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

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

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.

I would like to drop PageHead() check for Hugetlb to keep seem behavior,
which will keep seem error code if isolate fail or success on head/tail
page.

Thanks.

> BTW - hugetlb pages not migrated due to passing a tail page does not
> seem to contribute to a 'Positive return value' indicating the number of
> non-migrated pages.