Subject: [RFC PATCH RESEND 28/28] kernel/fork: throttle call_rcu() calls in vm_area_free

From: Suren Baghdasaryan <[email-masked]>
Date: Thu, 1 Sep 2022 10:35:16 -0700

call_rcu() can take a long time when callback offloading is enabled.
Its use in the vm_area_free can cause regressions in the exit path when
multiple VMAs are being freed. To minimize that impact, place VMAs into
a list and free them in groups using one call_rcu() call per group.

Signed-off-by: Suren Baghdasaryan <[email-masked]>
---
 include/linux/mm.h       |  1 +
 include/linux/mm_types.h | 11 ++++++-
 kernel/fork.c            | 68 +++++++++++++++++++++++++++++++++++-----
 mm/init-mm.c             |  3 ++
 mm/mmap.c                |  1 +
 5 files changed, 75 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a3cbaa7b9119..81dff694ac14 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -249,6 +249,7 @@ void setup_initial_init_mm(void *start_code, void *end_code,
 struct vm_area_struct *vm_area_alloc(struct mm_struct *);
 struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
 void vm_area_free(struct vm_area_struct *);
+void drain_free_vmas(struct mm_struct *mm);
 
 #ifndef CONFIG_MMU
 extern struct rb_root nommu_region_tree;
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 36562e702baf..6f3effc493b1 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -412,7 +412,11 @@ struct vm_area_struct {
 			struct vm_area_struct *vm_next, *vm_prev;
 		};
 #ifdef CONFIG_PER_VMA_LOCK
-		struct rcu_head vm_rcu;	/* Used for deferred freeing. */
+		struct {
+			struct list_head vm_free_list;
+			/* Used for deferred freeing. */
+			struct rcu_head vm_rcu;
+		};
 #endif
 	};
 
@@ -573,6 +577,11 @@ struct mm_struct {
 					  */
 #ifdef CONFIG_PER_VMA_LOCK
 		int mm_lock_seq;
+		struct {
+			struct list_head head;
+			spinlock_t lock;
+			int size;
+		} vma_free_list;
 #endif

diff --git a/kernel/fork.c b/kernel/fork.c
index b443ba3a247a..7c88710aed72 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -483,26 +483,75 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
-static void __vm_area_free(struct rcu_head *head)
+static inline void __vm_area_free(struct vm_area_struct *vma)
 {
-	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
-						  vm_rcu);
 	/* The vma should either have no lock holders or be write-locked. */
 	vma_assert_no_reader(vma);
 	kmem_cache_free(vm_area_cachep, vma);
 }
-#endif
+
+static void vma_free_rcu_callback(struct rcu_head *head)
+{
+	struct vm_area_struct *first_vma;
+	struct vm_area_struct *vma, *vma2;
+
+	first_vma = container_of(head, struct vm_area_struct, vm_rcu);
+	list_for_each_entry_safe(vma, vma2, &first_vma->vm_free_list, vm_free_list)
+		__vm_area_free(vma);
+	__vm_area_free(first_vma);
+}
+
+void drain_free_vmas(struct mm_struct *mm)
+{
+	struct vm_area_struct *first_vma;
+	LIST_HEAD(to_destroy);
+
+	spin_lock(&mm->vma_free_list.lock);
+	list_splice_init(&mm->vma_free_list.head, &to_destroy);
+	mm->vma_free_list.size = 0;
+	spin_unlock(&mm->vma_free_list.lock);
+
+	if (list_empty(&to_destroy))
+		return;
+
+	first_vma = list_first_entry(&to_destroy, struct vm_area_struct, vm_free_list);
+	/* Remove the head which is allocated on the stack */
+	list_del(&to_destroy);
+
+	call_rcu(&first_vma->vm_rcu, vma_free_rcu_callback);
+}
+
+#define VM_AREA_FREE_LIST_MAX	32
+
+void vm_area_free(struct vm_area_struct *vma)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	bool drain;
+
+	free_anon_vma_name(vma);
+
+	spin_lock(&mm->vma_free_list.lock);
+	list_add(&vma->vm_free_list, &mm->vma_free_list.head);
+	mm->vma_free_list.size++;
+	drain = mm->vma_free_list.size > VM_AREA_FREE_LIST_MAX;
+	spin_unlock(&mm->vma_free_list.lock);
+
+	if (drain)
+		drain_free_vmas(mm);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
+void drain_free_vmas(struct mm_struct *mm) {}
 
 void vm_area_free(struct vm_area_struct *vma)
 {
 	free_anon_vma_name(vma);
-#ifdef CONFIG_PER_VMA_LOCK
-	call_rcu(&vma->vm_rcu, __vm_area_free);
-#else
 	kmem_cache_free(vm_area_cachep, vma);
-#endif
 }
 
+#endif /* CONFIG_PER_VMA_LOCK */
+
 static void account_kernel_stack(struct task_struct *tsk, int account)
 {
 	if (IS_ENABLED(CONFIG_VMAP_STACK)) {
@@ -1137,6 +1186,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	INIT_LIST_HEAD(&mm->mmlist);
 #ifdef CONFIG_PER_VMA_LOCK
 	WRITE_ONCE(mm->mm_lock_seq, 0);
+	INIT_LIST_HEAD(&mm->vma_free_list.head);
+	spin_lock_init(&mm->vma_free_list.lock);
+	mm->vma_free_list.size = 0;
 #endif
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index 8399f90d631c..7b6d2460545f 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -39,6 +39,9 @@ struct mm_struct init_mm = {
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 #ifdef CONFIG_PER_VMA_LOCK
 	.mm_lock_seq	= 0,
+	.vma_free_list.head = LIST_HEAD_INIT(init_mm.vma_free_list.head),
+	.vma_free_list.lock =  __SPIN_LOCK_UNLOCKED(init_mm.vma_free_list.lock),
+	.vma_free_list.size = 0,
 #endif
 	.user_ns	= &init_user_ns,
 	.cpu_bitmap	= CPU_BITS_NONE,
diff --git a/mm/mmap.c b/mm/mmap.c
index 1edfcd384f5e..d61b7ef84ba6 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3149,6 +3149,7 @@ void exit_mmap(struct mm_struct *mm)
 	}
 	mm->mmap = NULL;
 	mmap_write_unlock(mm);
+	drain_free_vmas(mm);
 	vm_unacct_memory(nr_accounted);
 }
 
-- 
2.37.2.789.g6183377224-goog