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xen: lock pte pages while pinning/unpinning
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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>
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@jsgf
Jeremy Fitzhardinge authored and jsgf committed Oct 16, 2007
1 parent 9f79991 commit 7426071
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Showing 3 changed files with 103 additions and 41 deletions.
30 changes: 21 additions & 9 deletions arch/x86/xen/enlighten.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand All @@ -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();
Expand All @@ -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)));
}
}
}

Expand Down Expand Up @@ -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)));
}
}

Expand Down
113 changes: 82 additions & 31 deletions arch/x86/xen/mmu.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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;
Expand All @@ -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;
Expand All @@ -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);
Expand All @@ -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;
Expand All @@ -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;
Expand All @@ -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();

Expand All @@ -424,24 +472,21 @@ 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);
}

/* 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;
Expand All @@ -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 */
Expand All @@ -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);

Expand Down Expand Up @@ -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);
}
1 change: 0 additions & 1 deletion mm/Kconfig
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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"

#
Expand Down

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