netlink: do not enter direct reclaim from netlink_dump()

[ Upstream commit d35c99ff77ecb2eb239731b799386f3b3637a31e ]

Since linux-3.15, netlink_dump() can use up to 16384 bytes skb
allocations.

Due to struct skb_shared_info ~320 bytes overhead, we end up using
order-3 (on x86) page allocations, that might trigger direct reclaim and
add stress.

The intent was really to attempt a large allocation but immediately
fallback to a smaller one (order-1 on x86) in case of memory stress.

On recent kernels (linux-4.4), we can remove __GFP_DIRECT_RECLAIM to
meet the goal. Old kernels would need to remove __GFP_WAIT

While we are at it, since we do an order-3 allocation, allow to use
all the allocated bytes instead of 16384 to reduce syscalls during
large dumps.

iproute2 already uses 32KB recvmsg() buffer sizes.

Alexei provided an initial patch downsizing to SKB_WITH_OVERHEAD(16384)

Fixes: 9063e21fb026 ("netlink: autosize skb lengthes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Alexei Starovoitov <ast@kernel.org>
Cc: Greg Thelen <gthelen@google.com>
Reviewed-by: Greg Rose <grose@lightfleet.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 7a5fa0c98377643aadcb047184eb7a32c4463c5f..28fc283c1ec1fbf4b5049b8d52e087b48628aae6 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2557,7 +2557,7 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
        /* Record the max length of recvmsg() calls for future allocations */
        nlk->max_recvmsg_len = max(nlk->max_recvmsg_len, len);
        nlk->max_recvmsg_len = min_t(size_t, nlk->max_recvmsg_len,
-                                    16384);
+                                    SKB_WITH_OVERHEAD(32768));
 
        copied = data_skb->len;
        if (len < copied) {
@@ -2810,14 +2810,13 @@ static int netlink_dump(struct sock *sk)
        if (alloc_min_size < nlk->max_recvmsg_len) {
                alloc_size = nlk->max_recvmsg_len;
                skb = netlink_alloc_skb(sk, alloc_size, nlk->portid,
-                                       GFP_KERNEL |
-                                       __GFP_NOWARN |
-                                       __GFP_NORETRY);
+                                       (GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) |
+                                       __GFP_NOWARN | __GFP_NORETRY);
        }
        if (!skb) {
                alloc_size = alloc_min_size;
                skb = netlink_alloc_skb(sk, alloc_size, nlk->portid,
-                                       GFP_KERNEL);
+                                       (GFP_KERNEL & ~__GFP_DIRECT_RECLAIM));
        }
        if (!skb)
                goto errout_skb;