Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64

[Michal Hocko]

On Mon 28-09-20 13:53:58, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
> 
> Hi all,
> 
> This patchset adds support for 1GB PUD THP on x86_64. It is on top of
> v5.9-rc5-mmots-2020-09-18-21-23. It is also available at:
> https://github.com/x-y-z/linux-1gb-thp/tree/1gb_thp_v5.9-rc5-mmots-2020-09-18-21-23
> 
> Other than PUD THP, we had some discussion on generating THPs and contiguous
> physical memory via a synchronous system call [0]. I am planning to send out a
> separate patchset on it later, since I feel that it can be done independently of
> PUD THP support.

While the technical challenges for the kernel implementation can be
discussed before the user API is decided I believe we cannot simply add
something now and then decide about a proper interface. I have raised
few basic questions we should should find answers for before the any
interface is added. Let me copy them here for easier reference
- THP allocation time - #PF and/or madvise context
- lazy/sync instantiation
- huge page sizes controllable by the userspace?
- aggressiveness - how hard to try
- internal fragmentation - allow to create THPs on sparsely or unpopulated
  ranges
- do we need some sort of access control or privilege check as some THPs
  would be a really scarce (like those that require pre-reservation).
-- 
Michal Hocko
SUSE Labs