linux-kernel - Re: [PATCH 1/6] block: Introduce bio_for_each

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Message-ID: <20130925224755.GJ30372@lenny.home.zabbo.net>
Date:	Wed, 25 Sep 2013 15:47:55 -0700
From:	Zach Brown <zab@...hat.com>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	hch@...radead.org, axboe@...nel.dk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] block: Introduce bio_for_each_page()

On Wed, Sep 25, 2013 at 02:49:10PM -0700, Kent Overstreet wrote:
> On Wed, Sep 25, 2013 at 02:17:02PM -0700, Zach Brown wrote:
> > >  void zero_fill_bio(struct bio *bio)
> > >  {
> > > -	unsigned long flags;
> > >  	struct bio_vec bv;
> > >  	struct bvec_iter iter;
> > >  
> > > -	bio_for_each_segment(bv, bio, iter) {
> > > +#if defined(CONFIG_HIGHMEM) || defined(ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE)
> > > +	bio_for_each_page(bv, bio, iter) {
> > > +		unsigned long flags;
> > >  		char *data = bvec_kmap_irq(&bv, &flags);
> > >  		memset(data, 0, bv.bv_len);
> > >  		flush_dcache_page(bv.bv_page);
> > >  		bvec_kunmap_irq(data, &flags);
> > >  	}
> > > +#else
> > > +	bio_for_each_segment(bv, bio, iter)
> > > +		memset(page_address(bv.bv_page) + bv.bv_offset,
> > > +		       0, bv.bv_len);
> > > +#endif
> > 
> > This looks pretty sketchy.  I'd expect this to be doable with one loop
> > and that bvec_kmap_irq() and flush_dcache_page() would fall back to
> > page_address() and nops when they're not needed.
> > 
> > Where did this come from?
> 
> It's just that if we need the kmap or the flush_dcache_page we have to
> process the bio one 4k page at a time - if not, we can process 64k (or
> whatever) bvecs all at once. That doesn't just save us memcpy calls, we
> can also avoid all the machinery in bio_for_each_page() for chunking up
> large bvecs into single page bvecs.

Understood.  A comment would probably be wise as that ifdefery is going
to raise all the eyebrows.

- z
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