linux-kernel - Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter functions

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Date:   Tue, 6 Dec 2022 19:42:02 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        linux-kernel@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mina Almasry <almasrymina@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>, hughd@...gle.com,
        tsahu@...ux.ibm.com, jhubbard@...dia.com,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter
 functions

On 12/07/22 11:34, Muchun Song wrote:
> 
> 
> > On Nov 30, 2022, at 06:50, Sidhartha Kumar <sidhartha.kumar@...cle.com> wrote:
> > 
> > Add folio equivalents for set_compound_order() and set_compound_page_dtor().
> > 
> > Also remove extra new-lines introduced by mm/hugetlb: convert
> > move_hugetlb_state() to folios and mm/hugetlb_cgroup: convert
> > hugetlb_cgroup_uncharge_page() to folios.
> > 
> > Suggested-by: Mike Kravetz <mike.kravetz@...cle.com>
> > Suggested-by: Muchun Song <songmuchun@...edance.com>
> > Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
> > ---
> > include/linux/mm.h | 16 ++++++++++++++++
> > mm/hugetlb.c       |  4 +---
> > 2 files changed, 17 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index a48c5ad16a5e..2bdef8a5298a 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -972,6 +972,13 @@ static inline void set_compound_page_dtor(struct page *page,
> > page[1].compound_dtor = compound_dtor;
> > }
> > 
> > +static inline void folio_set_compound_dtor(struct folio *folio,
> > + enum compound_dtor_id compound_dtor)
> > +{
> > + VM_BUG_ON_FOLIO(compound_dtor >= NR_COMPOUND_DTORS, folio);
> > + folio->_folio_dtor = compound_dtor;
> > +}
> > +
> > void destroy_large_folio(struct folio *folio);
> > 
> > static inline int head_compound_pincount(struct page *head)
> > @@ -987,6 +994,15 @@ static inline void set_compound_order(struct page *page, unsigned int order)
> > #endif
> > }
> > 
> > +static inline void folio_set_compound_order(struct folio *folio,
> > + unsigned int order)
> > +{
> > + folio->_folio_order = order;
> > +#ifdef CONFIG_64BIT
> > + folio->_folio_nr_pages = order ? 1U << order : 0;
> 
> It seems that you think the user could pass 0 to order. However,
> ->_folio_nr_pages and ->_folio_order fields are invalid for order-0 pages.
> You should not touch it. So this should be:
> 
> static inline void folio_set_compound_order(struct folio *folio,
> 					    unsigned int order)
> {
> 	if (!folio_test_large(folio))
> 		return;
> 
> 	folio->_folio_order = order;
> #ifdef CONFIG_64BIT
> 	folio->_folio_nr_pages = 1U << order;
> #endif
> }

I believe this was changed to accommodate the code in
__destroy_compound_gigantic_page().  It is used in a subsequent patch.
Here is the v6.0 version of the routine.

static void __destroy_compound_gigantic_page(struct page *page,
					unsigned int order, bool demote)
{
	int i;
	int nr_pages = 1 << order;
	struct page *p = page + 1;

	atomic_set(compound_mapcount_ptr(page), 0);
	atomic_set(compound_pincount_ptr(page), 0);

	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
		p->mapping = NULL;
		clear_compound_head(p);
		if (!demote)
			set_page_refcounted(p);
	}

	set_compound_order(page, 0);
#ifdef CONFIG_64BIT
	page[1].compound_nr = 0;
#endif
	__ClearPageHead(page);
}


Might have been better to change this set_compound_order call to
folio_set_compound_order in this patch.
-- 
Mike Kravetz