futex: Implement FUTEX2_NUMA

Extend the futex2 interface to be numa aware.

When FUTEX2_NUMA is specified for a futex, the user value is extended
to two words (of the same size). The first is the user value we all
know, the second one will be the node to place this futex on.

  struct futex_numa_32 {
	u32 val;
	u32 node;
  };

When node is set to ~0, WAIT will set it to the current node_id such
that WAKE knows where to find it. If userspace corrupts the node value
between WAIT and WAKE, the futex will not be found and no wakeup will
happen.

When FUTEX2_NUMA is not set, the node is simply an extension of the
hash, such that traditional futexes are still interleaved over the
nodes.

This is done to avoid having to have a separate !numa hash-table.

[bigeasy: ensure to have at least hashsize of 4 in futex_init(), add
pr_info() for size and allocation information. Cast the naddr math to
void*]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-17-bigeasy@linutronix.de

1 files changed, 83 insertions, 17 deletions

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 8054fda94719..1490e6492993 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -36,6 +36,8 @@
 #include <linux/pagemap.h>
 #include <linux/debugfs.h>
 #include <linux/plist.h>
+#include <linux/gfp.h>
+#include <linux/vmalloc.h>
 #include <linux/memblock.h>
 #include <linux/fault-inject.h>
 #include <linux/slab.h>
@@ -51,11 +53,14 @@
  * reside in the same cacheline.
  */
 static struct {
-	struct futex_hash_bucket *queues;
 	unsigned long            hashmask;
+	unsigned int		 hashshift;
+	struct futex_hash_bucket *queues[MAX_NUMNODES];
 } __futex_data __read_mostly __aligned(2*sizeof(long));
-#define futex_queues   (__futex_data.queues)
-#define futex_hashmask (__futex_data.hashmask)
+
+#define futex_hashmask	(__futex_data.hashmask)
+#define futex_hashshift	(__futex_data.hashshift)
+#define futex_queues	(__futex_data.queues)
 
 struct futex_private_hash {
 	rcuref_t	users;
@@ -339,15 +344,35 @@ __futex_hash(union futex_key *key, struct futex_private_hash *fph)
 {
 	struct futex_hash_bucket *hb;
 	u32 hash;
+	int node;
 
 	hb = __futex_hash_private(key, fph);
 	if (hb)
 		return hb;
 
 	hash = jhash2((u32 *)key,
-		      offsetof(typeof(*key), both.offset) / 4,
+		      offsetof(typeof(*key), both.offset) / sizeof(u32),
 		      key->both.offset);
-	return &futex_queues[hash & futex_hashmask];
+	node = key->both.node;
+
+	if (node == FUTEX_NO_NODE) {
+		/*
+		 * In case of !FLAGS_NUMA, use some unused hash bits to pick a
+		 * node -- this ensures regular futexes are interleaved across
+		 * the nodes and avoids having to allocate multiple
+		 * hash-tables.
+		 *
+		 * NOTE: this isn't perfectly uniform, but it is fast and
+		 * handles sparse node masks.
+		 */
+		node = (hash >> futex_hashshift) % nr_node_ids;
+		if (!node_possible(node)) {
+			node = find_next_bit_wrap(node_possible_map.bits,
+						  nr_node_ids, node);
+		}
+	}
+
+	return &futex_queues[node][hash & futex_hashmask];
 }
 
 /**
@@ -454,25 +479,49 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
 	struct page *page;
 	struct folio *folio;
 	struct address_space *mapping;
-	int err, ro = 0;
+	int node, err, size, ro = 0;
 	bool fshared;
 
 	fshared = flags & FLAGS_SHARED;
+	size = futex_size(flags);
+	if (flags & FLAGS_NUMA)
+		size *= 2;
 
 	/*
 	 * The futex address must be "naturally" aligned.
 	 */
 	key->both.offset = address % PAGE_SIZE;
-	if (unlikely((address % sizeof(u32)) != 0))
+	if (unlikely((address % size) != 0))
 		return -EINVAL;
 	address -= key->both.offset;
 
-	if (unlikely(!access_ok(uaddr, sizeof(u32))))
+	if (unlikely(!access_ok(uaddr, size)))
 		return -EFAULT;
 
 	if (unlikely(should_fail_futex(fshared)))
 		return -EFAULT;
 
+	if (flags & FLAGS_NUMA) {
+		u32 __user *naddr = (void *)uaddr + size / 2;
+
+		if (futex_get_value(&node, naddr))
+			return -EFAULT;
+
+		if (node == FUTEX_NO_NODE) {
+			node = numa_node_id();
+			if (futex_put_value(node, naddr))
+				return -EFAULT;
+
+		} else if (node >= MAX_NUMNODES || !node_possible(node)) {
+			return -EINVAL;
+		}
+
+		key->both.node = node;
+
+	} else {
+		key->both.node = FUTEX_NO_NODE;
+	}
+
 	/*
 	 * PROCESS_PRIVATE futexes are fast.
 	 * As the mm cannot disappear under us and the 'key' only needs
@@ -1642,24 +1691,41 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 static int __init futex_init(void)
 {
 	unsigned long hashsize, i;
-	unsigned int futex_shift;
+	unsigned int order, n;
+	unsigned long size;
 
 #ifdef CONFIG_BASE_SMALL
 	hashsize = 16;
 #else
-	hashsize = roundup_pow_of_two(256 * num_possible_cpus());
+	hashsize = 256 * num_possible_cpus();
+	hashsize /= num_possible_nodes();
+	hashsize = max(4, hashsize);
+	hashsize = roundup_pow_of_two(hashsize);
 #endif
+	futex_hashshift = ilog2(hashsize);
+	size = sizeof(struct futex_hash_bucket) * hashsize;
+	order = get_order(size);
 
-	futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
-					       hashsize, 0, 0,
-					       &futex_shift, NULL,
-					       hashsize, hashsize);
-	hashsize = 1UL << futex_shift;
+	for_each_node(n) {
+		struct futex_hash_bucket *table;
 
-	for (i = 0; i < hashsize; i++)
-		futex_hash_bucket_init(&futex_queues[i], NULL);
+		if (order > MAX_PAGE_ORDER)
+			table = vmalloc_huge_node(size, GFP_KERNEL, n);
+		else
+			table = alloc_pages_exact_nid(n, size, GFP_KERNEL);
+
+		BUG_ON(!table);
+
+		for (i = 0; i < hashsize; i++)
+			futex_hash_bucket_init(&table[i], NULL);
+
+		futex_queues[n] = table;
+	}
 
 	futex_hashmask = hashsize - 1;
+	pr_info("futex hash table entries: %lu (%lu bytes on %d NUMA nodes, total %lu KiB, %s).\n",
+		hashsize, size, num_possible_nodes(), size * num_possible_nodes() / 1024,
+		order > MAX_PAGE_ORDER ? "vmalloc" : "linear");
 	return 0;
 }
 core_initcall(futex_init);