[PATCH net-next v4 07/27] net: introduce managed frags infrastructure

Pavel Begunkov
2022-07-07T11:52:12Z

Some users like io_uring can do page pinning more efficiently, so we
want a way to delegate referencing to other subsystems. For that add
a new flag called SKBFL_MANAGED_FRAG_REFS. When set, skb doesn't hold
page references and upper layers are responsivle to managing page
lifetime.

It's allowed to convert skbs from managed to normal by calling
skb_zcopy_downgrade_managed(). The function will take all needed
page references and clear the flag. It's needed, for instance,
to avoid mixing managed modes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
include/linux/skbuff.h | 25 +++++++++++++++++++++++--
net/core/skbuff.c | 29 +++++++++++++++++++++++++++--
2 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a8a2dd4cfdfd..07004593d7ca 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -688,11 +688,16 @@ enum {
SKBFL_PURE_ZEROCOPY = BIT(2),

SKBFL_DONT_ORPHAN = BIT(3),
+
+ /* page references are managed by the ubuf_info, so it's safe to
+ * use frags only up until ubuf_info is released
+ */
+ SKBFL_MANAGED_FRAG_REFS = BIT(4),
};

#define SKBFL_ZEROCOPY_FRAG (SKBFL_ZEROCOPY_ENABLE | SKBFL_SHARED_FRAG)
#define SKBFL_ALL_ZEROCOPY (SKBFL_ZEROCOPY_FRAG | SKBFL_PURE_ZEROCOPY | \
- SKBFL_DONT_ORPHAN)
+ SKBFL_DONT_ORPHAN | SKBFL_MANAGED_FRAG_REFS)

/*
* The callback notifies userspace to release buffers when skb DMA is done in
@@ -1810,6 +1815,11 @@ static inline bool skb_zcopy_pure(const struct sk_buff *skb)
return skb_shinfo(skb)->flags & SKBFL_PURE_ZEROCOPY;
}

+static inline bool skb_zcopy_managed(const struct sk_buff *skb)
+{
+ return skb_shinfo(skb)->flags & SKBFL_MANAGED_FRAG_REFS;
+}
+
static inline bool skb_pure_zcopy_same(const struct sk_buff *skb1,
const struct sk_buff *skb2)
{
@@ -1884,6 +1894,14 @@ static inline void skb_zcopy_clear(struct sk_buff *skb, bool zerocopy_success)
}
}

+void __skb_zcopy_downgrade_managed(struct sk_buff *skb);
+
+static inline void skb_zcopy_downgrade_managed(struct sk_buff *skb)
+{
+ if (unlikely(skb_zcopy_managed(skb)))
+ __skb_zcopy_downgrade_managed(skb);
+}
+
static inline void skb_mark_not_on_list(struct sk_buff *skb)
{
skb->next = NULL;
@@ -3499,7 +3517,10 @@ static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle)
*/
static inline void skb_frag_unref(struct sk_buff *skb, int f)
{
- __skb_frag_unref(&skb_shinfo(skb)->frags[f], skb->pp_recycle);
+ struct skb_shared_info *shinfo = skb_shinfo(skb);
+
+ if (!skb_zcopy_managed(skb))
+ __skb_frag_unref(&shinfo->frags[f], skb->pp_recycle);
}

/**
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f5a3ebbc1f7e..cf4107d80bc4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -666,11 +666,18 @@ static void skb_release_data(struct sk_buff *skb)
&shinfo->dataref))
goto exit;

- skb_zcopy_clear(skb, true);
+ if (skb_zcopy(skb)) {
+ bool skip_unref = shinfo->flags & SKBFL_MANAGED_FRAG_REFS;
+
+ skb_zcopy_clear(skb, true);
+ if (skip_unref)
+ goto free_head;
+ }

for (i = 0; i < shinfo->nr_frags; i++)
__skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);

+free_head:
if (shinfo->frag_list)
kfree_skb_list(shinfo->frag_list);

@@ -895,7 +902,10 @@ EXPORT_SYMBOL(skb_dump);
*/
void skb_tx_error(struct sk_buff *skb)
{
- skb_zcopy_clear(skb, true);
+ if (skb) {
+ skb_zcopy_downgrade_managed(skb);
+ skb_zcopy_clear(skb, true);
+ }
}
EXPORT_SYMBOL(skb_tx_error);

@@ -1375,6 +1385,16 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
}
EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);

+void __skb_zcopy_downgrade_managed(struct sk_buff *skb)
+{
+ int i;
+
+ skb_shinfo(skb)->flags &= ~SKBFL_MANAGED_FRAG_REFS;
+ for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
+ skb_frag_ref(skb, i);
+}
+EXPORT_SYMBOL_GPL(__skb_zcopy_downgrade_managed);
+
static int skb_zerocopy_clone(struct sk_buff *nskb, struct sk_buff *orig,
gfp_t gfp_mask)
{
@@ -1692,6 +1712,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,

BUG_ON(skb_shared(skb));

+ skb_zcopy_downgrade_managed(skb);
+
size = SKB_DATA_ALIGN(size);

if (skb_pfmemalloc(skb))
@@ -3488,6 +3510,8 @@ void skb_split(struct sk_buff *skb, struct sk_buff *skb1, const u32 len)
int pos = skb_headlen(skb);
const int zc_flags = SKBFL_SHARED_FRAG | SKBFL_PURE_ZEROCOPY;

+ skb_zcopy_downgrade_managed(skb);
+
skb_shinfo(skb1)->flags |= skb_shinfo(skb)->flags & zc_flags;
skb_zerocopy_clone(skb1, skb, 0);
if (len < pos) /* Split line is inside header. */
@@ -3841,6 +3865,7 @@ int skb_append_pagefrags(struct sk_buff *skb, struct page *page,
if (skb_can_coalesce(skb, i, page, offset)) {
skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], size);
} else if (i < MAX_SKB_FRAGS) {
+ skb_zcopy_downgrade_managed(skb);
get_page(page);
skb_fill_page_desc(skb, i, page, offset, size);
} else {
--
2.36.1