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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Eric Dumazet <edumazet@google.com>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>
Subject: [PATCH 5.16 0945/1039] ipv4: avoid quadratic behavior in netns dismantle
Date:   Mon, 24 Jan 2022 19:45:34 +0100
Message-Id: <20220124184157.064140950@linuxfoundation.org>
In-Reply-To: <20220124184125.121143506@linuxfoundation.org>
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From: Eric Dumazet <edumazet@google.com>

commit d07418afea8f1d9896aaf9dc5ae47ac4f45b220c upstream.

net/ipv4/fib_semantics.c uses an hash table of 256 slots,
keyed by device ifindexes: fib_info_devhash[DEVINDEX_HASHSIZE]

Problem is that with network namespaces, devices tend
to use the same ifindex.

lo device for instance has a fixed ifindex of one,
for all network namespaces.

This means that hosts with thousands of netns spend
a lot of time looking at some hash buckets with thousands
of elements, notably at netns dismantle.

Simply add a per netns perturbation (net_hash_mix())
to spread elements more uniformely.

Also change fib_devindex_hashfn() to use more entropy.

Fixes: aa79e66eee5d ("net: Make ifindex generation per-net namespace")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/fib_semantics.c |   36 +++++++++++++++++-------------------
 1 file changed, 17 insertions(+), 19 deletions(-)

--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -29,6 +29,7 @@
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/netlink.h>
+#include <linux/hash.h>
 
 #include <net/arp.h>
 #include <net/ip.h>
@@ -319,11 +320,15 @@ static inline int nh_comp(struct fib_inf
 
 static inline unsigned int fib_devindex_hashfn(unsigned int val)
 {
-	unsigned int mask = DEVINDEX_HASHSIZE - 1;
+	return hash_32(val, DEVINDEX_HASHBITS);
+}
+
+static struct hlist_head *
+fib_info_devhash_bucket(const struct net_device *dev)
+{
+	u32 val = net_hash_mix(dev_net(dev)) ^ dev->ifindex;
 
-	return (val ^
-		(val >> DEVINDEX_HASHBITS) ^
-		(val >> (DEVINDEX_HASHBITS * 2))) & mask;
+	return &fib_info_devhash[fib_devindex_hashfn(val)];
 }
 
 static unsigned int fib_info_hashfn_1(int init_val, u8 protocol, u8 scope,
@@ -433,12 +438,11 @@ int ip_fib_check_default(__be32 gw, stru
 {
 	struct hlist_head *head;
 	struct fib_nh *nh;
-	unsigned int hash;
 
 	spin_lock(&fib_info_lock);
 
-	hash = fib_devindex_hashfn(dev->ifindex);
-	head = &fib_info_devhash[hash];
+	head = fib_info_devhash_bucket(dev);
+
 	hlist_for_each_entry(nh, head, nh_hash) {
 		if (nh->fib_nh_dev == dev &&
 		    nh->fib_nh_gw4 == gw &&
@@ -1607,12 +1611,10 @@ link_it:
 	} else {
 		change_nexthops(fi) {
 			struct hlist_head *head;
-			unsigned int hash;
 
 			if (!nexthop_nh->fib_nh_dev)
 				continue;
-			hash = fib_devindex_hashfn(nexthop_nh->fib_nh_dev->ifindex);
-			head = &fib_info_devhash[hash];
+			head = fib_info_devhash_bucket(nexthop_nh->fib_nh_dev);
 			hlist_add_head(&nexthop_nh->nh_hash, head);
 		} endfor_nexthops(fi)
 	}
@@ -1964,8 +1966,7 @@ void fib_nhc_update_mtu(struct fib_nh_co
 
 void fib_sync_mtu(struct net_device *dev, u32 orig_mtu)
 {
-	unsigned int hash = fib_devindex_hashfn(dev->ifindex);
-	struct hlist_head *head = &fib_info_devhash[hash];
+	struct hlist_head *head = fib_info_devhash_bucket(dev);
 	struct fib_nh *nh;
 
 	hlist_for_each_entry(nh, head, nh_hash) {
@@ -1984,12 +1985,11 @@ void fib_sync_mtu(struct net_device *dev
  */
 int fib_sync_down_dev(struct net_device *dev, unsigned long event, bool force)
 {
-	int ret = 0;
-	int scope = RT_SCOPE_NOWHERE;
+	struct hlist_head *head = fib_info_devhash_bucket(dev);
 	struct fib_info *prev_fi = NULL;
-	unsigned int hash = fib_devindex_hashfn(dev->ifindex);
-	struct hlist_head *head = &fib_info_devhash[hash];
+	int scope = RT_SCOPE_NOWHERE;
 	struct fib_nh *nh;
+	int ret = 0;
 
 	if (force)
 		scope = -1;
@@ -2134,7 +2134,6 @@ out:
 int fib_sync_up(struct net_device *dev, unsigned char nh_flags)
 {
 	struct fib_info *prev_fi;
-	unsigned int hash;
 	struct hlist_head *head;
 	struct fib_nh *nh;
 	int ret;
@@ -2150,8 +2149,7 @@ int fib_sync_up(struct net_device *dev,
 	}
 
 	prev_fi = NULL;
-	hash = fib_devindex_hashfn(dev->ifindex);
-	head = &fib_info_devhash[hash];
+	head = fib_info_devhash_bucket(dev);
 	ret = 0;
 
 	hlist_for_each_entry(nh, head, nh_hash) {