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

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

Hi Mike

On 2023/8/8 2:45, Zi Yan wrote:
> On 7 Aug 2023, at 8:20, Kefeng Wang wrote:
> 
>> Hi Zi Yan and Matthew and Naoya,
>>
>> On 2023/8/4 13:54, Kefeng Wang wrote:
>>>
>>>
>>> On 2023/8/4 10:42, Zi Yan wrote:
>>>> On 3 Aug 2023, at 21:45, Kefeng Wang wrote:
>>>>
>>>>> On 2023/8/3 20:30, Matthew Wilcox wrote:
>>>>>> On Thu, Aug 03, 2023 at 03:13:21PM +0800, Kefeng Wang wrote:
>>>>>>>
>>
>> ...
>>
>>>>>
>>>>>
>>>>>     if (PageHuge(page))  // page must be a hugetlb page
>>>>>      if (PageHead(page)) // page must be a head page, not tail
>>>>>                isolate_hugetlb() // isolate the hugetlb page if head
>>>>>
>>>>> After using folio,
>>>>>
>>>>>     if (folio_test_hugetlb(folio)) // only check folio is hugetlb or not
>>>>>
>>>>> I don't check the page is head or not, since the follow_page could
>>>>> return a sub-page, so the check PageHead need be retained, right?
>>>>
>>>> Right. It will prevent the kernel from trying to isolate the same hugetlb page
>>>> twice when two pages are in the same hugetlb folio. But looking at the
>>>> code, if you try to isolate an already-isolated hugetlb folio, isolate_hugetlb()
>>>> would return false, no error would show up. But it changes err value
>>>> from -EACCES to -EBUSY and user will see a different page status than before.
>>>
>>
>> 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,
>> and move_pages() will return -ENOENT errno,but after that commit,
>> -EACCES is returned, which not match the manual,
>>
>>>
>>> When check man[1], the current -EACCES is not right, -EBUSY is not
>>> precise but more suitable for this scenario,
>>>
>>>        -EACCES
>>>                 The page is mapped by multiple processes and can be moved
>>>                 only if MPOL_MF_MOVE_ALL is specified.
>>>
>>>        -EBUSY The page is currently busy and cannot be moved.  Try again
>>>                 later.  This occurs if a page is undergoing I/O or another
>>>                 kernel subsystem is holding a reference to the page.
>>>       -ENOENT
>>>                 The page is not present.
>>>
>>>>
>>>> I wonder why we do not have follow_folio() and returns -ENOENT error pointer
>>>> when addr points to a non head page. It would make this patch more folio if
>>>> follow_folio() can be used in place of follow_page(). One caveat is that
>>>> user will see -ENOENT instead of -EACCES after this change.
>>>>
>>>
>>> -ENOENT is ok, but maybe the man need to be updated too.
>>
>> According to above analysis, -ENOENT is suitable when introduce the
>> follow_folio(), but when THP migrate support is introduced by
>> e8db67eb0ded ("mm: migrate: move_pages() supports thp migration") in
>> v4.14, the tail page will be turned into head page and return -EBUSY,
>>
>> So should we unify errno(maybe use -ENOENT) about the tail page?
>>
>>
>>>
>>>
>>>
>>> [1] https://man7.org/linux/man-pages/man2/move_pages.2.html
> 
> I think so. I think -EBUSY is more reasonable for tail pages. But there is
> some subtle difference between THP and hugetlb from current code:
> 
> For THP, compound_head() is used to get the head page for isolation, this means
> if user specifies a tail page address in move_pages(), the whole THP can be
> migrated.
> 
> For hugetlb, only if user specifies the head page address of a hugetlb page,
> the hugetlb page will be migrated. Otherwise, an error would show up.
> 
> 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

> 
> --
> Best Regards,
> Yan, Zi