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

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Message-ID: <91923595-7f02-3be0-9c59-9c1fd20c82a8@intel.com>
Date:   Wed, 20 Sep 2017 16:46:41 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tycho Andersen <tycho@...ker.com>,
        Yisheng Xie <xieyisheng1@...wei.com>
Cc:     Juerg Haefliger <juerg.haefliger@...onical.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>, x86@...nel.org
Subject: Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame
 Ownership (XPFO)

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"?

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.