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List:       linux-ext4
Subject:    Re: [PATCH v7 04/14] mm: add device coherent vma selection for memory migration
From:       David Hildenbrand <david () redhat ! com>
Date:       2022-06-30 20:56:01
Message-ID: 24577304-15ea-0c9c-9b73-946143bf2726 () redhat ! com
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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

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