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

[Ryan Roberts]

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?

> 
> 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
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".

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

> 
> 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(-)
>