Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

[Khalid Aziz]

On 1/18/22 14:46, Matthew Wilcox wrote:
> On Tue, Jan 18, 2022 at 01:41:40PM -0800, Dave Hansen wrote:
> > On 1/18/22 1:19 PM, Khalid Aziz wrote:
> > > - Starting address must be aligned to pgdir size (512GB on x86_64)
> >
> > How does this work on systems with 5-level paging where a top-level page
> > table entry covers 256TB?  Is the alignment requirement 512GB or 256TB?
> >   How does userspace figure out which alignment requirement it is subject to?
>
> The original idea was any power of two, naturally aligned, >= PAGE_SIZE,
> but I suspect Khalid has simplified it for this first implementation.

Hi Dave,

Yes, this is mostly to keep code somewhat simpler. Large regions make it easier to manage the separate set of shared 
VMAs. Part of the exploration here is to see what size regions work for other people. This initial prototype is x86 only 
and for now I am using PGDIR_SIZE. I see your point about how would userspace figure out alignment since this should 
across all architectures and PGDIR_SIZE/PMD_SIZE/PUD_SIZE are not the same across architectures. We can choose a fixed 
size and alignment. I would like to keep the region size at 2^20 pages or larger to minimize having to manage large 
number of small shared regions if those regions are not contiguous. Do you have any suggestion?

Thanks for your feedback.

--
Khalid