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

[David Hildenbrand]

> 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.

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