Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting THP tests.

[David Hildenbrand]

> Am 08.03.2021 um 22:25 schrieb Yang Shi <shy828301@xxxxxxxxx>:
> 
> On Mon, Mar 8, 2021 at 12:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>> 
>> 
>>>> Am 08.03.2021 um 21:18 schrieb Yang Shi <shy828301@xxxxxxxxx>:
>>> 
>>> On Mon, Mar 8, 2021 at 11:30 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>> 
>>>>> On 08.03.21 20:11, Yang Shi wrote:
>>>>> On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>>>>>> 
>>>>>> On 8 Mar 2021, at 13:11, David Hildenbrand wrote:
>>>>>> 
>>>>>>> On 08.03.21 18:49, Zi Yan wrote:
>>>>>>>> On 8 Mar 2021, at 11:17, David Hildenbrand wrote:
>>>>>>>> 
>>>>>>>>> On 08.03.21 16:22, Zi Yan wrote:
>>>>>>>>>> From: Zi Yan <ziy@xxxxxxxxxx>
>>>>>>>>>> 
>>>>>>>>>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>>>>>>>>>> <debugfs>/split_huge_pages_in_range_pid, THPs in the process with the
>>>>>>>>>> given pid and virtual address range are split. It is used to test
>>>>>>>>>> split_huge_page function. In addition, a selftest program is added to
>>>>>>>>>> tools/testing/selftests/vm to utilize the interface by splitting
>>>>>>>>>> PMD THPs and PTE-mapped THPs.
>>>>>>>>> 
>>>>>>>>> Won't something like
>>>>>>>>> 
>>>>>>>>> 1. MADV_HUGEPAGE
>>>>>>>>> 
>>>>>>>>> 2. Access memory
>>>>>>>>> 
>>>>>>>>> 3. MADV_NOHUGEPAGE
>>>>>>>>> 
>>>>>>>>> Have a similar effect? What's the benefit of this?
>>>>>>>> 
>>>>>>>> Thanks for checking the patch.
>>>>>>>> 
>>>>>>>> No, MADV_NOHUGEPAGE just replaces VM_HUGEPAGE with VM_NOHUGEPAGE,
>>>>>>>> nothing else will be done.
>>>>>>> 
>>>>>>> Ah, okay - maybe my memory was tricking me. There is some s390x KVM code that forces MADV_NOHUGEPAGE and force-splits everything.
>>>>>>> 
>>>>>>> I do wonder, though, if this functionality would be worth a proper user interface (e.g., madvise), though. There might be actual benefit in having this as a !debug interface.
>>>>>>> 
>>>>>>> I think you aware of the discussion in https://lkml.kernel.org/r/d098c392-273a-36a4-1a29-59731cdf5d3d@xxxxxxxxxx
>>>>>> 
>>>>>> Yes. Thanks for bringing this up.
>>>>>> 
>>>>>>> 
>>>>>>> If there will be an interface to collapse a THP -- "this memory area is worth extra performance now by collapsing a THP if possible" -- it might also be helpful to have the opposite functionality -- "this memory area is not worth a THP, rather use that somehwere else".
>>>>>>> 
>>>>>>> MADV_HUGE_COLLAPSE vs. MADV_HUGE_SPLIT
>>>>>> 
>>>>>> I agree that MADV_HUGE_SPLIT would be useful as the opposite of COLLAPSE when user might just want PAGESIZE mappings.
>>>>>> Right now, HUGE_SPLIT is implicit from mapping changes like mprotect or MADV_DONTNEED.
>>>>> 
>>>>> IMHO, it sounds not very useful. MADV_DONTNEED would split PMD for any
>>>>> partial THP. If the range covers the whole THP, the whole THP is going
>>>>> to be freed anyway. All other places in kernel which need split THP
>>>>> have been covered. So I didn't realize any usecase from userspace for
>>>>> just splitting PMD to PTEs.
>>>> 
>>>> THP are a limited resource. So indicating which virtual memory regions
>>>> are not performance sensitive right now (e.g., cold pages in a databse)
>>>> and not worth a THP might be quite valuable, no?
>>> 
>>> Such functionality could be achieved by MADV_COLD or MADV_PAGEOUT,
>>> right? Then a subsequent call to MADV_NOHUGEPAGE would prevent from
>>> collapsing or allocating THP for that area.
>>> 
>> 
>> I remember these deal with optimizing swapping. Not sure how they interact with THP, especially on systems without swap - I would guess they don‘t as of now.
> 
> Yes, MADV_PAGEOUT would just swap the THP or sub pages out. I think I
> just forgot to mention MADV_FREE which would be more suitable for this
> usecase.
> 
>> 

Can you elaborate? MADV_FREE is destructive, just like a delayed MADV_DONTNEED. How would that help here?

>>>> 
>>>> --
>>>> Thanks,
>>>> 
>>>> David / dhildenb
>>>> 
>>> 
>> 
>