Re: [PATCH v7 1/6] net: introduce helper sendpage_ok() in include/linux/net.h

[Christoph Hellwig <hch@lst.de>]

I think we should go for something simple like this instead:

---
From 4867e158ee86ebd801b4c267e8f8a4a762a71343 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Tue, 18 Aug 2020 18:19:23 +0200
Subject: net: bypass ->sendpage for slab pages

Sending Slab or tail pages into ->sendpage will cause really strange
delayed oops.  Prevent it right in the networking code instead of
requiring drivers to work around the fact.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 net/socket.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/socket.c b/net/socket.c
index dbbe8ea7d395da..fbc82eb96d18ce 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -3638,7 +3638,12 @@ EXPORT_SYMBOL(kernel_getpeername);
 int kernel_sendpage(struct socket *sock, struct page *page, int offset,
 		    size_t size, int flags)
 {
-	if (sock->ops->sendpage)
+	/*
+	 * sendpage does manipulates the refcount of the passed in page, which
+	 * does not work for Slab pages, or for tails of non-__GFP_COMP
+	 * high order pages.
+	 */
+	if (sock->ops->sendpage && !PageSlab(page) && page_count(page) > 0)
 		return sock->ops->sendpage(sock, page, offset, size, flags);
 
 	return sock_no_sendpage(sock, page, offset, size, flags);
-- 
2.28.0