Re: [PATCH 4/5] block: Fix zero_fill_bio()

[Ming Lei <ming.lei@redhat.com>]

On Fri, May 15, 2020 at 05:19:13PM -0700, Bart Van Assche wrote:
> Multiple block drivers use zero_fill_bio() to zero-initialize the data
> buffer used for read operations. Make sure that all pages are zeroed
> instead of only the first if one or more multi-page bvecs are used to
> describe the data buffer.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/bio.c         | 27 ++++++++++++++++++++++-----
>  include/linux/bio.h |  1 +
>  2 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 1594804fe8bc..48fcafbdae70 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -527,17 +527,34 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, unsigned int nr_iovecs,
>  }
>  EXPORT_SYMBOL(bio_alloc_bioset);
>  
> +void zero_fill_bvec(const struct bio_vec *bvec)
> +{
> +	struct page *page = bvec->bv_page;
> +	u32 offset = bvec->bv_offset;
> +	u32 left = bvec->bv_len;
> +
> +	while (left) {
> +		u32 len = min_t(u32, left, PAGE_SIZE - offset);
> +		void *kaddr;
> +
> +		kaddr = kmap_atomic(page);
> +		memset(kaddr + offset, 0, len);
> +		flush_dcache_page(page);
> +		kunmap_atomic(kaddr);
> +		page++;
> +		left -= len;
> +		offset = 0;
> +	}
> +}
> +EXPORT_SYMBOL(zero_fill_bvec);
> +
>  void zero_fill_bio_iter(struct bio *bio, struct bvec_iter start)
>  {
> -	unsigned long flags;
>  	struct bio_vec bv;
>  	struct bvec_iter iter;
>  
>  	__bio_for_each_segment(bv, bio, iter, start) {

The 'bv' from __bio_for_each_segment() is single page bvec, and so far
only [__]bio_for_each_bvec iterates over multi-page bvec.

So nothing to be fixed and this patch isn't necessary.

Thanks, 
Ming