linux-kernel - Re: [PATCH V10 01/19] block: introduce multi-page page bvec helpers

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Date:   Mon, 19 Nov 2018 10:25:26 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        Christoph Hellwig <hch@....de>, Theodore Ts'o <tytso@....edu>,
        linux-ext4@...r.kernel.org, Coly Li <colyli@...e.de>,
        linux-bcache@...r.kernel.org, Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V10 01/19] block: introduce multi-page page bvec helpers

On Thu, Nov 15, 2018 at 10:25:59AM -0800, Omar Sandoval wrote:
> On Thu, Nov 15, 2018 at 04:52:48PM +0800, Ming Lei wrote:
> > This patch introduces helpers of 'mp_bvec_iter_*' for multipage
> > bvec support.
> > 
> > The introduced helpers treate one bvec as real multi-page segment,
> > which may include more than one pages.
> > 
> > The existed helpers of bvec_iter_* are interfaces for supporting current
> > bvec iterator which is thought as single-page by drivers, fs, dm and
> > etc. These introduced helpers will build single-page bvec in flight, so
> > this way won't break current bio/bvec users, which needn't any change.
> > 
> > Cc: Dave Chinner <dchinner@...hat.com>
> > Cc: Kent Overstreet <kent.overstreet@...il.com>
> > Cc: Mike Snitzer <snitzer@...hat.com>
> > Cc: dm-devel@...hat.com
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: linux-fsdevel@...r.kernel.org
> > Cc: Shaohua Li <shli@...nel.org>
> > Cc: linux-raid@...r.kernel.org
> > Cc: linux-erofs@...ts.ozlabs.org
> > Cc: David Sterba <dsterba@...e.com>
> > Cc: linux-btrfs@...r.kernel.org
> > Cc: Darrick J. Wong <darrick.wong@...cle.com>
> > Cc: linux-xfs@...r.kernel.org
> > Cc: Gao Xiang <gaoxiang25@...wei.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: Theodore Ts'o <tytso@....edu>
> > Cc: linux-ext4@...r.kernel.org
> > Cc: Coly Li <colyli@...e.de>
> > Cc: linux-bcache@...r.kernel.org
> > Cc: Boaz Harrosh <ooo@...ctrozaur.com>
> > Cc: Bob Peterson <rpeterso@...hat.com>
> > Cc: cluster-devel@...hat.com
> 
> Reviewed-by: Omar Sandoval <osandov@...com>
> 
> But a couple of comments below.
> 
> > Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > ---
> >  include/linux/bvec.h | 63 +++++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 60 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> > index 02c73c6aa805..8ef904a50577 100644
> > --- a/include/linux/bvec.h
> > +++ b/include/linux/bvec.h
> > @@ -23,6 +23,44 @@
> >  #include <linux/kernel.h>
> >  #include <linux/bug.h>
> >  #include <linux/errno.h>
> > +#include <linux/mm.h>
> > +
> > +/*
> > + * What is multi-page bvecs?
> > + *
> > + * - bvecs stored in bio->bi_io_vec is always multi-page(mp) style
> > + *
> > + * - bvec(struct bio_vec) represents one physically contiguous I/O
> > + *   buffer, now the buffer may include more than one pages after
> > + *   multi-page(mp) bvec is supported, and all these pages represented
> > + *   by one bvec is physically contiguous. Before mp support, at most
> > + *   one page is included in one bvec, we call it single-page(sp)
> > + *   bvec.
> > + *
> > + * - .bv_page of the bvec represents the 1st page in the mp bvec
> > + *
> > + * - .bv_offset of the bvec represents offset of the buffer in the bvec
> > + *
> > + * The effect on the current drivers/filesystem/dm/bcache/...:
> > + *
> > + * - almost everyone supposes that one bvec only includes one single
> > + *   page, so we keep the sp interface not changed, for example,
> > + *   bio_for_each_segment() still returns bvec with single page
> > + *
> > + * - bio_for_each_segment*() will be changed to return single-page
> > + *   bvec too
> > + *
> > + * - during iterating, iterator variable(struct bvec_iter) is always
> > + *   updated in multipage bvec style and that means bvec_iter_advance()
> > + *   is kept not changed
> > + *
> > + * - returned(copied) single-page bvec is built in flight by bvec
> > + *   helpers from the stored multipage bvec
> > + *
> > + * - In case that some components(such as iov_iter) need to support
> > + *   multi-page bvec, we introduce new helpers(mp_bvec_iter_*) for
> > + *   them.
> > + */
> 
> This comment sounds more like a commit message (i.e., how were things
> before, and how are we changing them). In a couple of years when I read
> this code, I probably won't care how it was changed, just how it works.
> So I think a comment explaining the concepts of multi-page and
> single-page bvecs is very useful, but please move all of the "foo was
> changed" and "before mp support" type stuff to the commit message.

OK.

> 
> >  /*
> >   * was unsigned short, but we might as well be ready for > 64kB I/O pages
> > @@ -50,16 +88,35 @@ struct bvec_iter {
> >   */
> >  #define __bvec_iter_bvec(bvec, iter)	(&(bvec)[(iter).bi_idx])
> >  
> > -#define bvec_iter_page(bvec, iter)				\
> > +#define mp_bvec_iter_page(bvec, iter)				\
> >  	(__bvec_iter_bvec((bvec), (iter))->bv_page)
> >  
> > -#define bvec_iter_len(bvec, iter)				\
> > +#define mp_bvec_iter_len(bvec, iter)				\
> >  	min((iter).bi_size,					\
> >  	    __bvec_iter_bvec((bvec), (iter))->bv_len - (iter).bi_bvec_done)
> >  
> > -#define bvec_iter_offset(bvec, iter)				\
> > +#define mp_bvec_iter_offset(bvec, iter)				\
> >  	(__bvec_iter_bvec((bvec), (iter))->bv_offset + (iter).bi_bvec_done)
> >  
> > +#define mp_bvec_iter_page_idx(bvec, iter)			\
> > +	(mp_bvec_iter_offset((bvec), (iter)) / PAGE_SIZE)
> > +
> > +/*
> > + * <page, offset,length> of single-page(sp) segment.
> > + *
> > + * This helpers are for building sp bvec in flight.
> > + */
> > +#define bvec_iter_offset(bvec, iter)					\
> > +	(mp_bvec_iter_offset((bvec), (iter)) % PAGE_SIZE)
> > +
> > +#define bvec_iter_len(bvec, iter)					\
> > +	min_t(unsigned, mp_bvec_iter_len((bvec), (iter)),		\
> > +	    (PAGE_SIZE - (bvec_iter_offset((bvec), (iter)))))
> 
> The parentheses around (bvec_iter_offset((bvec), (iter))) and
> (PAGE_SIZE - (bvec_iter_offset((bvec), (iter)))) are unnecessary
> clutter. This looks easier to read to me:

Good catch!

Thanks,
Ming