Message-ID: <5b8b22e7-6355-4b08-b5b5-1e33ebae6f16@arm.com>
Date: Wed, 24 Apr 2024 11:01:11 +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 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
echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled

What does that mean?

> 
>>>> Of course the huge= mount option would also need to take a per-size option in
>>>> this case. e.g. huge=2048kB:advise,64kB:always
>>>
>>> IMO, I do not want to change the global 'huge=' mount option, which can control
>>> both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's
>>
>> How does huge= control anon shmem? I thought it was only for mounted
>> filesystems; so tmpfs? Perhaps my mental model for how this works is broken...
> 
> Sorry for noise, you are right. So this is still the reason I don't want to
> change the semantics of 'huge=', which is used to control tmpfs.
> 
>>> keep it be same with the global sysfs interface:
>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled.
>>>
>>> For tmpfs large folio strategy, I plan to address it later, and we may need more
>>> discussion to determine if it should follow the file large folio strategy or not
>>> (no investigation now).
>>
>> OK. But until you get to tmpfs, you'll need an interim definition for what it
>> means if a per-size control is set to "inherit" and the top-level control is set
>> to deny/force.
>>
>>>
>>> Thanks for reviewing.
>>
>> No problem! Thanks for doing the work!
>>
>>>
>>>>> TODO:
>>>>>    - More testing and provide some performance data.
>>>>>    - Need more discussion about the large folio allocation strategy for a
>>>>> 'regular
>>>>> file' operation created by memfd_create(), for example using ftruncate(fd) to
>>>>> specify
>>>>> the 'file' size, which need to follow the anonymous mTHP rule too?
>>>>>    - Do not split the large folio when share memory swap out.
>>>>>    - Can swap in a large folio for share memory.
>>>>>
>>>>> Baolin Wang (5):
>>>>>     mm: memory: extend finish_fault() to support large folio
>>>>>     mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio()
>>>>>     mm: shmem: add THP validation for PMD-mapped THP related statistics
>>>>>     mm: shmem: add mTHP support for anonymous share pages
>>>>>     mm: shmem: add anonymous share mTHP counters
>>>>>
>>>>>    include/linux/huge_mm.h |   4 +-
>>>>>    mm/huge_memory.c        |   8 ++-
>>>>>    mm/memory.c             |  25 +++++++---
>>>>>    mm/shmem.c              | 107 ++++++++++++++++++++++++++++++----------
>>>>>    4 files changed, 108 insertions(+), 36 deletions(-)
>>>>>

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