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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 3/7/2022 1:02 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> 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@huawei.com>
>>> ---
>>>    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?
> 
> Per my understanding, -ENOMEM should be returned to indicate an fatal
> error.
> 

Ah, right. Sorry for noise.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>