Date: Wed, 24 Apr 2024 14:55:26 +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 2024/4/23 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

>> 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'.

> 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 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).

Thanks for reviewing.

>> 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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