linux-kernel - Re: [PATCH v3 6/7] bio: add a helper calculating nr segments to alloc

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Date:   Mon, 11 Jan 2021 10:57:10 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 6/7] bio: add a helper calculating nr segments to alloc

On Sat, Jan 09, 2021 at 04:03:02PM +0000, Pavel Begunkov wrote:
> Add a helper function calculating the number of bvec segments we need to
> allocate to construct a bio. It doesn't change anything functionally,
> but will be used to not duplicate special cases in the future.
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  fs/block_dev.c       |  7 ++++---
>  fs/iomap/direct-io.c |  9 ++++-----
>  include/linux/bio.h  | 10 ++++++++++
>  3 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 3b8963e228a1..6f5bd9950baf 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -416,7 +416,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  		dio->size += bio->bi_iter.bi_size;
>  		pos += bio->bi_iter.bi_size;
>  
> -		nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
> +		nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES);
>  		if (!nr_pages) {
>  			bool polled = false;
>  
> @@ -481,9 +481,10 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  {
>  	int nr_pages;
>  
> -	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
> -	if (!nr_pages)
> +	if (!iov_iter_count(iter))
>  		return 0;
> +
> +	nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES + 1);
>  	if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
>  		return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
>  
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 933f234d5bec..ea1e8f696076 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -250,11 +250,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	orig_count = iov_iter_count(dio->submit.iter);
>  	iov_iter_truncate(dio->submit.iter, length);
>  
> -	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> -	if (nr_pages <= 0) {
> -		ret = nr_pages;
> +	if (!iov_iter_count(dio->submit.iter))
>  		goto out;
> -	}
>  
>  	if (need_zeroout) {
>  		/* zero out from the start of the block to the write offset */
> @@ -263,6 +260,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  			iomap_dio_zero(dio, iomap, pos - pad, pad);
>  	}
>  
> +	nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_PAGES);
>  	do {
>  		size_t n;
>  		if (dio->error) {
> @@ -308,7 +306,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		dio->size += n;
>  		copied += n;
>  
> -		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> +		nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter,
> +						 BIO_MAX_PAGES);
>  		iomap_dio_submit_bio(dio, iomap, bio, pos);
>  		pos += n;
>  	} while (nr_pages);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1edda614f7ce..d8f9077c43ef 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -10,6 +10,7 @@
>  #include <linux/ioprio.h>
>  /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
>  #include <linux/blk_types.h>
> +#include <linux/uio.h>
>  
>  #define BIO_DEBUG
>  
> @@ -441,6 +442,15 @@ static inline void bio_wouldblock_error(struct bio *bio)
>  	bio_endio(bio);
>  }
>  
> +/*
> + * Calculate number of bvec segments that should be allocated to fit data
> + * pointed by @iter.
> + */
> +static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
> +{
> +	return iov_iter_npages(iter, max_segs);
> +}
> +
>  struct request_queue;
>  
>  extern int submit_bio_wait(struct bio *bio);
> -- 
> 2.24.0
> 

Reviewed-by: Ming Lei <ming.lei@...hat.com>

-- 
Ming