Re: [PATCH v2 2/3] mm/hmm: allow snapshot of the special zero page

[Ralph Campbell <rcampbell@nvidia.com>]

On 10/21/19 11:49 AM, Jerome Glisse wrote:
> On Tue, Oct 15, 2019 at 01:48:13PM -0700, Ralph Campbell wrote:
>> Allow hmm_range_fault() to return success (0) when the CPU pagetable
>> entry points to the special shared zero page.
>> The caller can then handle the zero page by possibly clearing device
>> private memory instead of DMAing a zero page.
> 
> I do not understand why you are talking about DMA. GPU can work
> on main memory and migrating to GPU memory is optional and should
> not involve this function at all.

Good point. This is the device accessing the zero page over PCIe
or another bus, not migrating a zero page to device private memory.
I'll update the wording.

>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> Cc: "Jérôme Glisse" <jglisse@redhat.com>
>> Cc: Jason Gunthorpe <jgg@mellanox.com>
> 
> NAK please keep semantic or change it fully. See the alternative
> below.
> 
>> ---
>>   mm/hmm.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index 5df0dbf77e89..f62b119722a3 100644
>> --- a/mm/hmm.c
>> +++ b/mm/hmm.c
>> @@ -530,7 +530,9 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>>   			return -EBUSY;
>>   	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
>>   		*pfn = range->values[HMM_PFN_SPECIAL];
>> -		return -EFAULT;
>> +		if (!is_zero_pfn(pte_pfn(pte)))
>> +			return -EFAULT;
>> +		return 0;
> 
> An acceptable change would be to turn the branch into:
> 	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> 		if (!is_zero_pfn(pte_pfn(pte))) {
> 			*pfn = range->values[HMM_PFN_SPECIAL];
> 			return -EFAULT;
> 		}
> 		/* Fall-through for zero pfn (if write was needed the above
> 		 * hmm_pte_need_faul() would had catched it).
> 		 */
> 	}
> 

Except this will return the zero pfn with no indication that it is special
(i.e., doesn't have a struct page).