[PATCH net-next 08/12] net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)

[David Howells <dhowells@xxxxxxxxxx>]

If sendmsg() is passed MSG_SPLICE_PAGES and is given a buffer that contains
some data that's resident in the slab, copy/coalesce it rather than
returning EIO.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: David Ahern <dsahern@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: netdev@xxxxxxxxxxxxxxx
---
 include/linux/skbuff.h |  3 +++
 net/core/skbuff.c      | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index e11a765fe7fa..11d98990f5f1 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -5084,6 +5084,9 @@ static inline void skb_mark_for_recycle(struct sk_buff *skb)
 #endif
 }
 
+ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
+			     ssize_t maxsize, gfp_t gfp);
+
 ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
 			     ssize_t maxsize, gfp_t gfp);
 
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c2840b0dcad9..a16499b9942b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -6927,17 +6927,44 @@ ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
 			break;
 		}
 
+		if (space == 0 &&
+		    !skb_can_coalesce(skb, skb_shinfo(skb)->nr_frags,
+				      pages[0], off)) {
+			iov_iter_revert(iter, len);
+			break;
+		}
+
 		i = 0;
 		do {
 			struct page *page = pages[i++];
 			size_t part = min_t(size_t, PAGE_SIZE - off, len);
-
-			ret = -EIO;
-			if (WARN_ON_ONCE(!sendpage_ok(page)))
+			bool put = false;
+
+			if (PageSlab(page)) {
+				const void *p;
+				void *q;
+
+				p = kmap_local_page(page);
+				q = page_frag_memdup(NULL, p + off, part, gfp,
+						     ULONG_MAX);
+				kunmap_local(p);
+				if (!q) {
+					iov_iter_revert(iter, len);
+					ret = -ENOMEM;
+					goto out;
+				}
+				page = virt_to_page(q);
+				off = offset_in_page(q);
+				put = true;
+			} else if (WARN_ON_ONCE(!sendpage_ok(page))) {
+				ret = -EIO;
 				goto out;
+			}
 
 			ret = skb_append_pagefrags(skb, page, off, part,
 						   frag_limit);
+			if (put)
+				put_page(page);
 			if (ret < 0) {
 				iov_iter_revert(iter, len);
 				goto out;