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Date:   Fri, 4 Aug 2023 13:54:12 +0800
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Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()
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To:     Zi Yan <ziy@nvidia.com>
CC:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>
References: <20230802095346.87449-1-wangkefeng.wang@huawei.com>
 <20230802095346.87449-2-wangkefeng.wang@huawei.com>
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 <001ee9b0-ea25-a896-e3ae-9a9b05a46546@huawei.com>
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 <fb2a22cf-14ae-3594-f5f3-8680c2100d70@huawei.com>
 <F2621E68-F36E-493C-8619-ADFE05050823@nvidia.com>
From:   Kefeng Wang <wangkefeng.wang@huawei.com>
In-Reply-To: <F2621E68-F36E-493C-8619-ADFE05050823@nvidia.com>
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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


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