futex: Allow automatic allocation of process wide futex hash

Allocate a private futex hash with 16 slots if a task forks its first
thread.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-14-bigeasy@linutronix.de

include/linux/futex.h

@@ -80,6 +80,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4);
 
 #ifdef CONFIG_FUTEX_PRIVATE_HASH
+int futex_hash_allocate_default(void);
 void futex_hash_free(struct mm_struct *mm);
 
 static inline void futex_mm_init(struct mm_struct *mm)
@@ -88,6 +89,7 @@ static inline void futex_mm_init(struct mm_struct *mm)
 }
 
 #else /* !CONFIG_FUTEX_PRIVATE_HASH */
+static inline int futex_hash_allocate_default(void) { return 0; }
 static inline void futex_hash_free(struct mm_struct *mm) { }
 static inline void futex_mm_init(struct mm_struct *mm) { }
 #endif /* CONFIG_FUTEX_PRIVATE_HASH */
@@ -107,6 +109,10 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig
 {
 	return -EINVAL;
 }
+static inline int futex_hash_allocate_default(void)
+{
+	return 0;
+}
 static inline void futex_hash_free(struct mm_struct *mm) { }
 static inline void futex_mm_init(struct mm_struct *mm) { }
 

kernel/fork.c

@@ -2164,6 +2164,13 @@ static void rv_task_fork(struct task_struct *p)
 #define rv_task_fork(p) do {} while (0)
 #endif
 
+static bool need_futex_hash_allocate_default(u64 clone_flags)
+{
+	if ((clone_flags & (CLONE_THREAD | CLONE_VM)) != (CLONE_THREAD | CLONE_VM))
+		return false;
+	return true;
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -2544,6 +2551,21 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cancel_cgroup;
 
+	/*
+	 * Allocate a default futex hash for the user process once the first
+	 * thread spawns.
+	 */
+	if (need_futex_hash_allocate_default(clone_flags)) {
+		retval = futex_hash_allocate_default();
+		if (retval)
+			goto bad_fork_core_free;
+		/*
+		 * If we fail beyond this point we don't free the allocated
+		 * futex hash map. We assume that another thread will be created
+		 * and makes use of it. The hash map will be freed once the main
+		 * thread terminates.
+		 */
+	}
 	/*
 	 * From this point on we must avoid any synchronous user-space
 	 * communication until we take the tasklist-lock. In particular, we do

kernel/futex/core.c

@@ -1294,6 +1294,17 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
 	return 0;
 }
 
+int futex_hash_allocate_default(void)
+{
+	if (!current->mm)
+		return 0;
+
+	if (current->mm->futex_phash)
+		return 0;
+
+	return futex_hash_allocate(16, false);
+}
+
 static int futex_hash_get_slots(void)
 {
 	struct futex_private_hash *fph;