mm: introduce vma detached flag

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.

Link: https://lkml.kernel.org/r/20230227173632.3292573-23-surenb@google.comSigned-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -711,6 +711,14 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &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) {}
@@ -721,6 +729,8 @@ static inline void vma_start_write(struct vm_area_struct *vma) {}
 static inline bool vma_try_start_write(struct vm_area_struct *vma)
 		{ return true; }
 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 */
 
@@ -732,6 +742,7 @@ 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);
 }
 

include/linux/mm_types.h

@@ -506,6 +506,9 @@ 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
 
 	/*

mm/mmap.c

@@ -599,6 +599,7 @@ 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);
@@ -2276,6 +2277,7 @@ 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);