kernel: scale nf_conntrack_max more reasonably

Use the kernel's built-in formula for computing this value.
The value applied by OpenWRT's sysctl configuration file does not scale
with the available memory, under-using hardware capabilities.
Also, that formula also influences net.netfilter.nf_conntrack_buckets,
which should improve conntrack performance in average (fewer connections
per hashtable bucket).

Backport upstream commit for its effect on the number of connections per
hashtable bucket.

Apply a hack patch to set the RAM size divisor to a more reasonable value (2048,
down from 16384) for our use case, a typical router handling several thousands
of connections.

Signed-off-by: Vincent Pelletier <[email protected]>
Signed-off-by: Rui Salvaterra <[email protected]>

package/kernel/linux/files/sysctl-nf-conntrack.conf

@@ -3,7 +3,6 @@
 
 net.netfilter.nf_conntrack_acct=1
 net.netfilter.nf_conntrack_checksum=0
-net.netfilter.nf_conntrack_max=16384
 net.netfilter.nf_conntrack_tcp_timeout_established=7440
 net.netfilter.nf_conntrack_udp_timeout=60
 net.netfilter.nf_conntrack_udp_timeout_stream=180

target/linux/generic/backport-5.10/612-v5.15-netfilter-conntrack-sanitize-table-size-default-sett.patch

@@ -0,0 +1,100 @@
+From d532bcd0b2699d84d71a0c71d37157ac6eb3be25 Mon Sep 17 00:00:00 2001
+Message-Id: <d532bcd0b2699d84d71a0c71d37157ac6eb3be25.1645246598.git.plr.vincent@gmail.com>
+From: Florian Westphal <[email protected]>
+Date: Thu, 26 Aug 2021 15:54:19 +0200
+Subject: [PATCH] netfilter: conntrack: sanitize table size default settings
+
+conntrack has two distinct table size settings:
+nf_conntrack_max and nf_conntrack_buckets.
+
+The former limits how many conntrack objects are allowed to exist
+in each namespace.
+
+The second sets the size of the hashtable.
+
+As all entries are inserted twice (once for original direction, once for
+reply), there should be at least twice as many buckets in the table than
+the maximum number of conntrack objects that can exist at the same time.
+
+Change the default multiplier to 1 and increase the chosen bucket sizes.
+This results in the same nf_conntrack_max settings as before but reduces
+the average bucket list length.
+
+Signed-off-by: Florian Westphal <[email protected]>
+Signed-off-by: Pablo Neira Ayuso <[email protected]>
+---
+ .../networking/nf_conntrack-sysctl.rst        | 13 ++++----
+ net/netfilter/nf_conntrack_core.c             | 30 +++++++++----------
+ 2 files changed, 22 insertions(+), 21 deletions(-)
+
+--- a/Documentation/networking/nf_conntrack-sysctl.rst
++++ b/Documentation/networking/nf_conntrack-sysctl.rst
+@@ -17,9 +17,8 @@ nf_conntrack_acct - BOOLEAN
+ nf_conntrack_buckets - INTEGER
+ 	Size of hash table. If not specified as parameter during module
+ 	loading, the default size is calculated by dividing total memory
+-	by 16384 to determine the number of buckets but the hash table will
+-	never have fewer than 32 and limited to 16384 buckets. For systems
+-	with more than 4GB of memory it will be 65536 buckets.
++	by 16384 to determine the number of buckets. The hash table will
++	never have fewer than 1024 and never more than 262144 buckets.
+ 	This sysctl is only writeable in the initial net namespace.
+ 
+ nf_conntrack_checksum - BOOLEAN
+@@ -100,8 +99,12 @@ nf_conntrack_log_invalid - INTEGER
+ 	Log invalid packets of a type specified by value.
+ 
+ nf_conntrack_max - INTEGER
+-	Size of connection tracking table.  Default value is
+-	nf_conntrack_buckets value * 4.
++        Maximum number of allowed connection tracking entries. This value is set
++        to nf_conntrack_buckets by default.
++        Note that connection tracking entries are added to the table twice -- once
++        for the original direction and once for the reply direction (i.e., with
++        the reversed address). This means that with default settings a maxed-out
++        table will have a average hash chain length of 2, not 1.
+ 
+ nf_conntrack_tcp_be_liberal - BOOLEAN
+ 	- 0 - disabled (default)
+--- a/net/netfilter/nf_conntrack_core.c
++++ b/net/netfilter/nf_conntrack_core.c
+@@ -2575,26 +2575,24 @@ int nf_conntrack_init_start(void)
+ 		spin_lock_init(&nf_conntrack_locks[i]);
+ 
+ 	if (!nf_conntrack_htable_size) {
+-		/* Idea from tcp.c: use 1/16384 of memory.
+-		 * On i386: 32MB machine has 512 buckets.
+-		 * >= 1GB machines have 16384 buckets.
+-		 * >= 4GB machines have 65536 buckets.
+-		 */
+ 		nf_conntrack_htable_size
+ 			= (((nr_pages << PAGE_SHIFT) / 16384)
+ 			   / sizeof(struct hlist_head));
+-		if (nr_pages > (4 * (1024 * 1024 * 1024 / PAGE_SIZE)))
+-			nf_conntrack_htable_size = 65536;
++		if (BITS_PER_LONG >= 64 &&
++		    nr_pages > (4 * (1024 * 1024 * 1024 / PAGE_SIZE)))
++			nf_conntrack_htable_size = 262144;
+ 		else if (nr_pages > (1024 * 1024 * 1024 / PAGE_SIZE))
+-			nf_conntrack_htable_size = 16384;
+-		if (nf_conntrack_htable_size < 32)
+-			nf_conntrack_htable_size = 32;
++			nf_conntrack_htable_size = 65536;
+ 
+-		/* Use a max. factor of four by default to get the same max as
+-		 * with the old struct list_heads. When a table size is given
+-		 * we use the old value of 8 to avoid reducing the max.
+-		 * entries. */
+-		max_factor = 4;
++		if (nf_conntrack_htable_size < 1024)
++			nf_conntrack_htable_size = 1024;
++		/* Use a max. factor of one by default to keep the average
++		 * hash chain length at 2 entries.  Each entry has to be added
++		 * twice (once for original direction, once for reply).
++		 * When a table size is given we use the old value of 8 to
++		 * avoid implicit reduction of the max entries setting.
++		 */
++		max_factor = 1;
+ 	}
+ 
+ 	nf_conntrack_hash = nf_ct_alloc_hashtable(&nf_conntrack_htable_size, 1);

target/linux/generic/hack-5.10/661-kernel-ct-size-the-hashtable-more-adequately.patch

@@ -0,0 +1,25 @@
+From 804fbb3f2ec9283f7b778e057a68bfff440a0be6 Mon Sep 17 00:00:00 2001
+From: Rui Salvaterra <[email protected]>
+Date: Wed, 30 Mar 2022 22:51:55 +0100
+Subject: [PATCH] kernel: ct: size the hashtable more adequately
+
+To set the default size of the connection tracking hash table, a divider of
+16384 becomes inadequate for a router handling lots of connections. Divide by
+2048 instead, making the default size scale better with the available RAM.
+
+Signed-off-by: Rui Salvaterra <[email protected]>
+---
+ net/netfilter/nf_conntrack_core.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/net/netfilter/nf_conntrack_core.c
++++ b/net/netfilter/nf_conntrack_core.c
+@@ -2576,7 +2576,7 @@ int nf_conntrack_init_start(void)
+ 
+ 	if (!nf_conntrack_htable_size) {
+ 		nf_conntrack_htable_size
+-			= (((nr_pages << PAGE_SHIFT) / 16384)
++			= (((nr_pages << PAGE_SHIFT) / 2048)
+ 			   / sizeof(struct hlist_head));
+ 		if (BITS_PER_LONG >= 64 &&
+ 		    nr_pages > (4 * (1024 * 1024 * 1024 / PAGE_SIZE)))

target/linux/generic/hack-5.15/661-kernel-ct-size-the-hashtable-more-adequately.patch

@@ -0,0 +1,25 @@
+From 804fbb3f2ec9283f7b778e057a68bfff440a0be6 Mon Sep 17 00:00:00 2001
+From: Rui Salvaterra <[email protected]>
+Date: Wed, 30 Mar 2022 22:51:55 +0100
+Subject: [PATCH] kernel: ct: size the hashtable more adequately
+
+To set the default size of the connection tracking hash table, a divider of
+16384 becomes inadequate for a router handling lots of connections. Divide by
+2048 instead, making the default size scale better with the available RAM.
+
+Signed-off-by: Rui Salvaterra <[email protected]>
+---
+ net/netfilter/nf_conntrack_core.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/net/netfilter/nf_conntrack_core.c
++++ b/net/netfilter/nf_conntrack_core.c
+@@ -2676,7 +2676,7 @@ int nf_conntrack_init_start(void)
+ 
+ 	if (!nf_conntrack_htable_size) {
+ 		nf_conntrack_htable_size
+-			= (((nr_pages << PAGE_SHIFT) / 16384)
++			= (((nr_pages << PAGE_SHIFT) / 2048)
+ 			   / sizeof(struct hlist_head));
+ 		if (BITS_PER_LONG >= 64 &&
+ 		    nr_pages > (4 * (1024 * 1024 * 1024 / PAGE_SIZE)))