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

["Huang, Ying" <ying.huang@intel.com>]

Kefeng Wang <wangkefeng.wang@huawei.com> 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