[PATCH v1 4/7] arm64/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

[Will Deacon]

On Mon, Mar 21, 2022 at 02:38:02PM +0000, Will Deacon wrote:
> On Wed, Mar 16, 2022 at 06:27:01PM +0000, Catalin Marinas wrote:
> > On Tue, Mar 15, 2022 at 03:18:34PM +0100, David Hildenbrand wrote:
> > > diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
> > > index b1e1b74d993c..62e0ebeed720 100644
> > > --- a/arch/arm64/include/asm/pgtable-prot.h
> > > +++ b/arch/arm64/include/asm/pgtable-prot.h
> > > @@ -14,6 +14,7 @@
> > >   * Software defined PTE bits definition.
> > >   */
> > >  #define PTE_WRITE		(PTE_DBM)		 /* same as DBM (51) */
> > > +#define PTE_SWP_EXCLUSIVE	(_AT(pteval_t, 1) << 2)	 /* only for swp ptes */
> > 
> > I think we can use bit 1 here.
> > 
> > > @@ -909,12 +925,13 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> > >  /*
> > >   * Encode and decode a swap entry:
> > >   *	bits 0-1:	present (must be zero)
> > > - *	bits 2-7:	swap type
> > > + *	bits 2:		remember PG_anon_exclusive
> > > + *	bits 3-7:	swap type
> > >   *	bits 8-57:	swap offset
> > >   *	bit  58:	PTE_PROT_NONE (must be zero)
> > 
> > I don't remember exactly why we reserved bits 0 and 1 when, from the
> > hardware perspective, it's sufficient for bit 0 to be 0 and the whole
> > pte becomes invalid. We use bit 1 as the 'table' bit (when 0 at pmd
> > level, it's a huge page) but we shouldn't check for this on a swap
> > entry.
> 
> I'm a little worried that when we're dealing with huge mappings at the
> PMD level we might lose the ability to distinguish them from a pte-level
> mapping with this new flag set if we use bit 1. A similar issue to this
> was fixed a long time ago by 59911ca4325d ("ARM64: mm: Move PTE_PROT_NONE
> bit") when we used to use bit 1 for PTE_PROT_NONE.
> 
> Is something like:
> 
> 	pmd_to_swp_entry(swp_entry_to_pmd(pmd));
> 
> supposed to preserve the original pmd? I'm not sure that's guaranteed
> after this change if bit 1 can be cleared in the process -- we could end
> up with a pte, which the hardware would interpret as a table entry and
> end up with really bad things happening.

(I got this back to front: having the bit set rather than cleared would
be an issue, but the overall point remains).

Will