Re: [PATCH 12/16] mm/migration: fix potential page refcounts leak in migrate_pages

[Miaohe Lin]

On 2022/3/7 9:57, Baolin Wang wrote:
> Hi Miaohe,
> 
> On 3/4/2022 5:34 PM, Miaohe Lin wrote:
>> In -ENOMEM case, there might be some subpages of fail-to-migrate THPs
>> left in thp_split_pages list. We should move them back to migration
>> list so that they could be put back to the right list by the caller
>> otherwise the page refcnt will be leaked here. Also adjust nr_failed
>> and nr_thp_failed accordingly to make vm events account more accurate.
>>
>> Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>   mm/migrate.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index e0db06927f02..6c2dfed2ddb8 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1422,6 +1422,15 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>                   }
>>                     nr_failed_pages += nr_subpages;
>> +                /*
>> +                 * There might be some subpages of fail-to-migrate THPs
>> +                 * left in thp_split_pages list. Move them back to migration
>> +                 * list so that they could be put back to the right list by
>> +                 * the caller otherwise the page refcnt will be leaked.
>> +                 */
>> +                list_splice_init(&thp_split_pages, from);
>> +                nr_failed += retry;
>> +                nr_thp_failed += thp_retry;
> 
> Yes, I think we missed this case before, and your patch looks right. But we should also update the 'rc' to return the correct number of pages that were not migrated, right?

I'am not sure. -ENOMEM case always returns -ENOMEM since commit 95a402c3847c ("[PATCH] page migration:
use allocator function for migrate_pages()"). So I did not change rc. But I think you're right. We should
return the correct number of pages that were not migrated in this case.

Thanks.

> .