Messages in this thread Patch in this message | | | Date | Fri, 3 Jan 2014 20:55:47 +0100 | From | Oleg Nesterov <> | Subject | [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new_page() race |
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get/put_page(thp_tail) paths do get_page_unless_zero(page_head) + compound_lock(). In theory this page_head can be already freed and reallocated as alloc_pages(__GFP_COMP, smaller_order). In this case get_page_unless_zero() can succeed right after set_page_refcounted(), and compound_lock() can race with the non-atomic __SetPageHead().
Perhaps we should rework the thp locking (under discussion), but until then this patch moves set_page_refcounted() and adds wmb() to ensure that page->_count != 0 comes as a last change.
I am not sure about other callers of set_page_refcounted(), but at first glance they look fine to me.
Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Andrea Arcangeli <aarcange@redhat.com> --- mm/page_alloc.c | 12 ++++++++++-- 1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 115b23b..d323102 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -865,8 +865,6 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags) } set_page_private(page, 0); - set_page_refcounted(page); - arch_alloc_page(page, order); kernel_map_pages(page, 1 << order, 1); @@ -876,6 +874,16 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags) if (order && (gfp_flags & __GFP_COMP)) prep_compound_page(page, order); + /* + * Make sure the caller of get_page_unless_zero() will see the + * fully initialized page. Say, to ensure that compound_lock() + * can't race with the non-atomic __SetPage*() above. + */ +#ifdef CONFIG_TRANSPARENT_HUGEPAGE + smp_wmb(); +#endif + set_page_refcounted(page); + return 0; } -- 1.5.5.1
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