Re: [PATCH v1 12/16] iomap: add iomap_read_ops for read and readahead

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 29, 2025 at 04:56:23PM -0700, Joanne Koong wrote:
> Add a "struct iomap_read_ops" that contains a read_folio_range()
> callback that callers can provide as a custom handler for reading in a
> folio range, if the caller does not wish to issue bio read requests
> (which otherwise is the default behavior). read_folio_range() may read
> the request asynchronously or synchronously. The caller is responsible
> for calling iomap_start_folio_read()/iomap_finish_folio_read() when
> reading the folio range.
> 
> This makes it so that non-block based filesystems may use iomap for
> reads.

Also for things like checksumming in block based file systems.  I've
carried this patch originally from Goldwyn around for a while with
my PI support patches:

https://git.infradead.org/?p=users/hch/misc.git;a=commitdiff;h=54ad84fded1d954cb9ebf483008cb57421efc959

I'll see if we'll still need submit_bio with your version or if
that can be reworked on top of your callout.

> @@ -356,6 +356,12 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
>  	if (finished)
>  		folio_end_read(folio, uptodate);
>  }
> +
> +void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
> +		int error)
> +{
> +	return __iomap_finish_folio_read(folio, off, len, error, true);
> +}
>  EXPORT_SYMBOL_GPL(iomap_finish_folio_read);

..

> +	if (ifs)
> +		iomap_start_folio_read(folio, 1);

I don't fully understand these changes.  Any chance they could be split
into a prep patch with a detailed commit message so that the method
addition itself is mostly mechanical?

> +			if (read_ops && read_ops->read_folio_range) {
> +				ret = read_ops->read_folio_range(iter, folio, pos, plen);
> +				if (ret)
> +					break;
> +			} else {
> +				iomap_read_folio_range_async(iter, ctx, pos, plen);
> +			}

Overly long lines.