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

Xu Yu
2021-06-01T16:32:06Z

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