Re: [PATCH 13/16] mm: support THP migration to device private memory

["Huang\, Ying" <ying.huang@intel.com>]

Ralph Campbell <rcampbell@nvidia.com> writes:

> On 6/22/20 4:54 PM, Yang Shi wrote:
>> On Mon, Jun 22, 2020 at 4:02 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>>
>>> On 2020-06-22 15:33, Yang Shi wrote:
>>>> On Mon, Jun 22, 2020 at 3:30 PM Yang Shi <shy828301@gmail.com> wrote:
>>>>> On Mon, Jun 22, 2020 at 2:53 PM Zi Yan <ziy@nvidia.com> wrote:
>>>>>> On 22 Jun 2020, at 17:31, Ralph Campbell wrote:
>>>>>>> On 6/22/20 1:10 PM, Zi Yan wrote:
>>>>>>>> On 22 Jun 2020, at 15:36, Ralph Campbell wrote:
>>>>>>>>> On 6/21/20 4:20 PM, Zi Yan wrote:
>>>>>>>>>> On 19 Jun 2020, at 17:56, Ralph Campbell wrote:
>>> ...
>>>>>> Ying(cc’d) developed the code to swapout and swapin THP in one piece: https://lore.kernel.org/linux-mm/20181207054122.27822-1-ying.huang@intel.com/.
>>>>>> I am not sure whether the patchset makes into mainstream or not. It could be a good technical reference
>>>>>> for swapping in device private pages, although swapping in pages from disk and from device private
>>>>>> memory are two different scenarios.
>>>>>>
>>>>>> Since the device private memory swapin impacts core mm performance, we might want to discuss your patches
>>>>>> with more people, like the ones from Ying’s patchset, in the next version.
>>>>>
>>>>> I believe Ying will give you more insights about how THP swap works.
>>>>>
>>>>> But, IMHO device memory migration (migrate to system memory) seems
>>>>> like THP CoW more than swap.
>>>
>>>
>>> A fine point: overall, the desired behavior is "migrate", not CoW.
>>> That's important. Migrate means that you don't leave a page behind, even
>>> a read-only one. And that's exactly how device private migration is
>>> specified.
>>>
>>> We should try to avoid any erosion of clarity here. Even if somehow
>>> (really?) the underlying implementation calls this THP CoW, the actual
>>> goal is to migrate pages over to the device (and back).
>>>
>>>
>>>>>
>>>>> When migrating in:
>>>>
>>>> Sorry for my fat finger, hit sent button inadvertently, let me finish here.
>>>>
>>>> When migrating in:
>>>>
>>>>           - if THP is enabled: allocate THP, but need handle allocation
>>>> failure by falling back to base page
>>>>           - if THP is disabled: fallback to base page
>>>>
>>>
>>> OK, but *all* page entries (base and huge/large pages) need to be cleared,
>>> when migrating to device memory, unless I'm really confused here.
>>> So: not CoW.
>>
>> I realized the comment caused more confusion. I apologize for the
>> confusion. Yes, the trigger condition for swap/migration and CoW are
>> definitely different. Here I mean the fault handling part of migrating
>> into system memory.
>>
>> Swap-in just needs to handle the base page case since THP swapin is
>> not supported in upstream yet and the PMD is split in swap-out phase
>> (see shrink_page_list).
>>
>> The patch adds THP migration support to device memory, but you need to
>> handle migrate in (back to system memory) case correctly. The fault
>> handling should look like THP CoW fault handling behavior (before
>> 5.8):
>>      - if THP is enabled: allocate THP, fallback if allocation is failed
>>      - if THP is disabled: fallback to base page
>>
>> Swap fault handling doesn't look like the above. So, I said it seems
>> like more THP CoW (fault handling part only before 5.8). I hope I
>> articulate my mind.
>>
>> However, I didn't see such fallback is handled. It looks if THP
>> allocation is failed, it just returns SIGBUS; and no check about THP
>> status if I read the patches correctly. The THP might be disabled for
>> the specific vma or system wide before migrating from device memory
>> back to system memory.
>
> You are correct, the patch wasn't handling the fallback case.
> I'll add that in the next version.

For fallback, you need to split the THP in device firstly.  Because you
will migrate a base page instead a whole THP now.

Best Regards,
Huang, Ying

>>>
>>> thanks,
>>> --
>>> John Hubbard
>>> NVIDIA