Re: [PATCH v15 01/17] block: Add bio_add_folio()

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

On Mon, Jul 19, 2021 at 07:39:45PM +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@infradead.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..c64e35548fb2 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 that are 4GiB or larger.
> + *
> + * 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)
> +		return 0;

The added page shouldn't cross 4G boundary, so just wondering why not
check 'if (len > UINT_MAX - off)'?


Thanks, 
Ming