Re: [PATCH] iomap: Submit BIOs at the end of each extent

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Mar 19, 2020 at 08:07:20AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> By definition, an extent covers a range of consecutive blocks, so
> it would be quite rare to be able to just add pages to the BIO from
> a previous range.  The only case we can think of is a mapped extent
> followed by a hole extent, followed by another mapped extent which has
> been allocated immediately after the first extent.  We believe this to

Well... userspace can induce that with fallocate(INSERT_RANGE). :)

> be an unlikely layout for a filesystem to choose and, since the queue
> is plugged, those two BIOs would be merged by the block layer.
> 
> The reason we care is that ext2/ext4 choose to lay out blocks 0-11
> consecutively, followed by the indirect block, and we want to merge those
> two BIOs.  If we don't submit the data BIO before asking the filesystem
> for the next extent, then the indirect BIO will be submitted first,
> and waited for, leading to inefficient I/O patterns.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 83438b3257de..8d26920ddf00 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -388,6 +388,11 @@ iomap_readahead_actor(struct inode *inode, loff_t pos, loff_t length,
>  				ctx, iomap, srcmap);
>  	}
>  
> +	if (ctx->bio) {
> +		submit_bio(ctx->bio);
> +		ctx->bio = NULL;
> +	}

Makes sense, but could we have a quick comment here to capture why we're
submitting the bio here?

/*
 * Submit the bio now so that we neither combine IO requests for
 * non-adjacent ranges nor interleave data and metadata requests.
 */

--D

> +
>  	return done;
>  }
>  
> -- 
> 2.25.1
>