linux-kernel - Re: [v1 resend 02/12] mm/migrate_device: flags for selecting device private THP pages

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Message-ID: <6eba6c3e-a579-44ca-af59-b8adc7dfffe1@nvidia.com>
Date: Tue, 8 Jul 2025 17:31:49 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org, Karol Herbst <kherbst@...hat.com>,
 Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Jérôme Glisse <jglisse@...hat.com>,
 Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
 Peter Xu <peterx@...hat.com>, Zi Yan <ziy@...dia.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>,
 Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 02/12] mm/migrate_device: flags for selecting device
 private THP pages

On 7/7/25 15:31, Alistair Popple wrote:
> On Fri, Jul 04, 2025 at 09:35:01AM +1000, Balbir Singh wrote:
>> Add flags to mark zone device migration pages.
>>
>> MIGRATE_VMA_SELECT_COMPOUND will be used to select THP pages during
>> migrate_vma_setup() and MIGRATE_PFN_COMPOUND will make migrating
>> device pages as compound pages during device pfn migration.
>>
>> Cc: Karol Herbst <kherbst@...hat.com>
>> Cc: Lyude Paul <lyude@...hat.com>
>> Cc: Danilo Krummrich <dakr@...nel.org>
>> Cc: David Airlie <airlied@...il.com>
>> Cc: Simona Vetter <simona@...ll.ch>
>> Cc: "Jérôme Glisse" <jglisse@...hat.com>
>> Cc: Shuah Khan <shuah@...nel.org>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Barry Song <baohua@...nel.org>
>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Cc: Peter Xu <peterx@...hat.com>
>> Cc: Zi Yan <ziy@...dia.com>
>> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
>> Cc: Jane Chu <jane.chu@...cle.com>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: Donet Tom <donettom@...ux.ibm.com>
>>
>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>> ---
>>  include/linux/migrate.h | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>> index aaa2114498d6..1661e2d5479a 100644
>> --- a/include/linux/migrate.h
>> +++ b/include/linux/migrate.h
>> @@ -167,6 +167,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>>  #define MIGRATE_PFN_VALID	(1UL << 0)
>>  #define MIGRATE_PFN_MIGRATE	(1UL << 1)
>>  #define MIGRATE_PFN_WRITE	(1UL << 3)
>> +#define MIGRATE_PFN_COMPOUND	(1UL << 4)
> 
> Why is this necessary? Couldn't migrate_vma just use folio_order() to figure out
> if it's a compound page or not?
> 

I can definitely explore that angle. As we move towards mTHP, we'll need additional bits for the various order sizes as well.

Balbir Singh