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

[Christoph Hellwig <hch@infradead.org>]

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.

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