linux-ext4 - Re: [RFC] Documentation: Add initial iomap document

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Date: Fri, 3 May 2024 09:41:26 -0700
From: Christoph Hellwig <hch@...radead.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <willy@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFC] Documentation: Add initial iomap document

> +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?