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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.
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