Re: [PATCH 2/3] sunrpc: simplify xdr_partial_copy_from_skb

Jeff Layton
2025-05-15T15:45:10Z

On Thu, 2025-05-15 at 13:48 +0200, Christoph Hellwig wrote:
> csum_partial_copy_to_xdr can handle a checksumming and non-checksumming
> case and implements this using a callback, which leads to a lot of
> boilerplate code and indirect calls in the fast path.
>
> Switch to storing a need_checksum flag in struct xdr_skb_reader instead
> to remove the indirect call and simplify the code.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> net/sunrpc/socklib.c | 163 ++++++++++++++++---------------------------
> 1 file changed, 59 insertions(+), 104 deletions(-)
>
> diff --git a/net/sunrpc/socklib.c b/net/sunrpc/socklib.c
> index 1b2b84feeec6..75e31e3a3ced 100644
> --- a/net/sunrpc/socklib.c
> +++ b/net/sunrpc/socklib.c
> @@ -27,97 +27,60 @@
> struct xdr_skb_reader {
> struct sk_buff *skb;
> unsigned int offset;
> + bool need_checksum;
> size_t count;
> __wsum csum;
> };
>
> -typedef size_t (*xdr_skb_read_actor)(struct xdr_skb_reader *desc, void *to,
> - size_t len);
> -
> /**
> * xdr_skb_read_bits - copy some data bits from skb to internal buffer
> * @desc: sk_buff copy helper
> * @to: copy destination
> * @len: number of bytes to copy
> *
> - * Possibly called several times to iterate over an sk_buff and copy
> - * data out of it.
> + * Possibly called several times to iterate over an sk_buff and copy data out of
> + * it.
> */
> static size_t
> xdr_skb_read_bits(struct xdr_skb_reader *desc, void *to, size_t len)
> {
> - if (len > desc->count)
> - len = desc->count;
> - if (unlikely(skb_copy_bits(desc->skb, desc->offset, to, len)))
> - return 0;
> - desc->count -= len;
> - desc->offset += len;
> - return len;
> -}
> + len = min(len, desc->count);
> +
> + if (desc->need_checksum) {
> + __wsum csum;
> +
> + csum = skb_copy_and_csum_bits(desc->skb, desc->offset, to, len);
> + desc->csum = csum_block_add(desc->csum, csum, desc->offset);
> + } else {
> + if (unlikely(skb_copy_bits(desc->skb, desc->offset, to, len)))
> + return 0;
> + }
>
> -/**
> - * xdr_skb_read_and_csum_bits - copy and checksum from skb to buffer
> - * @desc: sk_buff copy helper
> - * @to: copy destination
> - * @len: number of bytes to copy
> - *
> - * Same as skb_read_bits, but calculate a checksum at the same time.
> - */
> -static size_t xdr_skb_read_and_csum_bits(struct xdr_skb_reader *desc, void *to, size_t len)
> -{
> - unsigned int pos;
> - __wsum csum2;
> -
> - if (len > desc->count)
> - len = desc->count;
> - pos = desc->offset;
> - csum2 = skb_copy_and_csum_bits(desc->skb, pos, to, len);
> - desc->csum = csum_block_add(desc->csum, csum2, pos);
> desc->count -= len;
> desc->offset += len;
> return len;
> }
>
> -/**
> - * xdr_partial_copy_from_skb - copy data out of an skb
> - * @xdr: target XDR buffer
> - * @base: starting offset
> - * @desc: sk_buff copy helper
> - * @copy_actor: virtual method for copying data
> - *
> - */
> static ssize_t
> -xdr_partial_copy_from_skb(struct xdr_buf *xdr, unsigned int base, struct xdr_skb_reader *desc, xdr_skb_read_actor copy_actor)
> +xdr_partial_copy_from_skb(struct xdr_buf *xdr, struct xdr_skb_reader *desc)
> {
> - struct page **ppage = xdr->pages;
> - unsigned int len, pglen = xdr->page_len;
> - ssize_t copied = 0;
> - size_t ret;
> -
> - len = xdr->head[0].iov_len;
> - if (base < len) {
> - len -= base;
> - ret = copy_actor(desc, (char *)xdr->head[0].iov_base + base, len);
> - copied += ret;
> - if (ret != len || !desc->count)
> - goto out;
> - base = 0;
> - } else
> - base -= len;
> -
> - if (unlikely(pglen == 0))
> - goto copy_tail;
> - if (unlikely(base >= pglen)) {
> - base -= pglen;
> - goto copy_tail;
> - }
> - if (base || xdr->page_base) {
> - pglen -= base;
> - base += xdr->page_base;
> - ppage += base >> PAGE_SHIFT;
> - base &= ~PAGE_MASK;
> - }
> - do {
> + struct page **ppage = xdr->pages + (xdr->page_base >> PAGE_SHIFT);
> + unsigned int poff = xdr->page_base & ~PAGE_MASK;
> + unsigned int pglen = xdr->page_len;
> + ssize_t copied = 0;
> + size_t ret;
> +
> + if (xdr->head[0].iov_len == 0)
> + return 0;
> +
> + ret = xdr_skb_read_bits(desc, xdr->head[0].iov_base,
> + xdr->head[0].iov_len);
> + if (ret != xdr->head[0].iov_len || !desc->count)
> + return ret;
> + copied += ret;
> +
> + while (pglen) {
> + unsigned int len = min(PAGE_SIZE - poff, pglen);
> char *kaddr;
>
> /* ACL likes to be lazy in allocating pages - ACLs
> @@ -126,36 +89,29 @@ xdr_partial_copy_from_skb(struct xdr_buf *xdr, unsigned int base, struct xdr_skb
> *ppage = alloc_page(GFP_NOWAIT | __GFP_NOWARN);
> if (unlikely(*ppage == NULL)) {
> if (copied == 0)
> - copied = -ENOMEM;
> - goto out;
> + return -ENOMEM;
> + return copied;
> }
> }
>
> - len = PAGE_SIZE;
> kaddr = kmap_atomic(*ppage);
> - if (base) {
> - len -= base;
> - if (pglen < len)
> - len = pglen;
> - ret = copy_actor(desc, kaddr + base, len);
> - base = 0;
> - } else {
> - if (pglen < len)
> - len = pglen;
> - ret = copy_actor(desc, kaddr, len);
> - }
> + ret = xdr_skb_read_bits(desc, kaddr + poff, len);
> flush_dcache_page(*ppage);
> kunmap_atomic(kaddr);
> +
> copied += ret;
> if (ret != len || !desc->count)
> - goto out;
> + return copied;
> ppage++;
> - } while ((pglen -= len) != 0);
> -copy_tail:
> - len = xdr->tail[0].iov_len;
> - if (base < len)
> - copied += copy_actor(desc, (char *)xdr->tail[0].iov_base + base, len - base);
> -out:
> + pglen -= len;
> + poff = 0;
> + }
> +
> + if (xdr->tail[0].iov_len) {
> + copied += xdr_skb_read_bits(desc, xdr->tail[0].iov_base,
> + xdr->tail[0].iov_len);
> + }
> +
> return copied;
> }
>
> @@ -169,17 +125,22 @@ xdr_partial_copy_from_skb(struct xdr_buf *xdr, unsigned int base, struct xdr_skb
> */
> int csum_partial_copy_to_xdr(struct xdr_buf *xdr, struct sk_buff *skb)
> {
> - struct xdr_skb_reader desc;
> -
> - desc.skb = skb;
> - desc.offset = 0;
> - desc.count = skb->len - desc.offset;
> + struct xdr_skb_reader desc = {
> + .skb = skb,
> + .count = skb->len - desc.offset,
> + };
>
> - if (skb_csum_unnecessary(skb))
> - goto no_checksum;
> + if (skb_csum_unnecessary(skb)) {
> + if (xdr_partial_copy_from_skb(xdr, &desc) < 0)
> + return -1;
> + if (desc.count)
> + return -1;
> + return 0;
> + }
>
> + desc.need_checksum = true;
> desc.csum = csum_partial(skb->data, desc.offset, skb->csum);
> - if (xdr_partial_copy_from_skb(xdr, 0, &desc, xdr_skb_read_and_csum_bits) < 0)
> + if (xdr_partial_copy_from_skb(xdr, &desc) < 0)
> return -1;
> if (desc.offset != skb->len) {
> __wsum csum2;
> @@ -194,12 +155,6 @@ int csum_partial_copy_to_xdr(struct xdr_buf *xdr, struct sk_buff *skb)
> !skb->csum_complete_sw)
> netdev_rx_csum_fault(skb->dev, skb);
> return 0;
> -no_checksum:
> - if (xdr_partial_copy_from_skb(xdr, 0, &desc, xdr_skb_read_bits) < 0)
> - return -1;
> - if (desc.count)
> - return -1;
> - return 0;
> }
> EXPORT_SYMBOL_GPL(csum_partial_copy_to_xdr);
>

Reviewed-by: Jeff Layton <jlayton@kernel.org>