On Tue, Apr 02, 2013 at 12:30:15PM -0700, Hugh Dickins wrote: > On Tue, 2 Apr 2013, Minchan Kim wrote: > > On Mon, Apr 01, 2013 at 04:35:38PM -0700, David Rientjes wrote: > > > On Mon, 1 Apr 2013, Minchan Kim wrote: > > > > > > > __do_huge_pmd_anonymous_page depends on page_add_new_anon_rmap's > > > > spinlock for making sure that clear_huge_page write become visible > > > > after set set_pmd_at() write. > > > > > > > > But lru_cache_add_lru uses pagevec so it could miss spinlock > > > > easily so above rule was broken so user may see inconsistent data. > > > > > > > > This patch fixes it with using explict barrier rather than depending > > > > on lru spinlock. > > > > > > > > > > Is this the same issue that Andrea responded to in the "thp and memory > > > barrier assumptions" thread at http://marc.info/?t=134333512700004 ? > > > > Yes and Peter pointed out further step. > > Thanks for pointing out. > > Not that I know that Andrea alreay noticed it, I don't care about this > > patch. > > > > Remaining question is Kame's one. > > > Hmm...how about do_anonymous_page() ? there are no comments/locks/barriers. > > > Users can see non-zero value after page fault in theory ? > > Isn't there anyone could answer it? > > See Nick's 2008 0ed361dec "mm: fix PageUptodate data race", which gave us > > static inline void __SetPageUptodate(struct page *page) > { > smp_wmb(); > __set_bit(PG_uptodate, &(page)->flags); > } > > So both do_anonymous_page() and __do_huge_pmd_anonymous_page() look safe > to me already, though the huge_memory one could do with a fixed comment. Thanks you very much! That's one everybody are really missing. Here it goes! ==================== 8< ===================== >From fb0b9f3df698547bfb70f81d85e0d1e00f19e1fc Mon Sep 17 00:00:00 2001 From: Minchan Kim <minchan@xxxxxxxxxx> Date: Wed, 3 Apr 2013 08:53:27 +0900 Subject: [PATCH] THP: fix comment about memory barrier 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 <mgorman@xxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> --- 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 after 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 after + * the set_pte_at() write. + */ __SetPageUptodate(page); if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) -- 1.8.2 -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>