Re: [PATCH v7 04/14] mm: add device coherent vma selection for memory migration

David Hildenbrand
2022-06-30T20:56:17Z

On 30.06.22 13:44, Alistair Popple wrote:
>
> David Hildenbrand <david@redhat.com> writes:
>
>> On 29.06.22 05:54, Alex Sierra wrote:
>>> This case is used to migrate pages from device memory, back to system
>>> memory. Device coherent type memory is cache coherent from device and CPU
>>> point of view.
>>>
>>> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
>>> Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
>>> Reviewed-by: Alistair Poppple <apopple@nvidia.com>
>>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>>
>>
>> I'm not too familiar with this code, please excuse my naive questions:
>>
>>> @@ -148,15 +148,21 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>> if (is_writable_device_private_entry(entry))
>>> mpfn |= MIGRATE_PFN_WRITE;
>>> } else {
>>> - if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
>>> - goto next;
>>
>> Why not exclude MIGRATE_VMA_SELECT_DEVICE_PRIVATE here? IIRC that would
>> have happened before this change.
>
> I might be missing something as I don't quite follow - this path is for
> normal system pages so we only want to skip selecting them if
> MIGRATE_VMA_SELECT_SYSTEM or MIGRATE_VMA_SELECT_DEVICE_COHERENT aren't
> set.
>
> Note that MIGRATE_VMA_SELECT_DEVICE_PRIVATE doesn't apply here because
> we already know it's not a device private page by virtue of
> pte_present(pte) == True.

Ah, stupid me, pte_present(pte) is the key.

>
>>> pfn = pte_pfn(pte);
>>> - if (is_zero_pfn(pfn)) {
>>> + if (is_zero_pfn(pfn) &&
>>> + (migrate->flags & MIGRATE_VMA_SELECT_SYSTEM)) {
>>> mpfn = MIGRATE_PFN_MIGRATE;
>>> migrate->cpages++;
>>> goto next;
>>> }
>>> page = vm_normal_page(migrate->vma, addr, pte);
>>> + if (page && !is_zone_device_page(page) &&
>>
>> I'm wondering if that check logically belongs into patch #2.
>
> I don't think so as it would break functionality until the below
> conditionals are added - we explicitly don't want to skip
> is_zone_device_page(page) == False here because that is the pages we are
> trying to select.
>
> You could add in this:
>
>>> + !(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
>
> But then in patch 2 we know this can never be true because we've already
> checked for !MIGRATE_VMA_SELECT_SYSTEM there.

Ah, okay, thanks

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb