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/20240905001204.1481-1-sj@kernel.org
Fixes: d0cf3dd4 ("damon: convert __damon_va_three_regions to use the VMA iterator")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/b83651a0-5b24-4206-b860-cb54ffdf209b@roeck-us.netTested-by: Guenter Roeck <linux@roeck-us.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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;