Commit 52f37629 authored Apr 29, 2013 by Minchan Kim Committed by Linus Torvalds Apr 29, 2013

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
0ed361de ("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>

parent f1cb0879

mm/huge_memory.c

+5 −6

Original line number	Diff line number	Diff line
		@@ -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);

mm/memory.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -3244,6 +3244,11 @@ 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))

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