Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2023/8/16 5:12, Mike Kravetz wrote:
> On 08/15/23 11:58, Huang, Ying wrote:
>> Mike Kravetz <mike.kravetz@oracle.com> writes:
>>
>>> On 08/10/23 09:49, Kefeng Wang wrote:
>>>>
>>>>
>>>> On 2023/8/10 6:44, Mike Kravetz wrote:
>>>>> On 08/09/23 13:53, Mike Kravetz wrote:
>>>>>> On 08/09/23 20:37, Kefeng Wang wrote:
>>>>>>>>
>>>>>>>> Cc Mike to help us clarify the expected behavior of hugetlb.
>>>>>>>>
>>>>>>>> Hi Mike, what is the expected behavior, if a user tries to use move_pages()
>>>>>>>> to migrate a non head page of a hugetlb page?
>>>>>>>
>>>>>>> Could you give some advise, thanks
>>>>>>>
>>>>>>
>>>>>> Sorry, I was away for a while.
>>>>>>
>>>>>> It seems unfortunate that move_pages says the passed user addresses
>>>>>> should be aligned to page boundaries.  However, IIUC this is not checked
>>>>>> or enforced.  Otherwise, passing a hugetlb page should return the same
>>>>>> error.
>>>>>>
>>>>>> One thought would be that hugetlb mappings should behave the same
>>>>>> non-hugetlb mappings.  If passed the address of a hugetlb tail page, align
>>>>>> the address to a hugetlb boundary and migrate the page.  This changes the
>>>>>> existing behavior.  However, it would be hard to imagine anyone depending
>>>>>> on this.
>>>>>>
>>>>>> After taking a closer look at the add_page_for_migration(), it seems to
>>>>>> just ignore passed tail pages and do nothing for such passed addresses.
>>>>>> Correct?  Or, am I missing something?  Perhaps that is behavior we want/
>>>>>> need to preserve?
>>>>>
>>>>> My mistake, status -EACCES is returned when passing a tail page of a
>>>>> hugetlb page.
>>>>>
>>>>
>>>> As mentioned in previous mail， before e66f17ff7177 ("mm/hugetlb: take
>>>> page table lock in follow_huge_pmd()") in v4.0, follow_page() will
>>>> return NULL on tail page for Huagetlb page, so move_pages() will return
>>>> -ENOENT errno, but after that commit, -EACCES is returned.
>>>>
>>>> Meanwhile, the behavior of THP/HUGETLB is different, the whole THP will be
>>>> migrated on a tail page, but HUGETLB will return -EACCES(after v4.0)
>>>> or -ENOENT(before v4.0) on tail page.
>>>>
>>>>> Back to the question of 'What is the expected behavior if a tail page is
>>>>> passed?'.  I do not think we have defined an expected behavior.  If
>>>>> anything is 'expected' I would say it is -EACCES as returned today.
>>>>>
>>>>
>>>> My question is,
>>>>
>>>> Should we keep seem behavior between HUGETLB and THP, or only change the
>>>> errno from -EACCES to -ENOENT/-EBUSY.
>>>
>>> Just to be clear.  When you say "keep seem behavior between HUGETLB and THP",
>>> are you saying that you would like hugetlb to perform migration of the entire
>>> hugetlb page if a tail page is passed?
>>>
>>> IMO, this would be ideal as it would mean that hugetlb and THP behave the same
>>> when passed the address of a tail page.  The fewer places where hugetlb
>>> behavior diverges, the better.  However, this does change behavior.
>>
>> A separate patch will be needed for behavior change.
>>
> 
> Correct.
> 
> Since the goal of this series is to convert to folios, we should maintain the
> existing behavior and errno (-EACCES).  In a subsequent patch, we can
> change behavior.
> 
> That would be my suggestion.


Thanks all, will following the suggestion and re-post.