THP: fix comment about memory barrier

Currently the 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 of 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 commit
0ed361dec369 ("mm: fix PageUptodate data race") 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 need a comment about that.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e2f7f5aaaafb..45eaae030628 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);