[Xen-devel] [PATCH 1/3] xen: disable highmem PTE allocation even when CONFIG_HIGHPTE=y

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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@xxxxxxxxxx>
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Pasi Kärkkäinen <pasik@xxxxxx>
Cc: <stable@xxxxxxxxxx> # .32.x: 14315592: Allow highmem user page tables to be 
disabled at boot time
Cc: <stable@xxxxxxxxxx> # .32.x
Cc: <xen-devel@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/xen/enlighten.c |    7 +++++++
 arch/x86/xen/mmu.c       |   11 ++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 36daccb..b607239 100644
--- a/arch/x86/xen/enlighten.c
+++ b/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();
 
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index bf4cd6b..350a3de 100644
--- a/arch/x86/xen/mmu.c
+++ b/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
-- 
1.5.6.5


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel