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

Christoph Hellwig
2013-09-24T11:00:20Z

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.
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