Re: [PATCH v2 5/5] mm/migrate.c: fix potential deadlock in NUMA balancing shared exec THP case

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/3/24 1:17, Yang Shi wrote:
> On Tue, Mar 23, 2021 at 6:55 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>
>> Since commit c77c5cbafe54 ("mm: migrate: skip shared exec THP for NUMA
>> balancing"), the NUMA balancing would skip shared exec transhuge page.
>> But this enhancement is not suitable for transhuge page. Because it's
>> required that page_mapcount() must be 1 due to no migration pte dance
>> is done here. On the other hand, the shared exec transhuge page will
>> leave the migrate_misplaced_page() with pte entry untouched and page
>> locked. Thus pagefault for NUMA will be triggered again and deadlock
>> occurs when we start waiting for the page lock held by ourselves.
> 
> Thanks for catching this. By relooking the code I think the other
> important reason for removing this is
> migrate_misplaced_transhuge_page() actually can't see shared exec file
> THP at all since page_lock_anon_vma_read() is called before and if
> page is not anonymous page it will just restore the PMD without
> migrating anything.
> 
> The pages for private mapped file vma may be anonymous pages due to
> COW but they can't be THP so it won't trigger THP numa fault at all. I
> think this is why no bug was reported. I overlooked this in the first
> place.
> 
> Your fix is correct, and please add the above justification to your commit log.
> 

Will do. Many thanks for your review!

> Reviewed-by: Yang Shi <shy828301@gmail.com>
> 
>>
>> Fixes: c77c5cbafe54 ("mm: migrate: skip shared exec THP for NUMA balancing")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/migrate.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 5357a8527ca2..68bfa1625898 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -2192,9 +2192,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>>         int page_lru = page_is_file_lru(page);
>>         unsigned long start = address & HPAGE_PMD_MASK;
>>
>> -       if (is_shared_exec_page(vma, page))
>> -               goto out;
>> -
>>         new_page = alloc_pages_node(node,
>>                 (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
>>                 HPAGE_PMD_ORDER);
>> @@ -2306,7 +2303,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>>
>>  out_unlock:
>>         unlock_page(page);
>> -out:
>>         put_page(page);
>>         return 0;
>>  }
>> --
>> 2.19.1
>>
> .
>