[dm-devel] Re: 2.6: drivers/md/dm-io.c partially copies bio.c

[Kevin Corry]

On Monday 06 December 2004 8:42 am, Jens Axboe wrote:
> On Mon, Dec 06 2004, Kevin Corry wrote:
> > If I recall correctly (and it's been a while since I've looked at that
> > code), the bio_set was added because a few DM modules like to initiate
> > their own I/O requests (things like snapshots and DM's kcopyd daemon),
> > and we felt it was better to allow these modules to each create their
> > own mempools to allocate bios from, rather than allocate from the
> > single kernel-wide bio pool used by the filesystem layer.
>
> That is fully correct and also something that eg the bouncing code needs
> to be 100% deadlock free.
>
> > Strictly speaking (and as you mentioned), the slabs in the bio_set are
> > unnecessary, and they could use the global bio_slab. But there's
> > probably some argument to be made for having separate bio mempools for
> > these modules.
>
> The mempools are needed, the duplicated slabs are not. But that's just a
> small part of it, basically the whole allocation and index stuff is 100%
> duplicated from bio.c. Even bio_init().
>
> > Actually, I also seem to recall discussions with Joe Thornber from
> > quite awhile ago about trying to move this bio_set functionality into
> > fs/bio.c, to allow other device drivers to create their own private
> > bio pools. If you think something like this would be desireable, I can
> > try to look into the specifics again. If you think that having the
> > single kernel-wide bio pool is sufficient for all filesystems and
> > device-drivers (you certainly understand the trade-offs better than I
> > do), then I can look into removing the necessary code from dm-io.c
>
> An abstraction for this would be nice, as there are two users of it then
> (dm-io and highmem.c). So if your team would do that, I would sure help
> you review and integrate it.
>
> That's not what I'm arguing against, it's the (almost) wholesale
> duplication that's a bit silly.

I'll be happy to look into whether this duplication can be removed, and also 
whether the bio_set stuff could be moved out to bio.c or somewhere else 
common to the other device-drivers and filesystems. However, I've got a lot 
of stuff going on right now, so I won't really have time to get to this until 
after the start of the year (might be able to start looking into it over the 
holidays if I get everything else done in the next couple weeks). If/When I 
come up with some patches, I'll let you know.

-- 
Kevin Corry
kevcorry at us.ibm.com
http://evms.sourceforge.net/