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

[Ralph Campbell <rcampbell@nvidia.com>]

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.

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