[PATCH v5 2/2] mm/oom_kill: Have the OOM reaper and exit_mmap() traverse the maple tree in opposite order

[zhongjinji <zhongjinji@honor.com>]

When a process is OOM killed without reaper delay, the oom reaper and the
exit_mmap() thread likely run simultaneously. They traverse the vma's maple
tree along the same path and may easily unmap the same vma, causing them to
compete for the pte spinlock.

When a process exits, exit_mmap() traverses the vma's maple tree from low
to high addresses. To reduce the chance of unmapping the same vma
simultaneously, the OOM reaper should traverse the vma's tree from high to
low address.

Signed-off-by: zhongjinji <zhongjinji@honor.com>
---
 mm/oom_kill.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4b4d73b1e00d..a0650da9ec9c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -516,7 +516,7 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
 {
 	struct vm_area_struct *vma;
 	bool ret = true;
-	VMA_ITERATOR(vmi, mm, 0);
+	MA_STATE(mas, &mm->mm_mt, ULONG_MAX, 0);
 
 	/*
 	 * Tell all users of get_user/copy_from_user etc... that the content
@@ -526,7 +526,12 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
 	 */
 	set_bit(MMF_UNSTABLE, &mm->flags);
 
-	for_each_vma(vmi, vma) {
+	/*
+	 * When two tasks unmap the same vma at the same time, they may contend for the
+	 * pte spinlock. To reduce the probability of them unmapping the same vma, the
+	 * oom reaper traverse the vma maple tree in reverse order.
+	 */
+	while ((vma = mas_find_rev(&mas, 0)) != NULL) {
 		if (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP))
 			continue;
 
-- 
2.17.1