Re: [PATCH v1 3/8] iomap: add buffered write support for IOMAP_IN_MEM iomaps

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jun 06, 2025 at 04:37:58PM -0700, Joanne Koong wrote:
> Add buffered write support for IOMAP_IN_MEM iomaps. This lets
> IOMAP_IN_MEM iomaps use some of the internal features in iomaps
> such as granular dirty tracking for large folios.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  fs/iomap/buffered-io.c | 24 +++++++++++++++++-------
>  include/linux/iomap.h  | 10 ++++++++++
>  2 files changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 1caeb4921035..fd2ea1306d88 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -300,7 +300,7 @@ static inline bool iomap_block_needs_zeroing(const struct iomap_iter *iter,
>  {
>  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
>  
> -	return srcmap->type != IOMAP_MAPPED ||
> +	return (srcmap->type != IOMAP_MAPPED && srcmap->type != IOMAP_IN_MEM) ||
>  		(srcmap->flags & IOMAP_F_NEW) ||
>  		pos >= i_size_read(iter->inode);
>  }
> @@ -583,16 +583,26 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
>  					 pos + len - 1);
>  }
>  
> -static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
> -		size_t poff, size_t plen, const struct iomap *iomap)
> +static int iomap_read_folio_sync(const struct iomap_iter *iter, loff_t block_start,
> +				 struct folio *folio, size_t poff, size_t plen)
>  {
> -	return iomap_bio_read_folio_sync(block_start, folio, poff, plen, iomap);
> +	const struct iomap_folio_ops *folio_ops = iter->iomap.folio_ops;
> +	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> +
> +	if (folio_ops && folio_ops->read_folio_sync)
> +		return folio_ops->read_folio_sync(block_start, folio,
> +						  poff, plen, srcmap,
> +						  iter->private);

Hmm, patch 6 hooks this up to fuse_do_readfolio, which means that iomap
provides the folios and manages their uptodate/dirty state.  You still
want fuse to handle the folio contents (aka poke the fuse server via
FUSE_READ/FUSE_WRITE), but this avoids the memcpy that IOMAP_INLINE
performs.

So I think you're effectively addding another IO path to buffered-io.c,
which explains why you moved the bio code to a separate file.  I wonder
if you could hook up this new IO path by checking for a non-NULL
->read_folio_sync function and calling it regardless of iomap::type?

--D

> +
> +	/* IOMAP_IN_MEM iomaps must always handle ->read_folio_sync() */
> +	WARN_ON_ONCE(iter->iomap.type == IOMAP_IN_MEM);
> +
> +	return iomap_bio_read_folio_sync(block_start, folio, poff, plen, srcmap);
>  }
>  
>  static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  		size_t len, struct folio *folio)
>  {
> -	const struct iomap *srcmap = iomap_iter_srcmap(iter);
>  	struct iomap_folio_state *ifs;
>  	loff_t block_size = i_blocksize(iter->inode);
>  	loff_t block_start = round_down(pos, block_size);
> @@ -640,8 +650,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  			if (iter->flags & IOMAP_NOWAIT)
>  				return -EAGAIN;
>  
> -			status = iomap_read_folio_sync(block_start, folio,
> -					poff, plen, srcmap);
> +			status = iomap_read_folio_sync(iter, block_start, folio,
> +						       poff, plen);
>  			if (status)
>  				return status;
>  		}
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index dbbf217eb03f..e748aeebe1a5 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -175,6 +175,16 @@ struct iomap_folio_ops {
>  	 * locked by the iomap code.
>  	 */
>  	bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
> +
> +	/*
> +	 * Required for IOMAP_IN_MEM iomaps. Otherwise optional if the caller
> +	 * wishes to handle reading in a folio.
> +	 *
> +	 * The read must be done synchronously.
> +	 */
> +	int (*read_folio_sync)(loff_t block_start, struct folio *folio,
> +			       size_t off, size_t len, const struct iomap *iomap,
> +			       void *private);
>  };
>  
>  /*
> -- 
> 2.47.1
> 
>