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Subject: Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero
 support
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <23fa4ffa-965a-da80-e8b5-73ae92dc5767@nvidia.com>
Date:   Wed, 14 Dec 2022 11:00:02 +0800
Cc:     Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>, tsahu@linux.ibm.com,
        David Hildenbrand <david@redhat.com>
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References: <20221207223731.32784-1-sidhartha.kumar@oracle.com>
 <92965844-c430-8b8e-d9f1-705d7578bceb@nvidia.com>
 <ec8f46ca-9ea6-4567-2038-22f6d3000ed5@oracle.com>
 <d17530ad-8e12-8069-d619-a2d72fe80e15@nvidia.com>
 <0187f9c2-e80a-9cde-68bc-c9bdbd96b6fe@oracle.com>
 <Y5I78soNmAFv7pi8@casper.infradead.org> <Y5JCi3h8bUzLf3cu@monkey>
 <2723541a-79aa-c6b5-d82c-53db76b78145@oracle.com>
 <e86ca90f-e59e-3851-7225-b5f596ad04b9@nvidia.com>
 <36ddac45-ecd0-e2d2-e974-8c85ca503053@oracle.com>
 <20cc2088-b66e-28d1-a529-414e82146336@nvidia.com>
 <434a111c-7f1a-0018-6bd2-561cb382deea@oracle.com>
 <7d72778e-7305-18e9-edf4-ed55a98aa7e2@nvidia.com>
 <00222280-DBDD-49A3-92A5-05112359AE30@linux.dev>
 <f4909342-eaca-8c55-ad95-359ab7a5e6db@nvidia.com>
 <23fa4ffa-965a-da80-e8b5-73ae92dc5767@nvidia.com>
To:     John Hubbard <jhubbard@nvidia.com>
Precedence: bulk


> On Dec 10, 2022, at 05:20, John Hubbard <jhubbard@nvidia.com> wrote:
>=20
> On 12/9/22 13:10, John Hubbard wrote:
>> On 12/9/22 06:27, Muchun Song wrote:
>>>  =46rom you advise, I think we can remove VM_BUG_ON and handle =
non-zero
>>> order page, something like:
>> Yes, and thanks for summarizing all the individual feedback into a
>> proposed solution.
>> If we go this route, then I'd suggest a little note above the =
function,
>> such as:
>> /*
>>  * For non-large folios, this will have no effect, other than =
possibly
>>  * generating a warning, if the caller attempts to set a non-zero =
folio order
>>  * for a non-large folio.
>>  */
>>> static inline void folio_set_order(struct folio *folio,
>>>                            unsigned int order)
>>> {
>>>     if (!folio_test_large(folio)) {
>>>         WARN_ON(order);
>=20
> Although, on second thought...I'm still a little confused about why
> keeping the same name is so important?

Just my personal preference. I like its simplicity. I'm not against
large_folio_set_order, but suggest folio_set_order.

Thanks.

>=20
> A very direct approach that has more accurate naming (and therefore no
> need for a strange comment explaining the behavior) would be:
>=20
>=20
> static inline void large_folio_set_order(struct folio *folio,
> unsigned int order)
> {
> 	if (WARN_ON_ONCE(!folio_test_large(folio)))
> 		return;
>=20
> 	folio->_folio_order =3D order;
> #ifdef CONFIG_64BIT
> 	folio->_folio_nr_pages =3D order ? 1U << order : 0;
> #endif
> }
>=20
>=20
> thanks,
> --=20
> John Hubbard
> NVIDIA