net: don't wait for order-3 page allocation

commit fb05e7a8 upstream.

We saw excessive direct memory compaction triggered by skb_page_frag_refill.
This causes performance issues and add latency. Commit 5640f768
introduces the order-3 allocation. According to the changelog, the order-3
allocation isn't a must-have but to improve performance. But direct memory
compaction has high overhead. The benefit of order-3 allocation can't
compensate the overhead of direct memory compaction.

This patch makes the order-3 page allocation atomic. If there is no memory
pressure and memory isn't fragmented, the alloction will still success, so we
don't sacrifice the order-3 benefit here. If the atomic allocation fails,
direct memory compaction will not be triggered, skb_page_frag_refill will
fallback to order-0 immediately, hence the direct memory compaction overhead is
avoided. In the allocation failure case, kswapd is waken up and doing
compaction, so chances are allocation could success next time.

alloc_skb_with_frags is the same.

The mellanox driver does similar thing, if this is accepted, we must fix
the driver too.

V3: fix the same issue in alloc_skb_with_frags as pointed out by Eric
V2: make the changelog clearer

Cc: Eric Dumazet <edumazet@google.com>
Cc: Chris Mason <clm@fb.com>
Cc: Debabrata Banerjee <dbavatar@gmail.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ luis: backported to 3.16: used davem's backport to 3.14 ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/core/skbuff.c

@@ -368,9 +368,11 @@ static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
 		for (order = NETDEV_FRAG_PAGE_MAX_ORDER; ;) {
 			gfp_t gfp = gfp_mask;
 
-			if (order)
+			if (order) {
 				gfp |= __GFP_COMP | __GFP_NOWARN |
 				       __GFP_NOMEMALLOC;
+				gfp &= ~__GFP_WAIT;
+			}
 			nc->frag.page = alloc_pages(gfp, order);
 			if (likely(nc->frag.page))
 				break;

net/core/sock.c

@@ -1914,8 +1914,10 @@ bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t prio)
 	do {
 		gfp_t gfp = prio;
 
-		if (order)
+		if (order) {
 			gfp |= __GFP_COMP | __GFP_NOWARN | __GFP_NORETRY;
+			gfp &= ~__GFP_WAIT;
+		}
 		pfrag->page = alloc_pages(gfp, order);
 		if (likely(pfrag->page)) {
 			pfrag->offset = 0;