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Subject: Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter
 functions
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <Y5ALigw0kUO/B3z2@monkey>
Date:   Wed, 7 Dec 2022 12:11:56 +0800
Cc:     Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        linux-kernel@vger.kernel.org,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mina Almasry <almasrymina@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>, hughd@google.com,
        tsahu@linux.ibm.com, jhubbard@nvidia.com,
        David Hildenbrand <david@redhat.com>
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Message-Id: <4161AF1A-9508-4DF8-B756-FEB476EB32B5@linux.dev>
References: <20221129225039.82257-1-sidhartha.kumar@oracle.com>
 <20221129225039.82257-2-sidhartha.kumar@oracle.com>
 <BAD34D59-187B-4BB3-B139-7983A8B62648@linux.dev> <Y5ALigw0kUO/B3z2@monkey>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Precedence: bulk


> On Dec 7, 2022, at 11:42, Mike Kravetz <mike.kravetz@oracle.com> =
wrote:
>=20
> On 12/07/22 11:34, Muchun Song wrote:
>>=20
>>=20
>>> On Nov 30, 2022, at 06:50, Sidhartha Kumar =
<sidhartha.kumar@oracle.com> wrote:
>>>=20
>>> Add folio equivalents for set_compound_order() and =
set_compound_page_dtor().
>>>=20
>>> 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.
>>>=20
>>> Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
>>> Suggested-by: Muchun Song <songmuchun@bytedance.com>
>>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
>>> ---
>>> include/linux/mm.h | 16 ++++++++++++++++
>>> mm/hugetlb.c       |  4 +---
>>> 2 files changed, 17 insertions(+), 3 deletions(-)
>>>=20
>>> 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 =3D compound_dtor;
>>> }
>>>=20
>>> +static inline void folio_set_compound_dtor(struct folio *folio,
>>> + enum compound_dtor_id compound_dtor)
>>> +{
>>> + VM_BUG_ON_FOLIO(compound_dtor >=3D NR_COMPOUND_DTORS, folio);
>>> + folio->_folio_dtor =3D compound_dtor;
>>> +}
>>> +
>>> void destroy_large_folio(struct folio *folio);
>>>=20
>>> 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
>>> }
>>>=20
>>> +static inline void folio_set_compound_order(struct folio *folio,
>>> + unsigned int order)
>>> +{
>>> + folio->_folio_order =3D order;
>>> +#ifdef CONFIG_64BIT
>>> + folio->_folio_nr_pages =3D order ? 1U << order : 0;
>>=20
>> 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:
>>=20
>> static inline void folio_set_compound_order(struct folio *folio,
>>     unsigned int order)
>> {
>> 	if (!folio_test_large(folio))
>> 		return;
>>=20
>> 	folio->_folio_order =3D order;
>> #ifdef CONFIG_64BIT
>> 	folio->_folio_nr_pages =3D 1U << order;
>> #endif
>> }
>=20
> 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.

Thanks for your clarification.

>=20
> static void __destroy_compound_gigantic_page(struct page *page,
> unsigned int order, bool demote)
> {
> 	int i;
> 	int nr_pages =3D 1 << order;
> 	struct page *p =3D page + 1;
>=20
> 	atomic_set(compound_mapcount_ptr(page), 0);
> 	atomic_set(compound_pincount_ptr(page), 0);
>=20
> 	for (i =3D 1; i < nr_pages; i++, p =3D mem_map_next(p, page, i)) =
{
> 		p->mapping =3D NULL;
> 		clear_compound_head(p);
> 		if (!demote)
> 			set_page_refcounted(p);
> 	}
>=20
> 	set_compound_order(page, 0);
> #ifdef CONFIG_64BIT
> 	page[1].compound_nr =3D 0;
> #endif
> 	__ClearPageHead(page);
> }
>=20
>=20
> Might have been better to change this set_compound_order call to
> folio_set_compound_order in this patch.
>=20

Agree. It has confused me a lot. I suggest changing the code to the
followings. The folio_test_large() check is still to avoid unexpected
users for OOB.

static inline void folio_set_compound_order(struct folio *folio,
					    unsigned int order)
{
	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
	// or
	// if (!folio_test_large(folio))
	// 	return;

	folio->_folio_order =3D order;
#ifdef CONFIG_64BIT
	folio->_folio_nr_pages =3D order ? 1U << order : 0;
#endif
}

Thanks.