Subject: [PATCH v4 2/7] mm: introduce vma->vm_flags wrapper functions

From: Suren Baghdasaryan <[email-masked]>
Date: Thu, 26 Jan 2023 11:37:47 -0800

vm_flags are among VMA attributes which affect decisions like VMA merging
and splitting. Therefore all vm_flags modifications are performed after
taking exclusive mmap_lock to prevent vm_flags updates racing with such
operations. Introduce modifier functions for vm_flags to be used whenever
flags are updated. This way we can better check and control correct
locking behavior during these updates.

Signed-off-by: Suren Baghdasaryan <[email-masked]>
Reviewed-by: Davidlohr Bueso <[email-masked]>
Acked-by: Michal Hocko <[email-masked]>
Acked-by: Mel Gorman <[email-masked]>
---
 include/linux/mm.h       | 40 ++++++++++++++++++++++++++++++++++++++++
 include/linux/mm_types.h | 10 +++++++++-
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8d636e895ee9..abb31103d060 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -627,6 +627,46 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 }
 
+/* Use when VMA is not part of the VMA tree and needs no locking */
+static inline void vm_flags_init(struct vm_area_struct *vma,
+				 vm_flags_t flags)
+{
+	ACCESS_PRIVATE(vma, __vm_flags) = flags;
+}
+
+/* Use when VMA is part of the VMA tree and modifications need coordination */
+static inline void vm_flags_reset(struct vm_area_struct *vma,
+				  vm_flags_t flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	vm_flags_init(vma, flags);
+}
+
+static inline void vm_flags_set(struct vm_area_struct *vma,
+				vm_flags_t flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	ACCESS_PRIVATE(vma, __vm_flags) |= flags;
+}
+
+static inline void vm_flags_clear(struct vm_area_struct *vma,
+				  vm_flags_t flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	ACCESS_PRIVATE(vma, __vm_flags) &= ~flags;
+}
+
+/*
+ * Use only when the order of set/clear operations is unimportant, otherwise
+ * use vm_flags_{set|clear} explicitly.
+ */
+static inline void vm_flags_mod(struct vm_area_struct *vma,
+				vm_flags_t set, vm_flags_t clear)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	vm_flags_init(vma, (vma->vm_flags | set) & ~clear);
+}
+
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
 {
 	vma->vm_ops = NULL;
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2d6d790d9bed..da983aedb741 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -491,7 +491,15 @@ struct vm_area_struct {
 	 * See vmf_insert_mixed_prot() for discussion.
 	 */
 	pgprot_t vm_page_prot;
-	unsigned long vm_flags;		/* Flags, see mm.h. */
+
+	/*
+	 * Flags, see mm.h.
+	 * To modify use vm_flags_{init|reset|set|clear|mod} functions.
+	 */
+	union {
+		const vm_flags_t vm_flags;
+		vm_flags_t __private __vm_flags;
+	};
 
 	/*
 	 * For areas with an address space and backing store,
-- 
2.39.1