[PATCH v4 22/33] mm: introduce vma detached flag

[Suren Baghdasaryan]

Per-vma locking mechanism will search for VMA under RCU protection and
then after locking it, has to ensure it was not removed from the VMA
tree after we found it. To make this check efficient, introduce a
vma->detached flag to mark VMAs which were removed from the VMA tree.

Signed-off-by: Suren Baghdasaryan <surenb at google.com>
---
 include/linux/mm.h       | 11 +++++++++++
 include/linux/mm_types.h |  3 +++
 mm/mmap.c                |  2 ++
 3 files changed, 16 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3d5e8666892d..895bb3950e8a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -693,6 +693,14 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 	VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
 }
 
+static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
+{
+	/* When detaching vma should be write-locked */
+	if (detached)
+		vma_assert_write_locked(vma);
+	vma->detached = detached;
+}
+
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_init_lock(struct vm_area_struct *vma) {}
@@ -701,6 +709,8 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 static inline void vma_end_read(struct vm_area_struct *vma) {}
 static inline void vma_start_write(struct vm_area_struct *vma) {}
 static inline void vma_assert_write_locked(struct vm_area_struct *vma) {}
+static inline void vma_mark_detached(struct vm_area_struct *vma,
+				     bool detached) {}
 
 #endif /* CONFIG_PER_VMA_LOCK */
 
@@ -712,6 +722,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	vma->vm_mm = mm;
 	vma->vm_ops = &dummy_vm_ops;
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
+	vma_mark_detached(vma, false);
 	vma_init_lock(vma);
 }
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index a4e7493bacd7..45a219d33c6b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -511,6 +511,9 @@ struct vm_area_struct {
 #ifdef CONFIG_PER_VMA_LOCK
 	int vm_lock_seq;
 	struct rw_semaphore lock;
+
+	/* Flag to indicate areas detached from the mm->mm_mt tree */
+	bool detached;
 #endif
 
 	/*
diff --git a/mm/mmap.c b/mm/mmap.c
index b947d82e8522..df13c33498db 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -600,6 +600,7 @@ static inline void vma_complete(struct vma_prepare *vp,
 
 	if (vp->remove) {
 again:
+		vma_mark_detached(vp->remove, true);
 		if (vp->file) {
 			uprobe_munmap(vp->remove, vp->remove->vm_start,
 				      vp->remove->vm_end);
@@ -2261,6 +2262,7 @@ static inline int munmap_sidetree(struct vm_area_struct *vma,
 	if (mas_store_gfp(mas_detach, vma, GFP_KERNEL))
 		return -ENOMEM;
 
+	vma_mark_detached(vma, true);
 	if (vma->vm_flags & VM_LOCKED)
 		vma->vm_mm->locked_vm -= vma_pages(vma);
 
-- 
2.39.2.722.g9855ee24e9-goog