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

Tycho Andersen
2017-09-21T00:02:14Z

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