[PATCH] futex: Move futex_hash_free() back to __mmput()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

Fixes:  e703b7e247503 ("futex: Move futex cleanup to __mmdrop()")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/fork.c       |  2 +-
 kernel/futex/core.c | 15 +++++++++++----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af673856499dc..c4ada32598bd5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -689,7 +689,6 @@ void __mmdrop(struct mm_struct *mm)
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
-	futex_hash_free(mm);
 
 	free_mm(mm);
 }
@@ -1138,6 +1137,7 @@ static inline void __mmput(struct mm_struct *mm)
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
+	futex_hash_free(mm);
 	mmdrop(mm);
 }
 
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb5567af0c5..fb63c13aa66fc 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1724,10 +1724,6 @@ int futex_mm_init(struct mm_struct *mm)
 	/* futex-ref */
 	atomic_long_set(&mm->futex_atomic, 0);
 	mm->futex_batches = get_state_synchronize_rcu();
-	mm->futex_ref = alloc_percpu(unsigned int);
-	if (!mm->futex_ref)
-		return -ENOMEM;
-	this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
 	return 0;
 }
 
@@ -1801,6 +1797,17 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		}
 	}
 
+	if (!mm->futex_ref) {
+		/*
+		 * This will always be allocated by the first thread and
+		 * therefore requires no locking.
+		 */
+		mm->futex_ref = alloc_percpu(unsigned int);
+		if (!mm->futex_ref)
+			return -ENOMEM;
+		this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
+	}
+
 	fph = kvzalloc(struct_size(fph, queues, hash_slots),
 		       GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (!fph)
-- 
2.50.1