mm/damon/vaddr: protect vma traversal in __damon_va_thre_regions() with rcu read lock

Traversing VMAs of a given maple tree should be protected by rcu read
lock.  However, __damon_va_three_regions() is not doing the protection. 
Hold the lock.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: d0cf3dd ("damon: convert __damon_va_three_regions to use the VMA iterator")
Signed-off-by: Liam R. Howlett <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
Reported-by: Guenter Roeck <[email protected]>
Closes: https://lore.kernel.org/[email protected]
Tested-by: Guenter Roeck <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: howlett

mm/damon/vaddr.c

@@ -126,6 +126,7 @@ static int __damon_va_three_regions(struct mm_struct *mm,
 	 * If this is too slow, it can be optimised to examine the maple
 	 * tree gaps.
 	 */
+	rcu_read_lock();
 	for_each_vma(vmi, vma) {
 		unsigned long gap;
 
@@ -146,6 +147,7 @@ static int __damon_va_three_regions(struct mm_struct *mm,
 next:
 		prev = vma;
 	}
+	rcu_read_unlock();
 
 	if (!sz_range(&second_gap) || !sz_range(&first_gap))
 		return -EINVAL;