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Subject[PATCH 08/12] 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>

---
arch/x86/xen/enlighten.c | 30 ++++++++----
arch/x86/xen/mmu.c | 113 +++++++++++++++++++++++++++++++++-------------
mm/Kconfig | 1
3 files changed, 103 insertions(+), 41 deletions(-)
===================================================================
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -687,6 +687,15 @@ static __init void xen_alloc_pt_init(str
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)
@@ -696,9 +705,10 @@ static void xen_alloc_pt(struct mm_struc
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();
@@ -711,8 +721,10 @@ 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)));
+ }
}
}

@@ -827,15 +839,15 @@ static __init void xen_pagetable_setup_d
/* 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)));
}
}

===================================================================
--- 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
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
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
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, u
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, u
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,
/* 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);
}
===================================================================
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -137,7 +137,6 @@ config SPLIT_PTLOCK_CPUS
int
default "4096" if ARM && !CPU_CACHE_VIPT
default "4096" if PARISC && !PA20
- default "4096" if XEN
default "4"

#
--

-
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