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

[Balbir Singh <balbirs@nvidia.com>]

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