linux-kernel - Re: [v1 resend 04/12] mm/migrate_device: THP migration of zone device pages

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Message-ID: <83354b1f-8170-4f73-933f-5127f1d12dc6@nvidia.com>
Date: Fri, 18 Jul 2025 17:04:39 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Matthew Brost <matthew.brost@...el.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>,
 Alistair Popple <apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 04/12] mm/migrate_device: THP migration of zone device
 pages

On 7/18/25 16:59, Matthew Brost wrote:
> On Fri, Jul 04, 2025 at 09:35:03AM +1000, Balbir Singh wrote:
>> +	if (thp_migration_supported() &&
>> +		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
>> +		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
>> +		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
>> +		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
>> +						MIGRATE_PFN_COMPOUND;
>> +		migrate->dst[migrate->npages] = 0;
>> +		migrate->npages++;
>> +		migrate->cpages++;
> 
> It's a bit unclear what cpages and npages actually represent when
> collecting a THP. In my opinion, they should reflect the total number of
> minimum sized pages collected—i.e., we should increment by the shifted
> order (512) here. I'm fairly certain the logic in migrate_device.c would
> break if a 4MB range was requested and a THP was found first, followed by a
> non-THP.
> 

cpages and npages represent entries in the array and when or'ed with MIGRATE_PFN_COMPOUND
represent the right number of entries populated. If you have a test that shows
the breakage, I'd be keen to see it. We do populate other entries in 4k size(s) when
collecting to allow for a split of the folio.

Thanks for the review,
Balbir Singh