[RFC PATCH 10/37] mm: rcu safe vma freeing

[Michel Lespinasse <michel@lespinasse.org>]

This prepares for speculative page faults looking up and copying vmas
under protection of an rcu read lock, instead of the usual mmap read lock.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
---
 include/linux/mm_types.h | 16 +++++++++++-----
 kernel/fork.c            | 11 ++++++++++-
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 70882e628908..024970635921 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -304,12 +304,18 @@ struct vm_userfaultfd_ctx {};
 struct vm_area_struct {
 	/* The first cache line has the info for VMA tree walking. */
 
-	unsigned long vm_start;		/* Our start address within vm_mm. */
-	unsigned long vm_end;		/* The first byte after our end address
-					   within vm_mm. */
+	union {
+		struct {
+			/* VMA covers [vm_start; vm_end) addresses within mm */
+			unsigned long vm_start, vm_end;
 
-	/* linked list of VM areas per task, sorted by address */
-	struct vm_area_struct *vm_next, *vm_prev;
+			/* linked list of VMAs per task, sorted by address */
+			struct vm_area_struct *vm_next, *vm_prev;
+		};
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+		struct rcu_head vm_rcu;	/* Used for deferred freeing. */
+#endif
+	};
 
 	struct rb_node vm_rb;
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 426cd0c51f9e..b6078e546114 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -369,11 +369,20 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 	return new;
 }
 
-void vm_area_free(struct vm_area_struct *vma)
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+static void __vm_area_free(struct rcu_head *head)
 {
+	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
+						  vm_rcu);
 	kmem_cache_free(vm_area_cachep, vma);
 }
 
+void vm_area_free(struct vm_area_struct *vma)
+{
+	call_rcu(&vma->vm_rcu, __vm_area_free);
+}
+#endif	/* CONFIG_SPECULATIVE_PAGE_FAULT */
+
 static void account_kernel_stack(struct task_struct *tsk, int account)
 {
 	void *stack = task_stack_page(tsk);
-- 
2.20.1