Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and arm64

[Matthew Wilcox]

On Tue, Jan 23, 2024 at 10:34:21AM +0000, Ryan Roberts wrote:
> > +#define PFN_PTE_SHIFT		PAGE_SHIFT
> 
> I think this is buggy. And so is the arm64 implementation of set_ptes(). It
> works fine for 48-bit output address, but for 52-bit OAs, the high bits are not
> kept contigously, so if you happen to be setting a mapping for which the
> physical memory block straddles bit 48, this won't work.

I'd like to see the folio allocation that can straddle bit 48 ...

agreed, it's not workable _in general_, but specifically for a memory
allocation from a power-of-two allocator, you'd have to do a 49-bit
allocation (half a petabyte) to care.