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

From: David Hildenbrand <david@redhat.com>
To: Yang Shi <shy828301@gmail.com>, Zi Yan <ziy@nvidia.com>
Cc: Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Shuah Khan <shuah@kernel.org>, John Hubbard <jhubbard@nvidia.com>,
	Sandipan Das <sandipan@linux.ibm.com>,
	David Rientjes <rientjes@google.com>,
	Alex Shi <alex.shi@linux.alibaba.com>
Subject: Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting THP tests.
Date: Mon, 8 Mar 2021 20:30:45 +0100	[thread overview]
Message-ID: <c530e051-9292-1b2c-3036-70fc1eac3bfa@redhat.com> (raw)
In-Reply-To: <CAHbLzkofsc4_wEGmMFKwkUrc1pg4Y7iZZ9QKAXggiyg8RuOMqA@mail.gmail.com>

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