Message-ID: <4204b5f6-21f0-4aa2-a625-3dd2f416b649@arm.com>
Date: Thu, 25 Apr 2024 09:17:44 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, david@...hat.com, wangkefeng.wang@...wei.com,
 21cnbao@...il.com, ying.huang@...el.com, shy828301@...il.com,
 ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages

On 25/04/2024 07:20, Baolin Wang wrote:
> 
> 
> On 2024/4/24 22:20, Ryan Roberts wrote:
>> On 24/04/2024 14:49, Baolin Wang wrote:
>>>
>>>
>>> On 2024/4/24 18:01, Ryan Roberts wrote:
>>>> On 24/04/2024 10:55, Baolin Wang wrote:
>>>>>
>>>>>
>>>>> On 2024/4/24 16:26, Ryan Roberts wrote:
>>>>>> On 24/04/2024 07:55, Baolin Wang wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote:
>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote:
>>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP)
>>>>>>>>> allocation
>>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured
>>>>>>>>> through the
>>>>>>>>> sysfs interface located at
>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
>>>>>>>>>
>>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule
>>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped
>>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through
>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
>>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous
>>>>>>>>> pages,
>>>>>>>>> also including the anonymous shared pages, in order to enjoy the
>>>>>>>>> benefits of
>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB
>>>>>>>>> miss
>>>>>>>>> etc.
>>>>>>>>
>>>>>>>> This sounds like a very useful addition!
>>>>>>>>
>>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf
>>>>>>>> benchmarks
>>>>>>>> for those workloads) that predominantly use shared anon memory?
>>>>>>>
>>>>>>> As far as I know, some database related workloads make extensive use of
>>>>>>> shared
>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely
>>>>>>> also
>>>>>>> uses shared anonymous memory. And I still need to do some investigation to
>>>>>>> measure the performance.
>>>>>>>
>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL
>>>>>>
>>>>>> Thanks for the pointer!
>>>>>>
>>>>>>>
>>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared
>>>>>>>>> pages
>>>>>>>>> still follows the global control determined by the mount option "huge="
>>>>>>>>> parameter
>>>>>>>>> or the sysfs interface at
>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
>>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is
>>>>>>>>> enabled.
>>>>>>>>> Subsequently, the mTHP sysfs interface
>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
>>>>>>>>> is checked to determine the mTHP size that can be used for large folio
>>>>>>>>> allocation
>>>>>>>>> for these anonymous shared pages.
>>>>>>>>
>>>>>>>> I'm not sure about this proposed control mechanism; won't it break
>>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP
>>>>>>>> used to
>>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it
>>>>>>>
>>>>>>> Yes, I realized this after more testing.
>>>>>>>
>>>>>>>> doesn't make sense to me that we now depend upon the
>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
>>>>>>>> default disables all sizes except 2M, which is set to "inherit" from
>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled).
>>>>>>>>
>>>>>>>> The other problem is that shmem_enabled has a different set of options
>>>>>>>> (always/never/within_size/advise/deny/force) to enabled
>>>>>>>> (always/madvise/never)
>>>>>>>>
>>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled;
>>>>>>>> Introduce
>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all
>>>>>>>> the
>>>>>>>> same values as the top-level
>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled,
>>>>>>>> plus the additional "inherit" option. By default all sizes will be set to
>>>>>>>> "never" except 2M, which is set to "inherit".
>>>>>>>
>>>>>>> Sounds good to me. But I do not want to copy all same values from top-level
>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled':
>>>>>>> always within_size advise never deny force
>>>>>>>
>>>>>>> For mTHP's shmem_enabled interface, we can just keep below values:
>>>>>>> always within_size advise never
>>>>>>>
>>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to
>>>>>>> 'never', and 'force' is equal to 'always'.
>>>>>>
>>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my
>>>>>> rough
>>>>>> understanding is:
>>>>>>
>>>>>>     - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls
>>>>>>       mmap(SHARED|ANON) allocations (mostly; see rule 3)
>>>>>>     - huge=... controls tmpfs allocations
>>>>>>     - deny and force in shmem_enabled are equivalent to never and always for
>>>>>>       mmap(SHARED|ANON) but additionally override all tmpfs mounts so they
>>>>>> act as
>>>>>>       if they were mounted with huge=never or huge=always
>>>>>>
>>>>>> Is that correct? If so, then I think it still makes sense to support per-size
>>>>>
>>>>> Correct.
>>>>>
>>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the
>>>>>> top-level control is set to deny or force, you would need that to mean
>>>>>> something.
>>>>>
>>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface
>>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs
>>>>> allocation,
>>>>> so we should not use anonymous control to override tmpfs control, which
>>>>> seems a
>>>>> little mess?
>>>>
>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave
>>>> the
>>>> tmpfs stuff for another time. But my point is that
>>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the
>>>> value is deny or force. So if you have:
>>>>
>>>> echo deny > /mm/transparent_hugepage/shmem_enabled
>>>
>>> IIUC, this global control will cause shmem_is_huge() to always return false, so
>>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set,
>>> anonymous shmem will not use mTHP. No?
>>
>> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and
>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow
>> the established pattern.
>>
>> For anon-private, each size is controlled by its
>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is
>> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used
>> for that size.
>>
>> That approach enables us to 1) maintain back-compat and 2) control each size
>> independently
>>
>> 1) is met because the default is that all sizes are initially set to "never",
>> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled)
>> which is initially set to inherit. So any mTHP unaware SW can still modify
>> /mm/transparent_hugepage/enabled and it will still only apply to PMD size.
>>
>> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size
>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to
>> modify the value in /mm/transparent_hugepage/enabled.
> 
> Thanks for explanation. Initially, I want to make
> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but
> I think it should follow the same strategy as anon mTHP as you said.
> 
>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled
>>>>
>>>> What does that mean?
>>
>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to
>> support the deny and force values. When applied to non-PMD sizes, "deny" can
>> just be a noop for now, because there was no way to configure a tmpfs mount for
>> non-PMD size THP in the first place. But I'm not sure what to do with "force"?
> 
> OK. And I also prefer that "force" should be a noop too, since anon shmem
> control should not configure tmpfs huge page allocation.

I guess technically they won't be noops, but (for the non-PMD-sizes) "force"
will be an alias for "always" and "deny" will be an alias for "never"?

I was just a bit concerned about later changing that behavior to also impact
tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it,
I don't see that as a problem.

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