Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM

[Ralph Campbell]

On 10/31/19 10:34 AM, Jason Gunthorpe wrote:
> On Thu, Oct 31, 2019 at 10:28:12AM -0700, Ralph Campbell wrote:
> > > > > > > It seems especially over-complicated to use a full page table layout
> > > > > > > for this, wouldn't something simple like an xarray be good enough for
> > > > > > > test purposes?
> > > > > >
> > > > > > Possibly. A page table is really just a lookup table from virtual address
> > > > > > to pfn/page. Part of the rationale was to mimic what a real device
> > > > > > might do.
> > > > >
> > > > > Well, but the details of the page table layout don't see really
> > > > > important to this testing, IMHO.
> > > >
> > > > One problem with XArray is that on 32-bit machines the value would
> > > > need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
> > > > I guess we could make the driver 64-bit only.
> > >
> > > Why would a 32 bit machine need a 64 bit pfn?
> >
> > On x86, Physical Address Extension (PAE) uses a 64 bit PTE.
> > See arch/x86/include/asm/pgtable_32_types.h which includes
> > arch/x86/include/asm/pgtable-3level_types.h.
>
> That is the content of the PTE, not the address of the PTE. In this
> case the xarray index is the 'virtual' address of the fictional device
> and it can easily be 32 bits with no problem
>
> Jason

Oh, I see. You mean use a 32-bit user virtual address for the index
and store a pointer to the 64-bit PTE which of course would be
32 bit. That should work.
I was stuck on thinking the PTE needed to be stored.