linux-kernel - Re: [PATCH 0/22] Immutable biovecs, block layer changes

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Date:	Tue, 24 Sep 2013 04:00:12 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
	neilb@...e.de, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org
Subject: Re: [PATCH 0/22] Immutable biovecs, block layer changes

Just curious, what's the state of the remaining immutable bio work?

On Thu, Aug 08, 2013 at 02:15:29PM -0700, Kent Overstreet wrote:
> > What is preventing you from sending those out as well?  While it's not
> > absolutely nessecary it would certainly be good if we'd avoid a struct
> > bio size regression.
> 
> There's still some fairly significant changes, and I don't want to make
> too many invasive changes at once.
> 
> Main thing is making generic_make_request() take arbitrary size bios.
> After this series that's just two simple patches, but then the changes
> to make use of that will be changing behaviour in non obvious ways.
> 
> The way the merging changes work is it enables multi page bvecs - so a
> bvec can point to many physically contiguous pages. This moves segment
> merging to bio_add_page(), and gets rid of bi_phys_segments - now
> bi_vcnt == bi_phys_segments, we just split the bio if it's too big.
> 
> Then, with bio_add_page() building up large bios and the block layer
> splitting them as necessary, there shouldn't be any need for segment
> merging across bios anymore (because generally when that would've
> happened before, now we'd just be sending one larger bio down).
> 
> The remaining patches aren't terribly complicated though (less
> complicated than this patch series). Trickiest bit is multipage bvecs,
> and that's mostly just lots of code auditing - the way I convert
> existing code is by adding bio_for_each_page() - analagous to
> bio_for_each_segment, but giving you bvecs that point to single pages.
> So it's an easy conversion, just have to make sure nothing's missed.
---end quoted text---
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