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

[James Bottomley]

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.

James


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