[PATCH net-next v7 2/2] net: add netmem to skb_frag_t

From: Mina Almasry
Date: Thu Feb 01 2024 - 16:35:40 EST

Use struct netmem* instead of page in skb_frag_t. Currently struct
netmem* is always a struct page underneath, but the abstraction
allows efforts to add support for skb frags not backed by pages.

There is unfortunately 1 instance where the skb_frag_t is assumed to be
a exactly a bio_vec in kcm. For this case, WARN_ON_ONCE and return error
before doing a cast.

Add skb[_frag]_fill_netmem_*() and skb_add_rx_frag_netmem() helpers so
that the API can be used to create netmem skbs.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>

---

v7:
- Move skb_rx_add_frag to header file (Paolo).
- Move combine kcm skb_frag_page() check with nr_frags check.

v6:
- Add static_asserts to validate skb_frag_t to bio_vec cast in
kcm_write_msg (Yunsheng)

v4:
- Handle error in kcm_write_msgs() instead of only warning (Willem)

v3:
- Renamed the fields in skb_frag_t.

v2:
- Add skb frag filling helpers.

---
include/linux/skbuff.h | 109 ++++++++++++++++++++++++++++++-----------
net/core/skbuff.c | 38 ++++++++------
net/kcm/kcmsock.c | 7 +--
3 files changed, 107 insertions(+), 47 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2dde34c29203..7d1c03e59017 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -37,6 +37,7 @@
#endif
#include <net/net_debug.h>
#include <net/dropreason-core.h>
+#include <net/netmem.h>

/**
* DOC: skb checksums
@@ -359,7 +360,11 @@ extern int sysctl_max_skb_frags;
*/
#define GSO_BY_FRAGS 0xFFFF

-typedef struct bio_vec skb_frag_t;
+typedef struct skb_frag {
+ netmem_ref netmem;
+ unsigned int len;
+ unsigned int offset;
+} skb_frag_t;

/**
* skb_frag_size() - Returns the size of a skb fragment
@@ -367,7 +372,7 @@ typedef struct bio_vec skb_frag_t;
*/
static inline unsigned int skb_frag_size(const skb_frag_t *frag)
{
- return frag->bv_len;
+ return frag->len;
}

/**
@@ -377,7 +382,7 @@ static inline unsigned int skb_frag_size(const skb_frag_t *frag)
*/
static inline void skb_frag_size_set(skb_frag_t *frag, unsigned int size)
{
- frag->bv_len = size;
+ frag->len = size;
}

/**
@@ -387,7 +392,7 @@ static inline void skb_frag_size_set(skb_frag_t *frag, unsigned int size)
*/
static inline void skb_frag_size_add(skb_frag_t *frag, int delta)
{
- frag->bv_len += delta;
+ frag->len += delta;
}

/**
@@ -397,7 +402,7 @@ static inline void skb_frag_size_add(skb_frag_t *frag, int delta)
*/
static inline void skb_frag_size_sub(skb_frag_t *frag, int delta)
{
- frag->bv_len -= delta;
+ frag->len -= delta;
}

/**
@@ -417,7 +422,7 @@ static inline bool skb_frag_must_loop(struct page *p)
* skb_frag_foreach_page - loop over pages in a fragment
*
* @f: skb frag to operate on
- * @f_off: offset from start of f->bv_page
+ * @f_off: offset from start of f->netmem
* @f_len: length from f_off to loop over
* @p: (temp var) current page
* @p_off: (temp var) offset from start of current page,
@@ -2429,22 +2434,37 @@ static inline unsigned int skb_pagelen(const struct sk_buff *skb)
return skb_headlen(skb) + __skb_pagelen(skb);
}

+static inline void skb_frag_fill_netmem_desc(skb_frag_t *frag,
+ netmem_ref netmem, int off,
+ int size)
+{
+ frag->netmem = netmem;
+ frag->offset = off;
+ skb_frag_size_set(frag, size);
+}
+
static inline void skb_frag_fill_page_desc(skb_frag_t *frag,
struct page *page,
int off, int size)
{
- frag->bv_page = page;
- frag->bv_offset = off;
- skb_frag_size_set(frag, size);
+ skb_frag_fill_netmem_desc(frag, page_to_netmem(page), off, size);
+}
+
+static inline void __skb_fill_netmem_desc_noacc(struct skb_shared_info *shinfo,
+ int i, netmem_ref netmem,
+ int off, int size)
+{
+ skb_frag_t *frag = &shinfo->frags[i];
+
+ skb_frag_fill_netmem_desc(frag, netmem, off, size);
}

static inline void __skb_fill_page_desc_noacc(struct skb_shared_info *shinfo,
int i, struct page *page,
int off, int size)
{
- skb_frag_t *frag = &shinfo->frags[i];
-
- skb_frag_fill_page_desc(frag, page, off, size);
+ __skb_fill_netmem_desc_noacc(shinfo, i, page_to_netmem(page), off,
+ size);
}

/**
@@ -2460,10 +2480,10 @@ static inline void skb_len_add(struct sk_buff *skb, int delta)
}

/**
- * __skb_fill_page_desc - initialise a paged fragment in an skb
+ * __skb_fill_netmem_desc - initialise a fragment in an skb
* @skb: buffer containing fragment to be initialised
- * @i: paged fragment index to initialise
- * @page: the page to use for this fragment
+ * @i: fragment index to initialise
+ * @netmem: the netmem to use for this fragment
* @off: the offset to the data with @page
* @size: the length of the data
*
@@ -2472,10 +2492,12 @@ static inline void skb_len_add(struct sk_buff *skb, int delta)
*
* Does not take any additional reference on the fragment.
*/
-static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
- struct page *page, int off, int size)
+static inline void __skb_fill_netmem_desc(struct sk_buff *skb, int i,
+ netmem_ref netmem, int off, int size)
{
- __skb_fill_page_desc_noacc(skb_shinfo(skb), i, page, off, size);
+ struct page *page = netmem_to_page(netmem);
+
+ __skb_fill_netmem_desc_noacc(skb_shinfo(skb), i, netmem, off, size);

/* Propagate page pfmemalloc to the skb if we can. The problem is
* that not all callers have unique ownership of the page but rely
@@ -2483,7 +2505,20 @@ static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
*/
page = compound_head(page);
if (page_is_pfmemalloc(page))
- skb->pfmemalloc = true;
+ skb->pfmemalloc = true;
+}
+
+static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
+ struct page *page, int off, int size)
+{
+ __skb_fill_netmem_desc(skb, i, page_to_netmem(page), off, size);
+}
+
+static inline void skb_fill_netmem_desc(struct sk_buff *skb, int i,
+ netmem_ref netmem, int off, int size)
+{
+ __skb_fill_netmem_desc(skb, i, netmem, off, size);
+ skb_shinfo(skb)->nr_frags = i + 1;
}

/**
@@ -2503,8 +2538,7 @@ static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
static inline void skb_fill_page_desc(struct sk_buff *skb, int i,
struct page *page, int off, int size)
{
- __skb_fill_page_desc(skb, i, page, off, size);
- skb_shinfo(skb)->nr_frags = i + 1;
+ skb_fill_netmem_desc(skb, i, page_to_netmem(page), off, size);
}

/**
@@ -2528,8 +2562,25 @@ static inline void skb_fill_page_desc_noacc(struct sk_buff *skb, int i,
shinfo->nr_frags = i + 1;
}

-void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page, int off,
- int size, unsigned int truesize);
+static inline void skb_add_rx_frag_netmem(struct sk_buff *skb, int i,
+ netmem_ref netmem, int off, int size,
+ unsigned int truesize)
+{
+ DEBUG_NET_WARN_ON_ONCE(size > truesize);
+
+ skb_fill_netmem_desc(skb, i, netmem, off, size);
+ skb->len += size;
+ skb->data_len += size;
+ skb->truesize += truesize;
+}
+
+static inline void skb_add_rx_frag(struct sk_buff *skb, int i,
+ struct page *page, int off, int size,
+ unsigned int truesize)
+{
+ skb_add_rx_frag_netmem(skb, i, page_to_netmem(page), off, size,
+ truesize);
+}

void skb_coalesce_rx_frag(struct sk_buff *skb, int i, int size,
unsigned int truesize);
@@ -3378,7 +3429,7 @@ static inline void skb_propagate_pfmemalloc(const struct page *page,
*/
static inline unsigned int skb_frag_off(const skb_frag_t *frag)
{
- return frag->bv_offset;
+ return frag->offset;
}

/**
@@ -3388,7 +3439,7 @@ static inline unsigned int skb_frag_off(const skb_frag_t *frag)
*/
static inline void skb_frag_off_add(skb_frag_t *frag, int delta)
{
- frag->bv_offset += delta;
+ frag->offset += delta;
}

/**
@@ -3398,7 +3449,7 @@ static inline void skb_frag_off_add(skb_frag_t *frag, int delta)
*/
static inline void skb_frag_off_set(skb_frag_t *frag, unsigned int offset)
{
- frag->bv_offset = offset;
+ frag->offset = offset;
}

/**
@@ -3409,7 +3460,7 @@ static inline void skb_frag_off_set(skb_frag_t *frag, unsigned int offset)
static inline void skb_frag_off_copy(skb_frag_t *fragto,
const skb_frag_t *fragfrom)
{
- fragto->bv_offset = fragfrom->bv_offset;
+ fragto->offset = fragfrom->offset;
}

/**
@@ -3420,7 +3471,7 @@ static inline void skb_frag_off_copy(skb_frag_t *fragto,
*/
static inline struct page *skb_frag_page(const skb_frag_t *frag)
{
- return frag->bv_page;
+ return netmem_to_page(frag->netmem);
}

/**
@@ -3524,7 +3575,7 @@ static inline void *skb_frag_address_safe(const skb_frag_t *frag)
static inline void skb_frag_page_copy(skb_frag_t *fragto,
const skb_frag_t *fragfrom)
{
- fragto->bv_page = fragfrom->bv_page;
+ fragto->netmem = fragfrom->netmem;
}

bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t prio);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index edbbef563d4d..a754978cbed8 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -115,6 +115,24 @@ static struct kmem_cache *skb_small_head_cache __ro_after_init;
int sysctl_max_skb_frags __read_mostly = MAX_SKB_FRAGS;
EXPORT_SYMBOL(sysctl_max_skb_frags);

+/* kcm_write_msgs() relies on casting paged frags to bio_vec to use
+ * iov_iter_bvec(). These static asserts ensure the cast is valid is long as the
+ * netmem is a page.
+ */
+static_assert(offsetof(struct bio_vec, bv_page) ==
+ offsetof(skb_frag_t, netmem));
+static_assert(sizeof_field(struct bio_vec, bv_page) ==
+ sizeof_field(skb_frag_t, netmem));
+
+static_assert(offsetof(struct bio_vec, bv_len) == offsetof(skb_frag_t, len));
+static_assert(sizeof_field(struct bio_vec, bv_len) ==
+ sizeof_field(skb_frag_t, len));
+
+static_assert(offsetof(struct bio_vec, bv_offset) ==
+ offsetof(skb_frag_t, offset));
+static_assert(sizeof_field(struct bio_vec, bv_offset) ==
+ sizeof_field(skb_frag_t, offset));
+
#undef FN
#define FN(reason) [SKB_DROP_REASON_##reason] = #reason,
static const char * const drop_reasons[] = {
@@ -845,18 +863,6 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
}
EXPORT_SYMBOL(__napi_alloc_skb);

-void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page, int off,
- int size, unsigned int truesize)
-{
- DEBUG_NET_WARN_ON_ONCE(size > truesize);
-
- skb_fill_page_desc(skb, i, page, off, size);
- skb->len += size;
- skb->data_len += size;
- skb->truesize += truesize;
-}
-EXPORT_SYMBOL(skb_add_rx_frag);
-
void skb_coalesce_rx_frag(struct sk_buff *skb, int i, int size,
unsigned int truesize)
{
@@ -1906,10 +1912,11 @@ int skb_copy_ubufs(struct sk_buff *skb, gfp_t gfp_mask)

/* skb frags point to kernel buffers */
for (i = 0; i < new_frags - 1; i++) {
- __skb_fill_page_desc(skb, i, head, 0, psize);
+ __skb_fill_netmem_desc(skb, i, page_to_netmem(head), 0, psize);
head = (struct page *)page_private(head);
}
- __skb_fill_page_desc(skb, new_frags - 1, head, 0, d_off);
+ __skb_fill_netmem_desc(skb, new_frags - 1, page_to_netmem(head), 0,
+ d_off);
skb_shinfo(skb)->nr_frags = new_frags;

release:
@@ -3647,7 +3654,8 @@ skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
if (plen) {
page = virt_to_head_page(from->head);
offset = from->data - (unsigned char *)page_address(page);
- __skb_fill_page_desc(to, 0, page, offset, plen);
+ __skb_fill_netmem_desc(to, 0, page_to_netmem(page),
+ offset, plen);
get_page(page);
j = 1;
len -= plen;
diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 1184d40167b8..73c200c5c8e4 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -627,7 +627,8 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
skb = txm->frag_skb;
}

- if (WARN_ON(!skb_shinfo(skb)->nr_frags)) {
+ if (WARN_ON(!skb_shinfo(skb)->nr_frags) ||
+ WARN_ON_ONCE(!skb_frag_page(&skb_shinfo(skb)->frags[0]))) {
ret = -EINVAL;
goto out;
}
@@ -637,8 +638,8 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
msize += skb_frag_size(&skb_shinfo(skb)->frags[i]);

iov_iter_bvec(&msg.msg_iter, ITER_SOURCE,
- skb_shinfo(skb)->frags, skb_shinfo(skb)->nr_frags,
- msize);
+ (const struct bio_vec *)skb_shinfo(skb)->frags,
+ skb_shinfo(skb)->nr_frags, msize);
iov_iter_advance(&msg.msg_iter, txm->frag_offset);

do {
--
2.43.0.594.gd9cf4e227d-goog