Date: Thu, 25 Apr 2024 14:20:11 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Ryan Roberts <ryan.roberts@....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 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.

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