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Date:   Mon, 19 Nov 2018 11:31:11 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
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>,
        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 02/19] block: introduce bio_for_each_bvec()

On Fri, Nov 16, 2018 at 02:30:28PM +0100, Christoph Hellwig wrote:
> > +static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> > +				      unsigned bytes, bool mp)
> 
> I think these magic 'bool np' arguments and wrappers over wrapper
> don't help anyone to actually understand the code.  I'd vote for
> removing as many wrappers as we really don't need, and passing the
> actual segment limit instead of the magic bool flag.  Something like
> this untested patch:

I think this way is fine, just a little comment.

> 
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 277921ad42e7..dcad0b69f57a 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -138,30 +138,21 @@ static inline bool bio_full(struct bio *bio)
>  		bvec_for_each_segment(bvl, &((bio)->bi_io_vec[iter_all.idx]), i, iter_all)
>  
>  static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> -				      unsigned bytes, bool mp)
> +				      unsigned bytes, unsigned max_segment)

The new parameter should have been named as 'max_segment_len' or
'max_seg_len'.

>  {
>  	iter->bi_sector += bytes >> 9;
>  
>  	if (bio_no_advance_iter(bio))
>  		iter->bi_size -= bytes;
>  	else
> -		if (!mp)
> -			bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> -		else
> -			mp_bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> +		__bvec_iter_advance(bio->bi_io_vec, iter, bytes, max_segment);
>  		/* TODO: It is reasonable to complete bio with error here. */
>  }
>  
>  static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
>  				    unsigned bytes)
>  {
> -	__bio_advance_iter(bio, iter, bytes, false);
> -}
> -
> -static inline void bio_advance_mp_iter(struct bio *bio, struct bvec_iter *iter,
> -				       unsigned bytes)
> -{
> -	__bio_advance_iter(bio, iter, bytes, true);
> +	__bio_advance_iter(bio, iter, bytes, PAGE_SIZE);
>  }
>  
>  #define __bio_for_each_segment(bvl, bio, iter, start)			\
> @@ -177,7 +168,7 @@ static inline void bio_advance_mp_iter(struct bio *bio, struct bvec_iter *iter,
>  	for (iter = (start);						\
>  	     (iter).bi_size &&						\
>  		((bvl = bio_iter_mp_iovec((bio), (iter))), 1);	\
> -	     bio_advance_mp_iter((bio), &(iter), (bvl).bv_len))
> +	     __bio_advance_iter((bio), &(iter), (bvl).bv_len, 0))

Even we might pass '-1' for multi-page segment.

>  
>  /* returns one real segment(multipage bvec) each time */
>  #define bio_for_each_bvec(bvl, bio, iter)			\
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index 02f26d2b59ad..5e2ed46c1c88 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -138,8 +138,7 @@ struct bvec_iter_all {
>  })
>  
>  static inline bool __bvec_iter_advance(const struct bio_vec *bv,
> -				       struct bvec_iter *iter,
> -				       unsigned bytes, bool mp)
> +		struct bvec_iter *iter, unsigned bytes, unsigned max_segment)
>  {
>  	if (WARN_ONCE(bytes > iter->bi_size,
>  		     "Attempted to advance past end of bvec iter\n")) {
> @@ -148,18 +147,18 @@ static inline bool __bvec_iter_advance(const struct bio_vec *bv,
>  	}
>  
>  	while (bytes) {
> -		unsigned len;
> +		unsigned segment_len = mp_bvec_iter_len(bv, *iter);
>  
> -		if (mp)
> -			len = mp_bvec_iter_len(bv, *iter);
> -		else
> -			len = bvec_iter_len(bv, *iter);
> +		if (max_segment) {
> +			max_segment -= bvec_iter_offset(bv, *iter);
> +			segment_len = min(segment_len, max_segment);

Looks 'max_segment' needs to be constant, shouldn't be updated.

If '-1' is passed for multipage case, the above change may become:

		segment_len = min_t(segment_len, max_seg_len - bvec_iter_offset(bv, *iter));

This way is more clean, but with extra cost of the above line for multipage
case.

Thanks,
Ming

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