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

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Date:   Mon, 8 Mar 2021 20:30:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>
Subject: Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting
 THP tests.

On 08.03.21 20:11, Yang Shi wrote:
> On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@...dia.com> 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@...dia.com>
>>>>>>
>>>>>> 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@google.com
>>
>> 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?

-- 
Thanks,

David / dhildenb