x86, xen: Disable highmem PTE allocation even when CONFIG_HIGHPTE=y

There's a path in the pagefault code where the kernel deliberately
breaks its own locking rules by kmapping a high pte page without
holding the pagetable lock (in at least page_check_address). This
breaks Xen's ability to track the pinned/unpinned state of the
page. There does not appear to be a viable workaround for this
behaviour so simply disable HIGHPTE for all Xen guests.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
LKML-Reference: <1267204562-11844-1-git-send-email-ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pasi Kärkkäinen <pasik@iki.fi>
Cc: <stable@kernel.org> # .32.x: 14315592: Allow highmem user page tables to be disabled at boot time
Cc: <stable@kernel.org> # .32.x
Cc: <xen-devel@lists.xensource.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/xen/enlighten.c

@@ -50,6 +50,7 @@
 #include <asm/traps.h>
 #include <asm/setup.h>
 #include <asm/desc.h>
+#include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
 #include <asm/reboot.h>
@@ -1094,6 +1095,12 @@ asmlinkage void __init xen_start_kernel(void)
 
 	__supported_pte_mask |= _PAGE_IOMAP;
 
+	/*
+	 * Prevent page tables from being allocated in highmem, even
+	 * if CONFIG_HIGHPTE is enabled.
+	 */
+	__userpte_alloc_gfp &= ~__GFP_HIGHMEM;
+
 	/* Work out if we support NX */
 	x86_configure_nx();
 

arch/x86/xen/mmu.c

@@ -1432,14 +1432,15 @@ static void *xen_kmap_atomic_pte(struct page *page, enum km_type type)
 {
 	pgprot_t prot = PAGE_KERNEL;
 
+	/*
+	 * We disable highmem allocations for page tables so we should never
+	 * see any calls to kmap_atomic_pte on a highmem page.
+	 */
+	BUG_ON(PageHighMem(page));
+
 	if (PagePinned(page))
 		prot = PAGE_KERNEL_RO;
 
-	if (0 && PageHighMem(page))
-		printk("mapping highpte %lx type %d prot %s\n",
-		       page_to_pfn(page), type,
-		       (unsigned long)pgprot_val(prot) & _PAGE_RW ? "WRITE" : "READ");
-
 	return kmap_atomic_prot(page, type, prot);
 }
 #endif