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Date: Thu, 25 Apr 2024 09:46:01 +0100
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Subject: Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages
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To: David Hildenbrand <david@redhat.com>,
 Baolin Wang <baolin.wang@linux.alibaba.com>, akpm@linux-foundation.org,
 hughd@google.com
Cc: willy@infradead.org, wangkefeng.wang@huawei.com, 21cnbao@gmail.com,
 ying.huang@intel.com, shy828301@gmail.com, ziy@nvidia.com,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <cover.1713755580.git.baolin.wang@linux.alibaba.com>
 <4b998e7d-153f-48cc-a9bb-8c84bb675581@arm.com>
 <c1f68109-7665-4905-996f-f1067dfa2cb6@linux.alibaba.com>
 <80b5f87e-c156-4ccc-98f0-96f1fd864273@arm.com>
 <ef4f15dd-da31-4a1e-bec5-62a7002c4f7c@linux.alibaba.com>
 <5b8b22e7-6355-4b08-b5b5-1e33ebae6f16@arm.com>
 <813fe7fd-3004-4e8b-801d-95c33559a025@linux.alibaba.com>
 <76f816dd-3bbf-48c9-a630-3787051cf289@arm.com>
 <8c0d6358-3c16-4a57-822c-04b3b3403fe6@linux.alibaba.com>
 <4204b5f6-21f0-4aa2-a625-3dd2f416b649@arm.com>
 <94ae96f7-79ce-4b3f-a272-6af62d01a3f8@redhat.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <94ae96f7-79ce-4b3f-a272-6af62d01a3f8@redhat.com>
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On 25/04/2024 09:26, David Hildenbrand wrote:
> On 25.04.24 10:17, Ryan Roberts wrote:
>> 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.
> 
> Is the question what should happen if we "inherit" "force" or if someone
> specifies "force" for a mTP size explicitly?

Well I think it amounts to the same thing; there isn't much point in forbidding
"force" to be set directly because it can still be set indirectly through
"inherit". We can't forbid indirectly setting it, because "inherit" could be set
first, then the top-level shmem_enabled changed to "force" after - and we
wouldn't want to fail that.

So I think the question is just 'what should happen when "force" is configured
for a non-PMD-sized mTHP'?

I think the answer is 'for now, it behaves like "always", but in future it will
also force any tmpfs mounts to consider that mTHP size even if that size wasn't
specified by mount='