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

Yu Xu
2021-06-02T13:20:40Z

On 6/2/21 12:55 AM, Hugh Dickins wrote:
> On Wed, 2 Jun 2021, Xu Yu wrote:
>
>> 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>
>
> Well caught: you're absolutely right that there's a bug there.
> But isn't cond_resched() just papering over the real bug, and
> what it should do is a "page = compound_head(page);" before the
> get_page_unless_zero()? How does that work out in your testing?

compound_head works. The patched kernel is alive for hours under
our reproducer, which usually makes the vanilla kernel hung after
tens of minutes at most.

If we use compound_head, the behavior of __migration_entry_wait(tail)
changes from "retry fault" to "prevent THP from being split". Is that
right? Then which is preferred? If it were me, I would prefer "retry
fault".

>
> Hugh
>
>> ---
>> 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
>>
>>
>>

--
Thanks,
Yu