linux-kernel - Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for

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Message-ID: <2E8CA295-CEA2-4093-8287-F68C3977A538@nvidia.com>
Date:   Tue, 15 Aug 2023 09:49:06 -0400
From:   Zi Yan <ziy@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()

On 14 Aug 2023, at 23:56, Huang, Ying wrote:

> Kefeng Wang <wangkefeng.wang@...wei.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.

Are you referring to a comment or the man page? The man page says
-ENOENT means the page is not present. Or you think it also implies
there is no need to migrate? If yes, we probably need to update the man
page.


--
Best Regards,
Yan, Zi

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