linux-kernel - Re: [PATCH v2] mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable

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Date: Fri, 31 May 2024 15:05:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Sidhartha Kumar <sidhartha.kumar@...cle.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, vishal.moola@...cle.com,
 muchun.song@...ux.dev, osalvador@...e.de, willy@...radead.org
Subject: Re: [PATCH v2] mm/hugetlb: mm/memory_hotplug: use a folio in
 scan_movable_pages()

On 31.05.24 15:04, David Hildenbrand wrote:
> On 30.05.24 19:14, Sidhartha Kumar wrote:
>> By using a folio in scan_movable_pages() we convert the last user of the
>> page-based hugetlb information macro functions to the folio version.
>> After this conversion, we can safely remove the page-based definitions
>> from include/linux/hugetlb.h.
>>
>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
>> ---
>>
>> v1 -> v2:
>> 	simplify pfn skipping logic with pfn |= folio_nr_pages(folio) - 1
>> 	per Matthew
>>
>>    include/linux/hugetlb.h |  6 +-----
>>    mm/memory_hotplug.c     | 11 +++++------
>>    2 files changed, 6 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 15a58f69782c..279aca379b95 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -616,9 +616,7 @@ static __always_inline						\
>>    bool folio_test_hugetlb_##flname(struct folio *folio)		\
>>    	{	void *private = &folio->private;		\
>>    		return test_bit(HPG_##flname, private);		\
>> -	}							\
>> -static inline int HPage##uname(struct page *page)		\
>> -	{ return test_bit(HPG_##flname, &(page->private)); }
>> +	}
>>    
>>    #define SETHPAGEFLAG(uname, flname)				\
>>    static __always_inline						\
>> @@ -637,8 +635,6 @@ void folio_clear_hugetlb_##flname(struct folio *folio)		\
>>    #define TESTHPAGEFLAG(uname, flname)				\
>>    static inline bool						\
>>    folio_test_hugetlb_##flname(struct folio *folio)		\
>> -	{ return 0; }						\
>> -static inline int HPage##uname(struct page *page)		\
>>    	{ return 0; }
>>    
>>    #define SETHPAGEFLAG(uname, flname)				\
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 431b1f6753c0..9c36eb3bbd3b 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1731,8 +1731,8 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>>    	unsigned long pfn;
>>    
>>    	for (pfn = start; pfn < end; pfn++) {
>> -		struct page *page, *head;
>> -		unsigned long skip;
>> +		struct page *page;
>> +		struct folio *folio;
>>    
>>    		if (!pfn_valid(pfn))
>>    			continue;
>> @@ -1753,7 +1753,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>>    
>>    		if (!PageHuge(page))
>>    			continue;
>> -		head = compound_head(page);
>> +		folio = page_folio(page);
>>    		/*
>>    		 * This test is racy as we hold no reference or lock.  The
>>    		 * hugetlb page could have been free'ed and head is no longer
>> @@ -1761,10 +1761,9 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>>    		 * cases false positives and negatives are possible.  Calling
>>    		 * code must deal with these scenarios.
>>    		 */
>> -		if (HPageMigratable(head))
>> +		if (folio_test_hugetlb_migratable(folio))
>>    			goto found;
>> -		skip = compound_nr(head) - (pfn - page_to_pfn(head));
>> -		pfn += skip - 1;
>> +		pfn |= folio_nr_pages(folio) - 1;
> 
> Likely not exactly what we want?
> 
> pfn |= folio_nr_pages(folio);
> 
> Would make sure that we are "one PFN before the start of the next
> folio". The pfn++ before the next loop iteration would move us to the
> next folio.
> 
> Or am I missing something?

Okay, I got it wrong.

"folio_nr_pages(folio) - 1" gives us the bitmask to land one PFN before 
the end.

Acked-by: David Hildenbrand <david@...hat.com>

> 
> It might be cleaner if we would handle the "pfn++;" on the "continue;"
> paths inmstead, and simply here do something like
> 
> 	pfn = ALIGN(pfn + 1, folio_nr_pages(folio));
> 
> instead.
> 

-- 
Cheers,

David / dhildenb