A messenger fix, marked for stable.

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Merge tag 'ceph-for-4.20-rc4' of https://github.com/ceph/ceph-client

Pullk ceph fix from Ilya Dryomov:
 "A messenger fix, marked for stable"

* tag 'ceph-for-4.20-rc4' of https://github.com/ceph/ceph-client:
  libceph: fall back to sendmsg for slab pages

net/ceph/messenger.c

@@ -580,9 +580,15 @@ static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
 	struct bio_vec bvec;
 	int ret;
 
-	/* sendpage cannot properly handle pages with page_count == 0,
-	 * we need to fallback to sendmsg if that's the case */
-	if (page_count(page) >= 1)
+	/*
+	 * sendpage cannot properly handle pages with page_count == 0,
+	 * we need to fall back to sendmsg if that's the case.
+	 *
+	 * Same goes for slab pages: skb_can_coalesce() allows
+	 * coalescing neighboring slab objects into a single frag which
+	 * triggers one of hardened usercopy checks.
+	 */
+	if (page_count(page) >= 1 && !PageSlab(page))
 		return __ceph_tcp_sendpage(sock, page, offset, size, more);
 
 	bvec.bv_page = page;