Re: [PATCH] likely cleanup: revert unlikely in ll_back_merge_fn

[Jens Axboe <axboe@suse.de>]

On Wed, Apr 26 2006, James Bottomley wrote:
> On Wed, 2006-04-26 at 15:55 +0200, Jens Axboe wrote:
> > On Wed, Apr 26 2006, James Bottomley wrote:
> > > On Wed, 2006-04-26 at 07:20 +0200, Jens Axboe wrote:
> > > > But blk_recount_segments() sets the BIO_SEG_VALID flag. Ugh ok
> > > > __bio_add_page() basically kills the flag. James, I think you are the
> > > > author of that addition, does it really need to be so restrictive?
> > > > 
> > > >         /* If we may be able to merge these biovecs, force a recount */
> > > >         if (bio->bi_vcnt && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec) ||
> > > >             BIOVEC_VIRT_MERGEABLE(bvec-1, bvec)))
> > > >                 bio->bi_flags &= ~(1 << BIO_SEG_VALID);
> > > 
> > > Help me out here ... I can't find this chunk of code in the current
> > > tree.  Where is it?
> > 
> > Sorry, should have mentioned that. Current git tree (or 2.6.16 should be
> > the same), fs/bio.c:__bio_add_page():401.
> 
> OK, that's this change.
> 
> http://www.kernel.org/git/?p=linux/kernel/git/tglx/history.git;a=commit;h=2fed84384a0b084d78252aa14d6bfae03deb268f
> 
> I think the reason for this is that the bi_hw_back_size and bi_hw_front
> size aren't updated without a segment recount, so you could get rid of
> the valid flag clearing if you introduce a heuristic to update them.

Hmm yes, I'll ponder that a bit. Do you have any ideas on how to best
handle that? I guess it's not a huge deal, would be nice to avoid the
repeat segment calculations though.

-- 
Jens Axboe