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

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

On Fri, Mar 20, 2020 at 07:40:14AM -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
> 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.  Buffer heads solve
> this with the BH_boundary flag, but iomap doesn't need that as long as
> we submit the bio here.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/iomap/buffered-io.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f080f542911b..417115bfaf6b 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -420,6 +420,16 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
>  				ctx, iomap, srcmap);
>  	}
>  
> +	/*
> +	 * Submitting the bio here leads to better I/O patterns for
> +	 * filesystems which need to do metadata reads to find the
> +	 * next extent.
> +	 */
> +	if (ctx->bio) {
> +		submit_bio(ctx->bio);
> +		ctx->bio = NULL;
> +	}
> +
>  	return done;
>  }
>  
> @@ -449,8 +459,6 @@ iomap_readpages(struct address_space *mapping, struct list_head *pages,
>  	}
>  	ret = 0;
>  done:
> -	if (ctx.bio)
> -		submit_bio(ctx.bio);
>  	if (ctx.cur_page) {
>  		if (!ctx.cur_page_in_bio)
>  			unlock_page(ctx.cur_page);
> -- 
> 2.25.1
>