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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
