Re: [PATCH v2 1/2] mm: use aligned address in clear_gigantic_page()

Huang, Ying
2024-10-30T01:09:00Z

David Hildenbrand <david@redhat.com> writes:

> On 29.10.24 14:04, Kefeng Wang wrote:
>>>>>>>>>
>>>>>>>>> That should all be cleaned up ... process_huge_page() likely
>>>>>>>>> shouldn't
>>>>>>>>
>>>>>>>> Yes, let's fix the bug firstly,
>>>>>>>>
>>>>>>>>> be even consuming "nr_pages".
>>>>>>>>
>>>>>>>> No sure about this part, it uses nr_pages as the end and calculate
>>>>>>>> the
>>>>>>>> 'base'.
>>>>>>>
>>>>>>> It should be using folio_nr_pages().
>>>>>>
>>>>>> But process_huge_page() without an explicit folio argument, I'd like to
>>>>>> move the aligned address calculate into the folio_zero_user and
>>>>>> copy_user_large_folio(will rename it to folio_copy_user()) in the
>>>>>> following cleanup patches, or do it in the fix patches?
>>>>>
>>>>> First, why does folio_zero_user() call process_huge_page() for *a small
>>>>> folio*? Because we like or code to be extra complicated to understand?
>>>>> Or am I missing something important?
>>>>
>>>> The folio_zero_user() used for PMD-sized THP and HugeTLB before, and
>>>> after anon mTHP supported, it is used for order-2~order-PMD-order THP
>>>> and HugeTLB, so it won't process a small folio if I understand correctly.
>>>
>>> And unfortunately neither the documentation nor the function name
>>> expresses that :(
>>>
>>> I'm happy to review any patches that improve the situation here :)
>>>
>> Actually, could we drop the process_huge_page() totally, from my
>> testcase[1], process_huge_page() is not better than clear/copy page
>> from start to last, and sequential clearing/copying maybe more
>> beneficial to the hardware prefetching, and is there a way to let lkp
>> to test to check the performance, since the process_huge_page()
>> was submitted by Ying, what's your opinion?

I don't think that it's a good idea to revert the commit without
studying and root causing the issues. I can work together with you on
that. If we have solid and well explained data to prove
process_huge_page() isn't benefitial, we can revert the commit.

> I questioned that just recently [1], and Ying assumed that it still
> applies [2].
>
> c79b57e462b5 ("mm: hugetlb: clear target
> sub-page last when clearing huge page”) documents the scenario where
> this matters -- anon-w-seq which you also run below.
>
> If there is no performance benefit anymore, we should rip that
> out. But likely we should check on multiple micro-architectures with
> multiple #CPU configs that are relevant. c79b57e462b5 used a Xeon E5
> v3 2699 with 72 processes on 2 NUMA nodes, maybe your test environment
> cannot replicate that?
>
>
> [1]
> https://lore.kernel.org/linux-mm/b8272cb4-aee8-45ad-8dff-353444b3fa74@redhat.com/
> [2]
> https://lore.kernel.org/linux-mm/878quv9lhf.fsf@yhuang6-desk2.ccr.corp.intel.com/
>
>> [1]https://lore.kernel.org/linux-mm/2524689c-08f5-446c-8cb9-924f9db0ee3a@huawei.com/
>> case-anon-w-seq-mt (tried 2M PMD THP/ 64K mTHP)
>> case-anon-w-seq-hugetlb (2M PMD HugeTLB)
>
> But these are sequential, not random. I'd have thought access +
> zeroing would be sequentially either way. Did you run with random
> access as well>

--
Best Regards,
Huang, Ying