Re: [RFC PATCH 07/26] hugetlb: add hugetlb_pte to track HugeTLB page table entries

[Dr. David Alan Gilbert]

* Mike Kravetz (mike.kravetz@xxxxxxxxxx) wrote:
> On 06/24/22 17:36, James Houghton wrote:
> > After high-granularity mapping, page table entries for HugeTLB pages can
> > be of any size/type. (For example, we can have a 1G page mapped with a
> > mix of PMDs and PTEs.) This struct is to help keep track of a HugeTLB
> > PTE after we have done a page table walk.
> 
> This has been rolling around in my head.
> 
> Will this first use case (live migration) actually make use of this
> 'mixed mapping' model where hugetlb pages could be mapped at the PUD,
> PMD and PTE level all within the same vma?  I only understand the use
> case from a high level.  But, it seems that we would want to only want
> to migrate PTE (or PMD) sized pages and not necessarily a mix.

I suspect we would pick one size and use that size for all transfers
when in postcopy; not sure if there are any side cases though.

> The only reason I ask is because the code might be much simpler if all
> mappings within a vma were of the same size.  Of course, the
> performance/latency of converting a large mapping may be prohibitively
> expensive.

Imagine we're migrating a few TB VM, backed by 1GB hugepages, I'm guessing it
would be nice to clean up the PTE/PMDs for split 1GB pages as they're
completed rather than having thousands of them for the whole VM.
(I'm not sure if that is already doable)

Dave

> Looking to the future when supporting memory error handling/page poisoning
> it seems like we would certainly want multiple size mappings.
> 
> Just a thought.
> -- 
> Mike Kravetz
> 
-- 
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK