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

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Message-ID: <fe37643c-93a0-4220-b547-a5cae36b3231@redhat.com>
Date: Fri, 31 May 2024 15:04:29 +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 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?

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