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>

2 files changed, 103 insertions, 40 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)));
         }
 }
 
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 72f08ab43a4d..b2e32f9d0071 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -303,7 +303,12 @@ pgd_t xen_make_pgd(unsigned long pgd)
 }
 #endif  /* CONFIG_X86_PAE */
 
-
+enum pt_level {
+        PT_PGD,
+        PT_PUD,
+        PT_PMD,
+        PT_PTE
+};
 
 /*
   (Yet another) pagetable walker.  This one is intended for pinning a
@@ -315,7 +320,7 @@ pgd_t xen_make_pgd(unsigned long pgd)
   FIXADDR_TOP.  But the important bit is that we don't pin beyond
   there, because then we start getting into Xen's ptes.
 */
-static int pgd_walk(pgd_t *pgd_base, int (*func)(struct page *, unsigned),
+static int pgd_walk(pgd_t *pgd_base, int (*func)(struct page *, enum pt_level),
                     unsigned long limit)
 {
         pgd_t *pgd = pgd_base;
@@ -340,7 +345,7 @@ static int pgd_walk(pgd_t *pgd_base, int (*func)(struct page *, unsigned),
                 pud = pud_offset(pgd, 0);
 
                 if (PTRS_PER_PUD > 1) /* not folded */
-                        flush |= (*func)(virt_to_page(pud), 0);
+                        flush |= (*func)(virt_to_page(pud), PT_PUD);
 
                 for (; addr != pud_limit; pud++, addr = pud_next) {
                         pmd_t *pmd;
@@ -359,7 +364,7 @@ static int pgd_walk(pgd_t *pgd_base, int (*func)(struct page *, unsigned),
                         pmd = pmd_offset(pud, 0);
 
                         if (PTRS_PER_PMD > 1) /* not folded */
-                                flush |= (*func)(virt_to_page(pmd), 0);
+                                flush |= (*func)(virt_to_page(pmd), PT_PMD);
 
                         for (; addr != pmd_limit; pmd++) {
                                 addr += (PAGE_SIZE * PTRS_PER_PTE);
@@ -371,17 +376,47 @@ static int pgd_walk(pgd_t *pgd_base, int (*func)(struct page *, unsigned),
                                 if (pmd_none(*pmd))
                                         continue;
 
-                                flush |= (*func)(pmd_page(*pmd), 0);
+                                flush |= (*func)(pmd_page(*pmd), PT_PTE);
                         }
                 }
         }
 
-        flush |= (*func)(virt_to_page(pgd_base), UVMF_TLB_FLUSH);
+        flush |= (*func)(virt_to_page(pgd_base), PT_PGD);
 
         return flush;
 }
 
-static int pin_page(struct page *page, unsigned flags)
+static spinlock_t *lock_pte(struct page *page)
+{
+        spinlock_t *ptl = NULL;
+
+#if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
+        ptl = __pte_lockptr(page);
+        spin_lock(ptl);
+#endif
+
+        return ptl;
+}
+
+static void do_unlock(void *v)
+{
+        spinlock_t *ptl = v;
+        spin_unlock(ptl);
+}
+
+static void xen_do_pin(unsigned level, unsigned long pfn)
+{
+        struct mmuext_op *op;
+        struct multicall_space mcs;
+
+        mcs = __xen_mc_entry(sizeof(*op));
+        op = mcs.args;
+        op->cmd = level;
+        op->arg1.mfn = pfn_to_mfn(pfn);
+        MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+}
+
+static int pin_page(struct page *page, enum pt_level level)
 {
         unsigned pgfl = test_and_set_bit(PG_pinned, &page->flags);
         int flush;
@@ -396,12 +431,26 @@ static int pin_page(struct page *page, unsigned flags)
                 void *pt = lowmem_page_address(page);
                 unsigned long pfn = page_to_pfn(page);
                 struct multicall_space mcs = __xen_mc_entry(0);
+                spinlock_t *ptl;
 
                 flush = 0;
 
+                ptl = NULL;
+                if (level == PT_PTE)
+                        ptl = lock_pte(page);
+
                 MULTI_update_va_mapping(mcs.mc, (unsigned long)pt,
                                         pfn_pte(pfn, PAGE_KERNEL_RO),
-                                        flags);
+                                        level == PT_PGD ? UVMF_TLB_FLUSH : 0);
+
+                if (level == PT_PTE)
+                        xen_do_pin(MMUEXT_PIN_L1_TABLE, pfn);
+
+                if (ptl) {
+                        /* Queue a deferred unlock for when this batch
+                           is completed. */
+                        xen_mc_callback(do_unlock, ptl);
+                }
         }
 
         return flush;
@@ -412,8 +461,7 @@ static int pin_page(struct page *page, unsigned flags)
    read-only, and can be pinned. */
 void xen_pgd_pin(pgd_t *pgd)
 {
-        struct multicall_space mcs;
-        struct mmuext_op *op;
+        unsigned level;
 
         xen_mc_batch();
 
@@ -424,16 +472,13 @@ void xen_pgd_pin(pgd_t *pgd)
                 xen_mc_batch();
         }
 
-        mcs = __xen_mc_entry(sizeof(*op));
-        op = mcs.args;
-
 #ifdef CONFIG_X86_PAE
-        op->cmd = MMUEXT_PIN_L3_TABLE;
+        level = MMUEXT_PIN_L3_TABLE;
 #else
-        op->cmd = MMUEXT_PIN_L2_TABLE;
+        level = MMUEXT_PIN_L2_TABLE;
 #endif
-        op->arg1.mfn = pfn_to_mfn(PFN_DOWN(__pa(pgd)));
-        MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+
+        xen_do_pin(level, PFN_DOWN(__pa(pgd)));
 
         xen_mc_issue(0);
 }
@@ -441,7 +486,7 @@ void xen_pgd_pin(pgd_t *pgd)
 /* The init_mm pagetable is really pinned as soon as its created, but
    that's before we have page structures to store the bits.  So do all
    the book-keeping now. */
-static __init int mark_pinned(struct page *page, unsigned flags)
+static __init int mark_pinned(struct page *page, enum pt_level level)
 {
         SetPagePinned(page);
         return 0;
@@ -452,18 +497,32 @@ void __init xen_mark_init_mm_pinned(void)
         pgd_walk(init_mm.pgd, mark_pinned, FIXADDR_TOP);
 }
 
-static int unpin_page(struct page *page, unsigned flags)
+static int unpin_page(struct page *page, enum pt_level level)
 {
         unsigned pgfl = test_and_clear_bit(PG_pinned, &page->flags);
 
         if (pgfl && !PageHighMem(page)) {
                 void *pt = lowmem_page_address(page);
                 unsigned long pfn = page_to_pfn(page);
-                struct multicall_space mcs = __xen_mc_entry(0);
+                spinlock_t *ptl = NULL;
+                struct multicall_space mcs;
+
+                if (level == PT_PTE) {
+                        ptl = lock_pte(page);
+
+                        xen_do_pin(MMUEXT_UNPIN_TABLE, pfn);
+                }
+
+                mcs = __xen_mc_entry(0);
 
                 MULTI_update_va_mapping(mcs.mc, (unsigned long)pt,
                                         pfn_pte(pfn, PAGE_KERNEL),
-                                        flags);
+                                        level == PT_PGD ? UVMF_TLB_FLUSH : 0);
+
+                if (ptl) {
+                        /* unlock when batch completed */
+                        xen_mc_callback(do_unlock, ptl);
+                }
         }
 
         return 0;               /* never need to flush on unpin */
@@ -472,18 +531,9 @@ static int unpin_page(struct page *page, unsigned flags)
 /* Release a pagetables pages back as normal RW */
 static void xen_pgd_unpin(pgd_t *pgd)
 {
-        struct mmuext_op *op;
-        struct multicall_space mcs;
-
         xen_mc_batch();
 
-        mcs = __xen_mc_entry(sizeof(*op));
-
-        op = mcs.args;
-        op->cmd = MMUEXT_UNPIN_TABLE;
-        op->arg1.mfn = pfn_to_mfn(PFN_DOWN(__pa(pgd)));
-
-        MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+        xen_do_pin(MMUEXT_UNPIN_TABLE, PFN_DOWN(__pa(pgd)));
 
         pgd_walk(pgd, unpin_page, TASK_SIZE);
 
@@ -585,5 +635,6 @@ void xen_exit_mmap(struct mm_struct *mm)
         /* pgd may not be pinned in the error exit path of execve */
         if (PagePinned(virt_to_page(mm->pgd)))
                 xen_pgd_unpin(mm->pgd);
+
         spin_unlock(&mm->page_table_lock);
 }