Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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?

> 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(-)
>>>>>>