Now, memory barrier in __do_huge_pmd_anonymous_page doesn't work.
Because lru_cache_add_lru uses pagevec so it could miss spinlock
easily so above rule was broken so user might see inconsistent data.
I was not first person who pointed out the problem. Mel and Peter
pointed out a few months ago and Peter pointed out further that
even spin_lock/unlock can't make sure it.
http://marc.info/?t=134333512700004
In particular:
*A = a;
LOCK
UNLOCK
*B = b;
may occur as:
LOCK, STORE *B, STORE *A, UNLOCK
At last, Hugh pointed out that even we don't need memory barrier
in there because __SetPageUpdate already have done it from
Nick's [1] explicitly.
So this patch fixes comment on THP and adds same comment for
do_anonymous_page, too because everybody except Hugh was missing
that. It means we needs COMMENT about that.
[1] 0ed361dec "mm: fix PageUptodate data race"
Cc: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kamezawa Hiroyuki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Acked-by: Andrea Arcangeli <[email protected]>
Signed-off-by: Minchan Kim <[email protected]>
---
* from v1
* Add Acked-by from Andrea
* Comment correction
mm/huge_memory.c | 11 +++++------
mm/memory.c | 5 +++++
2 files changed, 10 insertions(+), 6 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e2f7f5aa..f2f17ff 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -713,6 +713,11 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
return VM_FAULT_OOM;
clear_huge_page(page, haddr, HPAGE_PMD_NR);
+ /*
+ * The memory barrier inside __SetPageUptodate makes sure that
+ * clear_huge_page writes become visible before the set_pmd_at()
+ * write.
+ */
__SetPageUptodate(page);
spin_lock(&mm->page_table_lock);
@@ -724,12 +729,6 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
} else {
pmd_t entry;
entry = mk_huge_pmd(page, vma);
- /*
- * The spinlocking to take the lru_lock inside
- * page_add_new_anon_rmap() acts as a full memory
- * barrier to be sure clear_huge_page writes become
- * visible after the set_pmd_at() write.
- */
page_add_new_anon_rmap(page, vma, haddr);
set_pmd_at(mm, haddr, pmd, entry);
pgtable_trans_huge_deposit(mm, pgtable);
diff --git a/mm/memory.c b/mm/memory.c
index 494526a..d0da51e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3196,6 +3196,11 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
page = alloc_zeroed_user_highpage_movable(vma, address);
if (!page)
goto oom;
+ /*
+ * The memory barrier inside __SetPageUptodate makes sure that
+ * preceeding stores to the page contents become visible before
+ * the set_pte_at() write.
+ */
__SetPageUptodate(page);
if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL))
--
1.8.2
(2013/04/05 17:16), Minchan Kim wrote:
> Now, memory barrier in __do_huge_pmd_anonymous_page doesn't work.
> Because lru_cache_add_lru uses pagevec so it could miss spinlock
> easily so above rule was broken so user might see inconsistent data.
>
> I was not first person who pointed out the problem. Mel and Peter
> pointed out a few months ago and Peter pointed out further that
> even spin_lock/unlock can't make sure it.
> http://marc.info/?t=134333512700004
>
> In particular:
>
> *A = a;
> LOCK
> UNLOCK
> *B = b;
>
> may occur as:
>
> LOCK, STORE *B, STORE *A, UNLOCK
>
> At last, Hugh pointed out that even we don't need memory barrier
> in there because __SetPageUpdate already have done it from
> Nick's [1] explicitly.
>
> So this patch fixes comment on THP and adds same comment for
> do_anonymous_page, too because everybody except Hugh was missing
> that. It means we needs COMMENT about that.
>
> [1] 0ed361dec "mm: fix PageUptodate data race"
>
> Cc: Mel Gorman <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Kamezawa Hiroyuki <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Acked-by: Andrea Arcangeli <[email protected]>
> Signed-off-by: Minchan Kim <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
On Fri, 5 Apr 2013, Minchan Kim wrote:
> Now, memory barrier in __do_huge_pmd_anonymous_page doesn't work.
> Because lru_cache_add_lru uses pagevec so it could miss spinlock
> easily so above rule was broken so user might see inconsistent data.
>
> I was not first person who pointed out the problem. Mel and Peter
> pointed out a few months ago and Peter pointed out further that
> even spin_lock/unlock can't make sure it.
> http://marc.info/?t=134333512700004
>
> In particular:
>
> *A = a;
> LOCK
> UNLOCK
> *B = b;
>
> may occur as:
>
> LOCK, STORE *B, STORE *A, UNLOCK
>
> At last, Hugh pointed out that even we don't need memory barrier
> in there because __SetPageUpdate already have done it from
> Nick's [1] explicitly.
>
> So this patch fixes comment on THP and adds same comment for
> do_anonymous_page, too because everybody except Hugh was missing
> that. It means we needs COMMENT about that.
>
> [1] 0ed361dec "mm: fix PageUptodate data race"
>
> Cc: Mel Gorman <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Kamezawa Hiroyuki <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Acked-by: Andrea Arcangeli <[email protected]>
> Signed-off-by: Minchan Kim <[email protected]>
Acked-by: David Rientjes <[email protected]>