Re: [RFC] Documentation: Add initial iomap document

[Christoph Hellwig <hch@infradead.org>]

> +i.e. at a high level how iomap does write iter is [1]::
> +	user IO
> +	  loop for file IO range
> +	    loop for each mapped extent
> +	      if (buffered) {
> +		loop for each page/folio {
> +		  instantiate page cache
> +		  copy data to/from page cache
> +		  update page cache state
> +		}
> +	      } else { /* direct IO */
> +		loop for each bio {
> +		  pack user pages into bio
> +		  submit bio
> +		}
> +	      }
> +	    }
> +	  }

This would probably be easier to follow (and match the actual code)
with two separate flows for buffered vs direct I/O.

The loop for each bio in direct I/O is also not wrong, but maybe a 
bit confusing as we really map over a pinned memory range.  The code
just has a little optimization to directly place that into a bio.

> +Guideline for filesystem conversion to iomap
> +=============================================

Maybe split this into a separate file?