Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> writes:
> 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:
>>>>>
>>>>>
>>>>> On 2023/8/2 20:21, Matthew Wilcox wrote:
>>>>>> On Wed, Aug 02, 2023 at 05:53:43PM +0800, Kefeng Wang wrote:
>>>>>>> err = -EACCES;
>>>>>>> - if (page_mapcount(page) > 1 && !migrate_all)
>>>>>>> - goto out_putpage;
>>>>>>> + if (folio_estimated_sharers(folio) > 1 && !migrate_all)
>>>>>>> + goto out_putfolio;
>>>>>>
>>>>>> I do not think this is the correct change. Maybe leave this line
>>>>>> alone.
>>>>>
>>>>> Ok, I am aware of the discussion about this in other mail, will not
>>>>> change it(also the next two patch about this function), or wait the
>>>>> new work of David.
>>>>>>
>>>>>>> - if (PageHuge(page)) {
>>>>>>> - if (PageHead(page)) {
>>>>>>> - isolated = isolate_hugetlb(page_folio(page), pagelist);
>>>>>>> + if (folio_test_hugetlb(folio)) {
>>>>>>> + if (folio_test_large(folio)) {
>>>>>>
>>>>>> This makes no sense when you read it. All hugetlb folios are large,
>>>>>> by definition. Think about what this code used to do, and what it
>>>>>> should be changed to.
>>>>>
>>>>> hugetlb folio is self large folio, will drop redundant check
>>>>
>>>> No, that's not the difference. Keep thinking about it. This is not
>>>> a mechanical translation!
>>>
>>>
>>> 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.
>
>
> 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.
>
>
>
> [1] https://man7.org/linux/man-pages/man2/move_pages.2.html
>
I don't think -ENOENT is appropriate. IIUC, -ENOENT means no need to
migrate. Which isn't the case here apparently.
--
Best Regards,
Huang, Ying
