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

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

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