af_unix: improve STREAM behavior with fragmented memory

unix_stream_sendmsg() currently uses order-2 allocations,
and we had numerous reports this can fail.

The __GFP_REPEAT flag present in sock_alloc_send_pskb() is
not helping.

This patch extends the work done in commit eb6a24816b
("af_unix: reduce high order page allocations) for
datagram sockets.

This opens the possibility of zero copy IO (splice() and
friends)

The trick is to not use skb_pull() anymore in recvmsg() path,
and instead add a @consumed field in UNIXCB() to track amount
of already read payload in the skb.

There is a performance regression for large sends
because of extra page allocations that will be addressed
in a follow-up patch, allowing sock_alloc_send_pskb()
to attempt high order page allocations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/af_unix.h

@@ -35,6 +35,7 @@ struct unix_skb_parms {
 #ifdef CONFIG_SECURITY_NETWORK
 	u32			secid;		/* Security ID		*/
 #endif
+	u32			consumed;
 };
 
 #define UNIXCB(skb) 	(*(struct unix_skb_parms *)&((skb)->cb))