linux-kernel - Re: [PATCH v14 090/138] block: Add bio_add

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Date:   Thu, 15 Jul 2021 13:59:17 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v14 090/138] block: Add bio_add_folio()

On Thu, Jul 15, 2021 at 04:36:16AM +0100, Matthew Wilcox (Oracle) wrote:
> This is a thin wrapper around bio_add_page().  The main advantage here
> is the documentation that the submitter can expect to see folios in the
> completion handler, and that stupidly large folios are not supported.
> It's not currently possible to allocate stupidly large folios, but if
> it ever becomes possible, this function will fail gracefully instead of
> doing I/O to the wrong bytes.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  block/bio.c         | 21 +++++++++++++++++++++
>  include/linux/bio.h |  3 ++-
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 1fab762e079b..1b500611d25c 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -933,6 +933,27 @@ int bio_add_page(struct bio *bio, struct page *page,
>  }
>  EXPORT_SYMBOL(bio_add_page);
>  
> +/**
> + * bio_add_folio - Attempt to add part of a folio to a bio.
> + * @bio: Bio to add to.
> + * @folio: Folio to add.
> + * @len: How many bytes from the folio to add.
> + * @off: First byte in this folio to add.
> + *
> + * Always uses the head page of the folio in the bio.  If a submitter
> + * only uses bio_add_folio(), it can count on never seeing tail pages
> + * in the completion routine.  BIOs do not support folios larger than 2GiB.
> + *
> + * Return: The number of bytes from this folio added to the bio.
> + */
> +size_t bio_add_folio(struct bio *bio, struct folio *folio, size_t len,
> +		size_t off)
> +{
> +	if (len > UINT_MAX || off > UINT_MAX)

Er... if bios don't support folios larger than 2GB, then why check @off
and @len against UINT_MAX, which is ~4GB?

--D

> +		return 0;
> +	return bio_add_page(bio, &folio->page, len, off);
> +}
> +
>  void bio_release_pages(struct bio *bio, bool mark_dirty)
>  {
>  	struct bvec_iter_all iter_all;
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 2203b686e1f0..ade93e2de6a1 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -462,7 +462,8 @@ extern void bio_uninit(struct bio *);
>  extern void bio_reset(struct bio *);
>  void bio_chain(struct bio *, struct bio *);
>  
> -extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
> +int bio_add_page(struct bio *, struct page *, unsigned len, unsigned off);
> +size_t bio_add_folio(struct bio *, struct folio *, size_t len, size_t off);
>  extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
>  			   unsigned int, unsigned int);
>  int bio_add_zone_append_page(struct bio *bio, struct page *page,
> -- 
> 2.30.2
>