[PATCH v9 04/26] Add a function to kmap one page of a multipage bio_vec

[Matthew Wilcox]

On Mon, Aug 10, 2026 at 03:47:21PM +0100, David Howells wrote:
> Add a function to kmap one page of a multipage bio_vec by offset (which is
> added to the offset in the bio_vec internally).  The caller is responsible
> for calculating how much of the page is then available.

I think this should be kmap_local_bvec().  Living in bvec.h makes sense,
but keeping it in the kmap_local_ namespace is better than putting it in
the bvec_ namespace.

The bvec should be const-qualified.  We're not very good at
being const-clean, but let's try harder ;-)

I think it's better to steal the same terminology for the kernel-doc
used in highmem.h, just for consistency:

/**
 * kmap_local_bvec - Map a page in this bvec for temporary usage
 * @bvec: The bvec containing the page.
 * @offset: The first byte within the bvec to map.
 *
 * This will not map the entire bvec, only the page containing @offset
 * from the start of @bvec.  Unmap the address with kunmap_local().
 * See kmap_local_page() for nesting rules.
 *
 * Context: Interrupt or process context.
 * Return: A pointer to a temporary mapping.
 */

> +/**
> + * bvec_kmap_partial - Map part of a bvec into the kernel virtual address space
> + * @bvec: bvec to map
> + * @offset: Offset into bvec
> + *
> + * Map the page containing the byte at @offset into the kernel virtual address
> + * space.  The caller is responsible for making sure this doesn't overrun.
> + *
> + * Call kunmap_local on the returned address to unmap.
> + */
> +static inline void *bvec_kmap_partial(struct bio_vec *bvec, size_t offset)
> +{
> +	offset += bvec->bv_offset;
> +
> +	return kmap_local_page(bvec->bv_page + (offset >> PAGE_SHIFT)) +
> +		(offset & ~PAGE_MASK);
> +}
> +
>  #endif /* __LINUX_BVEC_H */
> 
>