[PATCH net-next v2 11/18] xdp: add generic xdp_buff_add_frag()

Alexander Lobakin
2024-10-15T14:54:59Z

The code piece which would attach a frag to &xdp_buff is almost
identical across the drivers supporting XDP multi-buffer on Rx.
Make it a generic elegant onelner.
Also, I see lots of drivers calculating frags_truesize as
`xdp->frame_sz * nr_frags`. I can't say this is fully correct, since
frags might be backed by chunks of different sizes, especially with
stuff like the header split. Even page_pool_alloc() can give you two
different truesizes on two subsequent requests to allocate the same
buffer size. Add a field to &skb_shared_info (unionized as there's no
free slot currently on x6_64) to track the "true" truesize. It can be
used later when updating an skb.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
include/linux/skbuff.h | 16 ++++++--
include/net/xdp.h | 90 +++++++++++++++++++++++++++++++++++++++++-
2 files changed, 101 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c867df5b1051..6ec78c1598fe 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -607,11 +607,19 @@ struct skb_shared_info {
* Warning : all fields before dataref are cleared in __alloc_skb()
*/
atomic_t dataref;
- unsigned int xdp_frags_size;

- /* Intermediate layers must ensure that destructor_arg
- * remains valid until skb destructor */
- void * destructor_arg;
+ union {
+ struct {
+ u32 xdp_frags_size;
+ u32 xdp_frags_truesize;
+ };
+
+ /*
+ * Intermediate layers must ensure that destructor_arg
+ * remains valid until skb destructor.
+ */
+ void *destructor_arg;
+ };

/* must be last field, see pskb_expand_head() */
skb_frag_t frags[MAX_SKB_FRAGS];
diff --git a/include/net/xdp.h b/include/net/xdp.h
index c4b408d22669..19d2b283b845 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -167,6 +167,88 @@ xdp_get_buff_len(const struct xdp_buff *xdp)
return len;
}

+/**
+ * __xdp_buff_add_frag - attach a frag to an &xdp_buff
+ * @xdp: XDP buffer to attach the frag to
+ * @page: page containing the frag
+ * @offset: page offset at which the frag starts
+ * @size: size of the frag
+ * @truesize: truesize (page / page frag size) of the frag
+ * @try_coalesce: whether to try coalescing the frags
+ *
+ * Attach a frag to an XDP buffer. If it currently has no frags attached,
+ * initialize the related fields, otherwise check that the frag number
+ * didn't reach the limit of ``MAX_SKB_FRAGS``. If possible, try coalescing
+ * the frag with the previous one.
+ * The function doesn't check/update the pfmemalloc bit. Please use the
+ * non-underscored wrapper in drivers.
+ *
+ * Return: true on success, false if there's no space for the frag in
+ * the shared info struct.
+ */
+static inline bool __xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
+ u32 offset, u32 size, u32 truesize,
+ bool try_coalesce)
+{
+ struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
+ skb_frag_t *prev;
+ u32 nr_frags;
+
+ if (!xdp_buff_has_frags(xdp)) {
+ xdp_buff_set_frags_flag(xdp);
+
+ nr_frags = 0;
+ sinfo->xdp_frags_size = 0;
+ sinfo->xdp_frags_truesize = 0;
+
+ goto fill;
+ }
+
+ nr_frags = sinfo->nr_frags;
+ if (unlikely(nr_frags == MAX_SKB_FRAGS))
+ return false;
+
+ prev = &sinfo->frags[nr_frags - 1];
+ if (try_coalesce && page == skb_frag_page(prev) &&
+ offset == skb_frag_off(prev) + skb_frag_size(prev))
+ skb_frag_size_add(prev, size);
+ else
+fill:
+ __skb_fill_page_desc_noacc(sinfo, nr_frags++, page,
+ offset, size);
+
+ sinfo->nr_frags = nr_frags;
+ sinfo->xdp_frags_size += size;
+ sinfo->xdp_frags_truesize += truesize;
+
+ return true;
+}
+
+/**
+ * xdp_buff_add_frag - attach a frag to an &xdp_buff
+ * @xdp: XDP buffer to attach the frag to
+ * @page: page containing the frag
+ * @offset: page offset at which the frag starts
+ * @size: size of the frag
+ * @truesize: truesize (page / page frag size) of the frag
+ *
+ * Version of __xdp_buff_add_frag() which takes care of the pfmemalloc bit.
+ *
+ * Return: true on success, false if there's no space for the frag in
+ * the shared info struct.
+ */
+static inline bool xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
+ u32 offset, u32 size, u32 truesize)
+{
+ if (!__xdp_buff_add_frag(xdp, page, offset, size, truesize, true))
+ return false;
+
+ if (unlikely(page_is_pfmemalloc(page)))
+ xdp_buff_set_frag_pfmemalloc(xdp);
+
+ return true;
+}
+
struct xdp_frame {
void *data;
u32 len;
@@ -230,7 +312,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
unsigned int size, unsigned int truesize,
bool pfmemalloc)
{
- skb_shinfo(skb)->nr_frags = nr_frags;
+ struct skb_shared_info *sinfo = skb_shinfo(skb);
+
+ sinfo->nr_frags = nr_frags;
+ /* ``destructor_arg`` is unionized with ``xdp_frags_{,true}size``,
+ * reset it after that these fields aren't used anymore.
+ */
+ sinfo->destructor_arg = NULL;

skb->len += size;
skb->data_len += size;
--
2.46.2