[PATCH net-next v1 2/5] net: skbuff: refcount spliced page_pool frags via skb_pipe_buf_ops

[Mina Almasry]

Update skb_splice_bits() (spd_fill_page(), sock_spd_release(), and
skb_pipe_buf_ops) and skb_append_pagefrags() to acquire and release
references via netmem_inc_ref(), netmem_dec_ref(), and skb_frag_ref()
so page_pool RX fragments spliced into a pipe remain pinned in the pool
while in the pipe and pair with skb_append_pagefrags() and
skb_release_data() when spliced into a TX socket once netmem_inc_ref()
and netmem_dec_ref() handle page_pool netmems.

Caveats:
- Because pipe buffers created by skb_splice_bits() hold pp_ref_count
  until released (including across tee()), user pipes holding
  unconsumed spliced data keep those page_pool pages in-flight and
  DMA-mapped in the pool (bounded by pipe capacity and RLIMIT_NOFILE
  rather than sk_rcvbuf) until the pipe buffers are consumed or closed,
  with page_pool_destroy() deferring pool release via its periodic
  workqueue and page_pool_scrub() unmapping DMA.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
---
 net/core/skbuff.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 48f6764117243..32a9dcdf2cb22 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3114,9 +3114,27 @@ EXPORT_SYMBOL(skb_copy_bits);
  */
 static void sock_spd_release(struct splice_pipe_desc *spd, unsigned int i)
 {
-	put_page(spd->pages[i]);
+	netmem_dec_ref(page_to_netmem(spd->pages[i]));
 }
 
+static void skb_pipe_buf_release(struct pipe_inode_info *pipe,
+				 struct pipe_buffer *buf)
+{
+	netmem_dec_ref(page_to_netmem(buf->page));
+}
+
+static bool skb_pipe_buf_get(struct pipe_inode_info *pipe,
+			     struct pipe_buffer *buf)
+{
+	netmem_inc_ref(page_to_netmem(buf->page));
+	return true;
+}
+
+static const struct pipe_buf_operations skb_pipe_buf_ops = {
+	.release	= skb_pipe_buf_release,
+	.get		= skb_pipe_buf_get,
+};
+
 static struct page *linear_to_page(struct page *page, unsigned int *len,
 				   unsigned int *offset,
 				   struct sock *sk)
@@ -3165,7 +3183,7 @@ static bool spd_fill_page(struct splice_pipe_desc *spd, struct page *page,
 		spd->partial[spd->nr_pages - 1].len += *len;
 		return false;
 	}
-	get_page(page);
+	netmem_inc_ref(page_to_netmem(page));
 	spd->pages[spd->nr_pages] = page;
 	spd->partial[spd->nr_pages].len = *len;
 	spd->partial[spd->nr_pages].offset = offset;
@@ -3281,7 +3299,7 @@ int skb_splice_bits(struct sk_buff *skb, struct sock *sk, unsigned int offset,
 		.pages = pages,
 		.partial = partial,
 		.nr_pages_max = MAX_SKB_FRAGS,
-		.ops = &nosteal_pipe_buf_ops,
+		.ops = &skb_pipe_buf_ops,
 		.spd_release = sock_spd_release,
 	};
 	int ret = 0;
@@ -4692,8 +4710,8 @@ int skb_append_pagefrags(struct sk_buff *skb, struct page *page,
 		skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], size);
 	} else if (i < max_frags) {
 		skb_zcopy_downgrade_managed(skb);
-		get_page(page);
 		skb_fill_page_desc_noacc(skb, i, page, offset, size);
+		skb_frag_ref(skb, i);
 	} else {
 		return -EMSGSIZE;
 	}
-- 
2.56.0.385.gac671c819a-goog