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

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Date:   Fri, 4 Aug 2023 09:45:32 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Huang Ying <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()



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?