[PATCH] mm, thp: relax migration wait when failed to get tail page

[Xu Yu <xuyu@linux.alibaba.com>]

We notice that hung task happens in a conner but practical scenario when
CONFIG_PREEMPT_NONE is enabled, as follows.

Process 0                       Process 1                     Process 2..Inf
split_huge_page_to_list
    unmap_page
        split_huge_pmd_address
                                __migration_entry_wait(head)
                                                              __migration_entry_wait(tail)
    remap_page (roll back)
        remove_migration_ptes
            rmap_walk_anon
                cond_resched

Where __migration_entry_wait(tail) is occurred in kernel space, e.g.,
copy_to_user, which will immediately fault again without rescheduling,
and thus occupy the cpu fully.

When there are too many processes performing __migration_entry_wait on
tail page, remap_page will never be done after cond_resched.

This relaxes __migration_entry_wait on tail page, thus gives remap_page
a chance to complete.

Signed-off-by: Gang Deng <gavin.dg@linux.alibaba.com>
Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
---
 mm/migrate.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index b234c3f3acb7..df2dc39fe566 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -301,8 +301,11 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
 	 * is zero; but we must not call put_and_wait_on_page_locked() without
 	 * a ref. Use get_page_unless_zero(), and just fault again if it fails.
 	 */
-	if (!get_page_unless_zero(page))
-		goto out;
+	if (!get_page_unless_zero(page)) {
+		pte_unmap_unlock(ptep, ptl);
+		cond_resched();
+		return;
+	}
 	pte_unmap_unlock(ptep, ptl);
 	put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE);
 	return;
-- 
2.20.1.2432.ga663e714