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

[Christoph Hellwig <hch@infradead.org>]

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 good,

Reviewed-by: Christoph Hellwig <hch@lst.de>