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

Alistair Popple
2022-06-30T12:07:30Z

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.

>> 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.

>> + goto next;
>> + else if (page && is_device_coherent_page(page) &&
>> + (!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
>> + page->pgmap->owner != migrate->pgmap_owner))
>
>
> In general LGTM