netvm: set PF_MEMALLOC as appropriate during SKB processing

In order to make sure pfmemalloc packets receive all memory needed to
proceed, ensure processing of pfmemalloc SKBs happens under PF_MEMALLOC.
This is limited to a subset of protocols that are expected to be used for
writing to swap.  Taps are not allowed to use PF_MEMALLOC as these are
expected to communicate with userspace processes which could be paged out.

[a.p.zijlstra@chello.nl: Ideas taken from various patches]
[jslaby@suse.cz: Lock imbalance fix]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: Eric B Munson <emunson@mgebm.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 63 insertions, 6 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 0ebaea16632f..ce132443d5d1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3155,6 +3155,23 @@ void netdev_rx_handler_unregister(struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(netdev_rx_handler_unregister);
 
+/*
+ * Limit the use of PFMEMALLOC reserves to those protocols that implement
+ * the special handling of PFMEMALLOC skbs.
+ */
+static bool skb_pfmemalloc_protocol(struct sk_buff *skb)
+{
+        switch (skb->protocol) {
+        case __constant_htons(ETH_P_ARP):
+        case __constant_htons(ETH_P_IP):
+        case __constant_htons(ETH_P_IPV6):
+        case __constant_htons(ETH_P_8021Q):
+                return true;
+        default:
+                return false;
+        }
+}
+
 static int __netif_receive_skb(struct sk_buff *skb)
 {
         struct packet_type *ptype, *pt_prev;
@@ -3164,14 +3181,27 @@ static int __netif_receive_skb(struct sk_buff *skb)
         bool deliver_exact = false;
         int ret = NET_RX_DROP;
         __be16 type;
+        unsigned long pflags = current->flags;
 
         net_timestamp_check(!netdev_tstamp_prequeue, skb);
 
         trace_netif_receive_skb(skb);
 
+        /*
+         * PFMEMALLOC skbs are special, they should
+         * - be delivered to SOCK_MEMALLOC sockets only
+         * - stay away from userspace
+         * - have bounded memory usage
+         *
+         * Use PF_MEMALLOC as this saves us from propagating the allocation
+         * context down to all allocation sites.
+         */
+        if (sk_memalloc_socks() && skb_pfmemalloc(skb))
+                current->flags |= PF_MEMALLOC;
+
         /* if we've gotten here through NAPI, check netpoll */
         if (netpoll_receive_skb(skb))
-                return NET_RX_DROP;
+                goto out;
 
         orig_dev = skb->dev;
 
@@ -3191,7 +3221,7 @@ another_round:
         if (skb->protocol == cpu_to_be16(ETH_P_8021Q)) {
                 skb = vlan_untag(skb);
                 if (unlikely(!skb))
-                        goto out;
+                        goto unlock;
         }
 
 #ifdef CONFIG_NET_CLS_ACT
@@ -3201,6 +3231,9 @@ another_round:
         }
 #endif
 
+        if (sk_memalloc_socks() && skb_pfmemalloc(skb))
+                goto skip_taps;
+
         list_for_each_entry_rcu(ptype, &ptype_all, list) {
                 if (!ptype->dev || ptype->dev == skb->dev) {
                         if (pt_prev)
@@ -3209,13 +3242,18 @@ another_round:
                 }
         }
 
+skip_taps:
 #ifdef CONFIG_NET_CLS_ACT
         skb = handle_ing(skb, &pt_prev, &ret, orig_dev);
         if (!skb)
-                goto out;
+                goto unlock;
 ncls:
 #endif
 
+        if (sk_memalloc_socks() && skb_pfmemalloc(skb)
+                                && !skb_pfmemalloc_protocol(skb))
+                goto drop;
+
         rx_handler = rcu_dereference(skb->dev->rx_handler);
         if (vlan_tx_tag_present(skb)) {
                 if (pt_prev) {
@@ -3225,7 +3263,7 @@ ncls:
                 if (vlan_do_receive(&skb, !rx_handler))
                         goto another_round;
                 else if (unlikely(!skb))
-                        goto out;
+                        goto unlock;
         }
 
         if (rx_handler) {
@@ -3235,7 +3273,7 @@ ncls:
                 }
                 switch (rx_handler(&skb)) {
                 case RX_HANDLER_CONSUMED:
-                        goto out;
+                        goto unlock;
                 case RX_HANDLER_ANOTHER:
                         goto another_round;
                 case RX_HANDLER_EXACT:
@@ -3268,6 +3306,7 @@ ncls:
                 else
                         ret = pt_prev->func(skb, skb->dev, pt_prev, orig_dev);
         } else {
+drop:
                 atomic_long_inc(&skb->dev->rx_dropped);
                 kfree_skb(skb);
                 /* Jamal, now you will not able to escape explaining
@@ -3276,8 +3315,10 @@ ncls:
                 ret = NET_RX_DROP;
         }
 
-out:
+unlock:
         rcu_read_unlock();
+out:
+        tsk_restore_flags(current, pflags, PF_MEMALLOC);
         return ret;
 }
 
diff --git a/net/core/sock.c b/net/core/sock.c
index c8c5816289fe..32fdcd2d6e8f 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -298,6 +298,22 @@ void sk_clear_memalloc(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(sk_clear_memalloc);
 
+int __sk_backlog_rcv(struct sock *sk, struct sk_buff *skb)
+{
+        int ret;
+        unsigned long pflags = current->flags;
+
+        /* these should have been dropped before queueing */
+        BUG_ON(!sock_flag(sk, SOCK_MEMALLOC));
+
+        current->flags |= PF_MEMALLOC;
+        ret = sk->sk_backlog_rcv(sk, skb);
+        tsk_restore_flags(current, pflags, PF_MEMALLOC);
+
+        return ret;
+}
+EXPORT_SYMBOL(__sk_backlog_rcv);
+
 #if defined(CONFIG_CGROUPS)
 #if !defined(CONFIG_NET_CLS_CGROUP)
 int net_cls_subsys_id = -1;