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

[Omar Sandoval]

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@xxxxxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> Cc: dm-devel@xxxxxxxxxx
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: linux-raid@xxxxxxxxxxxxxxx
> Cc: linux-erofs@xxxxxxxxxxxxxxxx
> Cc: David Sterba <dsterba@xxxxxxxx>
> Cc: linux-btrfs@xxxxxxxxxxxxxxx
> Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Cc: linux-xfs@xxxxxxxxxxxxxxx
> Cc: Gao Xiang <gaoxiang25@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Cc: linux-ext4@xxxxxxxxxxxxxxx
> Cc: Coly Li <colyli@xxxxxxx>
> Cc: linux-bcache@xxxxxxxxxxxxxxx
> Cc: Boaz Harrosh <ooo@xxxxxxxxxxxxxxx>
> Cc: Bob Peterson <rpeterso@xxxxxxxxxx>
> Cc: cluster-devel@xxxxxxxxxx

Reviewed-by: Omar Sandoval <osandov@xxxxxx>

But a couple of comments below.

> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
>  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.

>  /*
>   * 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:

#define bvec_iter_len(bvec, iter)					\
	min_t(unsigned, mp_bvec_iter_len((bvec), (iter)),		\
	      PAGE_SIZE - bvec_iter_offset((bvec), (iter)))

> +
> +#define bvec_iter_page(bvec, iter)					\
> +	nth_page(mp_bvec_iter_page((bvec), (iter)),		\
> +		 mp_bvec_iter_page_idx((bvec), (iter)))
> +
>  #define bvec_iter_bvec(bvec, iter)				\
>  ((struct bio_vec) {						\
>  	.bv_page	= bvec_iter_page((bvec), (iter)),	\
> -- 
> 2.9.5
>