Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

[Tycho Andersen]

On Wed, Sep 20, 2017 at 04:46:41PM -0700, Dave Hansen wrote:
> On 09/12/2017 11:13 AM, Tycho Andersen wrote:
> > -void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp)
> > +void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp, bool will_map)
> >  {
> >  	int i, flush_tlb = 0;
> >  	struct xpfo *xpfo;
> > @@ -116,8 +116,14 @@ void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp)
> >  			 * Tag the page as a user page and flush the TLB if it
> >  			 * was previously allocated to the kernel.
> >  			 */
> > -			if (!test_and_set_bit(XPFO_PAGE_USER, &xpfo->flags))
> > +			bool was_user = !test_and_set_bit(XPFO_PAGE_USER,
> > +							  &xpfo->flags);
> > +
> > +			if (was_user || !will_map) {
> > +				set_kpte(page_address(page + i), page + i,
> > +					 __pgprot(0));
> >  				flush_tlb = 1;
> > +			}
> 
> Shouldn't the "was_user" be "was_kernel"?

Oof, yes, thanks.

> Also, the way this now works, let's say we have a nice, 2MB pmd_t (page
> table entry) mapping a nice, 2MB page in the allocator.  Then it gets
> allocated to userspace.  We do
> 
> 	for (i = 0; i < (1 << order); i++)  {
> 		...
> 		set_kpte(page_address(page + i), page+i, __pgprot(0));
> 	}
> 
> The set_kpte() will take the nice, 2MB mapping and break it down into
> 512 4k mappings, all pointing to a non-present PTE, in a newly-allocated
> PTE page.  So, you get the same result and waste 4k of memory in the
> process, *AND* make it slower because we added a level to the page tables.
> 
> I think you actually want to make a single set_kpte() call at the end of
> the function.  That's faster and preserves the large page in the direct
> mapping.

...and makes it easier to pair tlb flushes with changing the
protections. I guess we still need the for loop, because we need to
set/unset the xpfo bits as necessary, but I'll switch it to a single
set_kpte(). This also implies that the xpfo bits should all be the
same on every page in the mapping, which I think is true.

This will be a nice change, thanks!

Tycho