Re: [PATCH v2] mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable_pages()

[David Hildenbrand <david@xxxxxxxxxx>]

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@xxxxxxxxxx>
> > ---
> >
> > 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@xxxxxxxxxx>

> 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