Re: [PATCH] likely cleanup: revert unlikely in ll_back_merge_fn
From: Jens Axboe
Date: Thu Apr 27 2006 - 10:40:08 EST
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.
> 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.
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