Re: [PATCH 0/8] add mTHP support for anonymous shmem

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

On 2024/5/8 22:28, Daniel Gomez wrote:
> On Wed, May 08, 2024 at 01:58:19PM +0200, David Hildenbrand wrote:
>> On 08.05.24 13:39, Daniel Gomez wrote:
>>> On Mon, May 06, 2024 at 04:46:24PM +0800, 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.
>>>>
>>>> The primary strategy is similar to supporting anonymous mTHP. Introduce
>>>> a new interface '/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', with adding a new
>>>> additional "inherit" option. By default all sizes will be set to "never"
>>>> except PMD size, which is set to "inherit". This ensures backward compatibility
>>>> with the shmem enabled of the top level, meanwhile also allows independent
>>>> control of shmem enabled for each mTHP.
>>>
>>> I'm trying to understand the adoption of mTHP and how it fits into the adoption
>>> of (large) folios that the kernel is moving towards. Can you, or anyone involved
>>> here, explain this? How much do they overlap, and can we benefit from having
>>> both? Is there any argument against the adoption of large folios here that I
>>> might have missed?
>>
>> mTHP are implemented using large folios, just like traditional PMD-sized THP
>> are. (you really should explore the history of mTHP and how it all works
>> internally)
> 
> I'll check more in deep the code. By any chance are any of you going to be at
> LSFMM this year? I have this session [1] scheduled for Wednesday and it would
> be nice to get your feedback on it and if you see this working together with
> mTHP/THP.
> 
> [1] https://lore.kernel.org/all/4ktpayu66noklllpdpspa3vm5gbmb5boxskcj2q6qn7md3pwwt@kvlu64pqwjzl/

Great. I'm also interested in tmpfs support for large folios (or mTHP), 
so please CC me if you plan to send a new version.

As David mentioned, this patchset is mainly about adding mTHP support 
for anonymous shmem, and I think that some of the swap support for large 
folios could work together.