PATCH RFC 1/4] iomap: allow passing a folio into write begin path

[Brian Foster <bfoster@redhat.com>]

On Wed, Nov 20, 2024 at 12:38:41AM -0800, Christoph Hellwig wrote:
> On Tue, Nov 19, 2024 at 10:46:53AM -0500, Brian Foster wrote:
> > To facilitate batch processing of dirty folios for zero range, tweak
> > the write begin path to allow the caller to optionally pass in its
> > own folio/pos combination.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >  fs/iomap/buffered-io.c | 20 +++++++++++++-------
> >  1 file changed, 13 insertions(+), 7 deletions(-)
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index ce73d2a48c1e..d1a86aea1a7a 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -781,7 +781,7 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
> >  {
> >  	const struct iomap_folio_ops *folio_ops = iter->iomap.folio_ops;
> >  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> > -	struct folio *folio;
> > +	struct folio *folio = *foliop;
> >  	int status = 0;
> >  
> >  	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
> > @@ -794,9 +794,15 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
> >  	if (!mapping_large_folio_support(iter->inode->i_mapping))
> >  		len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));
> >  
> > -	folio = __iomap_get_folio(iter, pos, len);
> > -	if (IS_ERR(folio))
> > -		return PTR_ERR(folio);
> > +	/*
> > +	 * XXX: Might want to plumb batch handling down through here. For now
> > +	 * let the caller do it.
> 
> Yeah, plumbing in the batch here would be nicer.
> 

Ok, I'll take a closer look at that. IIRC I punted on it initially
because we'll also have to fix up pos/len down in this path when the
next folio is not contiguous. This was easier to just get the basics
working.

Brian

> I suspect doing batch processing might actually be a neat thing for
> the normal write patch as well.
>