Re: [PATCH] x86: hold mm->page_table_lock while doing vmalloc_sync

Andrea Arcangeli
2011-02-03T02:49:12Z

Hello,

Larry (CC'ed) found a problem with the patch in subject. When
USE_SPLIT_PTLOCKS is not defined (NR_CPUS == 2) it will deadlock in
ptep_clear_flush_notify in rmap.c because it's sending IPIs with the
page_table_lock already held, and the other CPUs now spins on the
page_table_lock with irq disabled, so the IPI never runs. With
CONFIG_TRANSPARENT_HUGEPAGE=y this deadlocks happens even with
USE_SPLIT_PTLOCKS defined so it become visible but it needs to be
fixed regardless (for NR_CPUS == 2).

I'd like to understand why the pgd_lock needs irq disabled, it sounds
too easy that I can just remove the _irqsave, doesn't it?

A pgd_free comment says it can run from irq. page_table_lock having to
be taken there is for Xen only, but other archs also uses
spin_lock_irqsave(pgd_lock) so I guess it's either common code, or
it's superfluous and not another Xen special requirement.

If we could remove that _irqsave like below it'd solve it... But
clearly something must be taking the pgd_lock from irq. (using a
rwlock would also be possible as long as nobody takes it in write mode
during irq, but if it's pgd_free that really runs in irq, that would
need the write_lock so it wouldn't be a solution).

I'm trying this fix and the VM_BUG_ON never triggered yet.

In short: who takes the pgd_lock from an irq? (__mmdrop shouldn't but
maybe I'm overlooking some aio bit?)

======
Subject: fix pgd_lock deadlock

From: Andrea Arcangeli <aarcange@redhat.com>

It's forbidden to take the page_table_lock with the irq disabled or if there's
contention the IPIs (for tlb flushes) sent with the page_table_lock held will
never run leading to a deadlock.

Apparently nobody takes the pgd_lock from irq so the _irqsave can be removed.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
arch/x86/mm/fault.c | 7 ++++---
arch/x86/mm/init_64.c | 7 ++++---
arch/x86/mm/pageattr.c | 21 +++++++++++----------
arch/x86/mm/pgtable.c | 10 ++++++----
arch/x86/xen/mmu.c | 10 ++++------
5 files changed, 29 insertions(+), 26 deletions(-)

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -230,14 +230,15 @@ void vmalloc_sync_all(void)
address >= TASK_SIZE && address < FIXADDR_TOP;
address += PMD_SIZE) {

- unsigned long flags;
struct page *page;

- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
list_for_each_entry(page, &pgd_list, lru) {
spinlock_t *pgt_lock;
pmd_t *ret;

+ /* the pgt_lock only for Xen */
pgt_lock = &pgd_page_get_mm(page)->page_table_lock;

spin_lock(pgt_lock);
@@ -247,7 +248,7 @@ void vmalloc_sync_all(void)
if (!ret)
break;
}
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock, flags);
}
}

--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -105,18 +105,19 @@ void sync_global_pgds(unsigned long star

for (address = start; address <= end; address += PGDIR_SIZE) {
const pgd_t *pgd_ref = pgd_offset_k(address);
- unsigned long flags;
struct page *page;

if (pgd_none(*pgd_ref))
continue;

- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
list_for_each_entry(page, &pgd_list, lru) {
pgd_t *pgd;
spinlock_t *pgt_lock;

pgd = (pgd_t *)page_address(page) + pgd_index(address);
+ /* the pgt_lock only for Xen */
pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
spin_lock(pgt_lock);

@@ -128,7 +129,7 @@ void sync_global_pgds(unsigned long star

spin_unlock(pgt_lock);
}
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}
}

--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -57,12 +57,11 @@ static unsigned long direct_pages_count[

void update_page_count(int level, unsigned long pages)
{
- unsigned long flags;
-
/* Protect against CPA */
- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
direct_pages_count[level] += pages;
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

static void split_page_count(int level)
@@ -402,7 +401,7 @@ static int
try_preserve_large_page(pte_t *kpte, unsigned long address,
struct cpa_data *cpa)
{
- unsigned long nextpage_addr, numpages, pmask, psize, flags, addr, pfn;
+ unsigned long nextpage_addr, numpages, pmask, psize, addr, pfn;
pte_t new_pte, old_pte, *tmp;
pgprot_t old_prot, new_prot, req_prot;
int i, do_split = 1;
@@ -411,7 +410,8 @@ try_preserve_large_page(pte_t *kpte, uns
if (cpa->force_split)
return 1;

- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
/*
* Check for races, another CPU might have split this page
* up already:
@@ -506,14 +506,14 @@ try_preserve_large_page(pte_t *kpte, uns
}

out_unlock:
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);

return do_split;
}

static int split_large_page(pte_t *kpte, unsigned long address)
{
- unsigned long flags, pfn, pfninc = 1;
+ unsigned long pfn, pfninc = 1;
unsigned int i, level;
pte_t *pbase, *tmp;
pgprot_t ref_prot;
@@ -527,7 +527,8 @@ static int split_large_page(pte_t *kpte,
if (!base)
return -ENOMEM;

- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
/*
* Check for races, another CPU might have split this page
* up for us already:
@@ -599,7 +600,7 @@ out_unlock:
*/
if (base)
__free_page(base);
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);

return 0;
}
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -126,9 +126,10 @@ static void pgd_dtor(pgd_t *pgd)
if (SHARED_KERNEL_PMD)
return;

- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);
pgd_list_del(pgd);
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

/*
@@ -280,12 +281,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
* respect to anything walking the pgd_list, so that they
* never see a partially populated pgd.
*/
- spin_lock_irqsave(&pgd_lock, flags);
+ VM_BUG_ON(in_interrupt());
+ spin_lock(&pgd_lock);

pgd_ctor(mm, pgd);
pgd_prepopulate_pmd(mm, pgd, pmds);

- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);

return pgd;

--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -986,10 +986,9 @@ static void xen_pgd_pin(struct mm_struct
*/
void xen_mm_pin_all(void)
{
- unsigned long flags;
struct page *page;

- spin_lock_irqsave(&pgd_lock, flags);
+ spin_lock(&pgd_lock);

list_for_each_entry(page, &pgd_list, lru) {
if (!PagePinned(page)) {
@@ -998,7 +997,7 @@ void xen_mm_pin_all(void)
}
}

- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

/*
@@ -1099,10 +1098,9 @@ static void xen_pgd_unpin(struct mm_stru
*/
void xen_mm_unpin_all(void)
{
- unsigned long flags;
struct page *page;

- spin_lock_irqsave(&pgd_lock, flags);
+ spin_lock(&pgd_lock);

list_for_each_entry(page, &pgd_list, lru) {
if (PageSavePinned(page)) {
@@ -1112,7 +1110,7 @@ void xen_mm_unpin_all(void)
}
}

- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

void xen_activate_mm(struct mm_struct *prev, struct mm_struct *next)