xen: lock pte pages while pinning/unpinning

When a pagetable is created, it is made globally visible in the rmap
prio tree before it is pinned via arch_dup_mmap(), and remains in the
rmap tree while it is unpinned with arch_exit_mmap().

This means that other CPUs may race with the pinning/unpinning
process, and see a pte between when it gets marked RO and actually
pinned, causing any pte updates to fail with write-protect faults.

As a result, all pte pages must be properly locked, and only unlocked
once the pinning/unpinning process has finished.

In order to avoid taking spinlocks for the whole pagetable - which may
overflow the PREEMPT_BITS portion of preempt counter - it locks and pins
each pte page individually, and then finally pins the whole pagetable.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickens <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <ak@suse.de>
Cc: Keir Fraser <keir@xensource.com>
Cc: Jan Beulich <jbeulich@novell.com>

1 files changed, 21 insertions, 9 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index e334bf7cb327..4186cb6a7f5a 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -666,6 +666,15 @@ static __init void xen_alloc_pt_init(struct mm_struct *mm, u32 pfn)
         make_lowmem_page_readonly(__va(PFN_PHYS(pfn)));
 }
 
+static void pin_pagetable_pfn(unsigned level, unsigned long pfn)
+{
+        struct mmuext_op op;
+        op.cmd = level;
+        op.arg1.mfn = pfn_to_mfn(pfn);
+        if (HYPERVISOR_mmuext_op(&op, 1, NULL, DOMID_SELF))
+                BUG();
+}
+
 /* This needs to make sure the new pte page is pinned iff its being
    attached to a pinned pagetable. */
 static void xen_alloc_pt(struct mm_struct *mm, u32 pfn)
@@ -675,9 +684,10 @@ static void xen_alloc_pt(struct mm_struct *mm, u32 pfn)
         if (PagePinned(virt_to_page(mm->pgd))) {
                 SetPagePinned(page);
 
-                if (!PageHighMem(page))
+                if (!PageHighMem(page)) {
                         make_lowmem_page_readonly(__va(PFN_PHYS(pfn)));
-                else
+                        pin_pagetable_pfn(MMUEXT_PIN_L1_TABLE, pfn);
+                } else
                         /* make sure there are no stray mappings of
                            this page */
                         kmap_flush_unused();
@@ -690,8 +700,10 @@ static void xen_release_pt(u32 pfn)
         struct page *page = pfn_to_page(pfn);
 
         if (PagePinned(page)) {
-                if (!PageHighMem(page))
+                if (!PageHighMem(page)) {
+                        pin_pagetable_pfn(MMUEXT_UNPIN_TABLE, pfn);
                         make_lowmem_page_readwrite(__va(PFN_PHYS(pfn)));
+                }
         }
 }
 
@@ -806,15 +818,15 @@ static __init void xen_pagetable_setup_done(pgd_t *base)
         /* Actually pin the pagetable down, but we can't set PG_pinned
            yet because the page structures don't exist yet. */
         {
-                struct mmuext_op op;
+                unsigned level;
+
 #ifdef CONFIG_X86_PAE
-                op.cmd = MMUEXT_PIN_L3_TABLE;
+                level = MMUEXT_PIN_L3_TABLE;
 #else
-                op.cmd = MMUEXT_PIN_L3_TABLE;
+                level = MMUEXT_PIN_L2_TABLE;
 #endif
-                op.arg1.mfn = pfn_to_mfn(PFN_DOWN(__pa(base)));
-                if (HYPERVISOR_mmuext_op(&op, 1, NULL, DOMID_SELF))
-                        BUG();
+
+                pin_pagetable_pfn(level, PFN_DOWN(__pa(base)));
         }
 }