Re: [RFC PATCH 1/6] sunrpc: Add helpers to build bvecs from contiguous pages

["Chuck Lever" <cel@kernel.org>]

On Tue, Oct 6, 2026, at 5:00 AM, J Louis Kaplan wrote:
> Add helpers to describe contiguous runs with multipage bvecs. These
> functions have no callers. They are used in later commits.
>
> Accept a caller-supplied maximum bvec length so transport callers
> can enforce direct memory access (DMA) mapping and device segment
> length limits. Also support counting entries without populating
> an array so callers can size their allocations using the same
> coalescing rules.
>
> Coalescing requires contiguous physical memory and page descriptors,
> and is disabled under KMSAN. Xen constraint handling remains WIP.
>
> Signed-off-by: J Louis Kaplan <Louis.Kaplan@arm.com>
> Assisted-by: LLM
> ---
>  include/linux/sunrpc/svc.h | 90 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 90 insertions(+)
>
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index dfadea50e0c6d..e651c2312a912 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -11,6 +11,7 @@
>  #ifndef SUNRPC_SVC_H
>  #define SUNRPC_SVC_H
> 
> +#include <linux/bvec.h>
>  #include <linux/in.h>
>  #include <linux/in6.h>
>  #include <linux/sunrpc/types.h>
> @@ -376,6 +377,95 @@ static inline void svc_thread_init_status(struct 
> svc_rqst *rqstp, int err)
>  		kthread_exit(1);
>  }
> 
> +/**
> + * svc_pages_to_bvec - build one bvec from physically contiguous pages
> + * @bv: bio_vec to initialize, or NULL to only measure the next extent
> + * @pages: first page-array slot in the range
> + * @nr_pages: number of page-array slots available from @pages
> + * @offset: byte offset in @pages[0]
> + * @len: maximum byte count to describe
> + * @max_bvec_len: maximum byte count in one bvec
> + *
> + * Return: The number of bytes described by @bv. The scan stops at a missing
> + * or physically non-contiguous page.
> + */
> +static inline unsigned int
> +svc_pages_to_bvec(struct bio_vec *bv, struct page *const *pages,
> +		  unsigned int nr_pages, unsigned int offset,
> +		  unsigned int len, unsigned int max_bvec_len)

Generally any function larger than two or three lines is to be
kept out of line. net/sunrpc/svc.c, perhaps, would be appropriate
here.


> +{
> +	unsigned long first_pfn;
> +	unsigned int bytes, used = 1;
> +
> +	if (WARN_ON_ONCE(!nr_pages || offset >= PAGE_SIZE || !len ||
> +			 !max_bvec_len || !pages[0]))
> +		return 0;
> +	first_pfn = page_to_pfn(pages[0]);
> +
> +	bytes = min3(len, max_bvec_len,
> +		     (unsigned int)(PAGE_SIZE - offset));
> +	while (bytes < len && bytes < max_bvec_len && used < nr_pages) {
> +		// TODO: handle Xen merge constraints, see `bvec_try_merge_page`
> +		//	 for reference, or unify helper functionality
> +		if (IS_ENABLED(CONFIG_KMSAN))
> +			break;
> +
> +		if (!pages[used] ||
> +		    page_to_pfn(pages[used]) != first_pfn + used ||
> +		    pages[0] + used != pages[used])
> +			break;
> +		used++;
> +		bytes = min3(len, max_bvec_len,
> +			     (used << PAGE_SHIFT) - offset);
> +	}
> +
> +	if (bv)
> +		bvec_set_page(bv, pages[0], bytes, offset);
> +	return bytes;
> +}
> +
> +/**
> + * svc_pages_to_bvecs - build bvecs for a service page-array range
> + * @bvecs: bio_vec array to populate, or NULL to only count the entries
> + * @pages: first page-array slot in the range
> + * @nr_pages: number of page-array slots available from @pages
> + * @offset: byte offset in @pages[0]
> + * @len: byte count to describe
> + * @max_bvec_len: maximum byte count in one bvec
> + *
> + * If @bvecs is not NULL, it must have room for the count returned by a
> + * preceding count-only call.
> + *
> + * Return: The number of populated or required bvecs, or zero if the range
> + * cannot be described from the supplied page-array slots.
> + */
> +static inline unsigned int
> +svc_pages_to_bvecs(struct bio_vec *bvecs, struct page *const *pages,
> +		   unsigned int nr_pages, unsigned int offset,
> +		   unsigned int len, unsigned int max_bvec_len)
> +{
> +	unsigned int remaining = len;
> +	unsigned int nents = 0;
> +
> +	while (remaining && nr_pages) {
> +		struct bio_vec *bv = bvecs ? &bvecs[nents] : NULL;
> +		unsigned int advanced;
> +		unsigned int bytes;
> +
> +		bytes = svc_pages_to_bvec(bv, pages, nr_pages, offset,
> +					  remaining, max_bvec_len);
> +		if (!bytes)
> +			break;
> +		advanced = (offset + bytes) >> PAGE_SHIFT;
> +		offset = offset_in_page(offset + bytes);
> +		pages += advanced;
> +		nr_pages -= advanced;
> +		remaining -= bytes;
> +		nents++;
> +	}
> +	return remaining ? 0 : nents;
> +}
> +
>  struct svc_deferred_req {
>  	u32			prot;	/* protocol (UDP or TCP) */
>  	bool			secure;	/* RQ_SECURE of the original request */
> -- 
> 2.43.0

-- 
Chuck Lever (Come to NFS bake-a-thon! https://nfsv4bat.org)