Re: [PATCH v3 0/6] add mTHP support for anonymous shmem

[David Hildenbrand <david@redhat.com>]

On 06.06.24 05:31, Baolin Wang wrote:
> 
> 
> On 2024/6/4 20:05, Daniel Gomez wrote:
>> On Tue, Jun 04, 2024 at 05:45:20PM +0800, Baolin Wang wrote:
>>>
>>>
>>> On 2024/6/4 16:18, Daniel Gomez wrote:
>>>> On Fri, May 31, 2024 at 01:13:48PM +0200, David Hildenbrand wrote:
>>>>>>>
>>>>>>> As a default, we should not be using large folios / mTHP for any shmem,
>>>>>>> just like we did with THP via shmem_enabled. This is what this series
>>>>>>> currently does, and is aprt of the whole mTHP user-space interface design.
>>>>>>>
>>>>>>> Further, the mTHP controls should control all of shmem, not only
>>>>>>> "anonymous shmem".
>>>>>>
>>>>>> Yes, that's what I thought and in my TODO list.
>>>>>
>>>>> Good, it would be helpful to coordinate with Daniel and Pankaj.
>>>>
>>>> I've integrated patches 11 and 12 from the lsf RFC thread [1] on top of Baolin's
>>>> v3 patches. You may find a version in my integration branch here [2]. I can
>>>> attach them here if it's preferred.
>>>>
>>>> [1] https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@samsung.com/
>>>> [2] https://protect2.fireeye.com/v1/url?k=a23e7c06-c3b56926-a23ff749-74fe485fb347-371ca2bfd5d9869f&q=1&e=6974304e-a786-4255-93a7-57498540241c&u=https%3A%2F%2Fgitlab.com%2Fdkruces%2Flinux-next%2F-%2Fcommits%2Fnext-20240604-shmem-mthp
>>>>
>>>> The point here is to combine the large folios strategy I proposed with mTHP
>>>> user controls. Would it make sense to limit the orders to the mapping order
>>>> calculated based on the size and index?
>>>
>>> IMO, for !anon shmem, this change makes sense to me. We should respect the
>>> size and mTHP should act as a order filter.
>>
>> What about respecing the size when within_size flag is enabled? Then, 'always'
>> would allocate mTHP enabled folios, regardless of the size. And 'never'
>> would ignore mTHP and size. So, 'never' can be used for this 'safe' boot case
>> mentioned in the discussion.
> 
> Looks reasonable to me. What do you think, David?
> 

That mimics existing PMD-THP behavior, right?

With "within_size", we must not exceed the size, with "always", we may 
exceed the size.

> And what about 'advise' option? Silimar to 'within_size'?

Good question. What's the behavior of PMD-THP? I would assume it behaves 
like "within_size", because in the common case we mmap (+advise) only 
within the size of the file, not exceeding it.

(the always option, as I learned during the meeting, is primarily 
helpful when writing to tmpfs files in an append-fashion. With 
mmap()+madvise() that doesn't quite happen)

-- 
Cheers,

David / dhildenb