[PATCH net] udp: fix memory schedule error

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Quoting from the commit 7c80b038d23e ("net: fix sk_wmem_schedule()
and sk_rmem_schedule() errors"):

"If sk->sk_forward_alloc is 150000, and we need to schedule 150001 bytes,
we want to allocate 1 byte more (rounded up to one page),
instead of 150001"

After applied this patch, we could avoid receive path scheduling
extra amount of memory.

Fixes: f970bd9e3a06 ("udp: implement memory accounting helpers")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/ipv4/udp.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 9592fe3e444a..a13f622cfa36 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1567,16 +1567,16 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 		goto uncharge_drop;
 
 	spin_lock(&list->lock);
-	if (size >= sk->sk_forward_alloc) {
-		amt = sk_mem_pages(size);
-		delta = amt << PAGE_SHIFT;
+	if (size > sk->sk_forward_alloc) {
+		delta = size - sk->sk_forward_alloc;
+		amt = sk_mem_pages(delta);
 		if (!__sk_mem_raise_allocated(sk, delta, amt, SK_MEM_RECV)) {
 			err = -ENOBUFS;
 			spin_unlock(&list->lock);
 			goto uncharge_drop;
 		}
 
-		sk->sk_forward_alloc += delta;
+		sk->sk_forward_alloc += amt << PAGE_SHIFT;
 	}
 
 	sk->sk_forward_alloc -= size;
-- 
2.37.3