From def52acc90faab583b124f3177d55c15d125e2d1 Mon Sep 17 00:00:00 2001 From: David Rientjes Date: Mon, 3 Mar 2014 15:38:18 -0800 Subject: mm: close PageTail race commit 668f9abbd4334e6c29fa8acd71635c4f9101caa7 upstream. Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned pages") introduces page_count(page) into memory compaction which dereferences page->first_page if PageTail(page). This results in a very rare NULL pointer dereference on the aforementioned page_count(page). Indeed, anything that does compound_head(), including page_count() is susceptible to racing with prep_compound_page() and seeing a NULL or dangling page->first_page pointer. This patch uses Andrea's implementation of compound_trans_head() that deals with such a race and makes it the default compound_head() implementation. This includes a read memory barrier that ensures that if PageTail(head) is true that we return a head page that is neither NULL nor dangling. The patch then adds a store memory barrier to prep_compound_page() to ensure page->first_page is set. This is the safest way to ensure we see the head page that we are expecting, PageTail(page) is already in the unlikely() path and the memory barriers are unfortunately required. Hugetlbfs is the exception, we don't enforce a store memory barrier during init since no race is possible. Signed-off-by: David Rientjes Cc: Holger Kiehl Cc: Christoph Lameter Cc: Rafael Aquini Cc: Vlastimil Babka Cc: Michal Hocko Cc: Mel Gorman Cc: Andrea Arcangeli Cc: Rik van Riel Cc: "Kirill A. Shutemov" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/ksm.c | 2 +- mm/memory-failure.c | 2 +- mm/page_alloc.c | 4 +++- mm/swap.c | 4 ++-- 4 files changed, 7 insertions(+), 5 deletions(-) (limited to 'mm') diff --git a/mm/ksm.c b/mm/ksm.c index b6afe0c440d8..784d1e4bc385 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -444,7 +444,7 @@ static void break_cow(struct rmap_item *rmap_item) static struct page *page_trans_compound_anon(struct page *page) { if (PageTransCompound(page)) { - struct page *head = compound_trans_head(page); + struct page *head = compound_head(page); /* * head may actually be splitted and freed from under * us but it's ok here. diff --git a/mm/memory-failure.c b/mm/memory-failure.c index e386beefc994..59c62fa75c5a 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -1544,7 +1544,7 @@ int soft_offline_page(struct page *page, int flags) { int ret; unsigned long pfn = page_to_pfn(page); - struct page *hpage = compound_trans_head(page); + struct page *hpage = compound_head(page); if (PageHWPoison(page)) { pr_info("soft offline: %#lx page already poisoned\n", pfn); diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 2ee0fd313f03..0ab02fb8e9b1 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -360,9 +360,11 @@ void prep_compound_page(struct page *page, unsigned long order) __SetPageHead(page); for (i = 1; i < nr_pages; i++) { struct page *p = page + i; - __SetPageTail(p); set_page_count(p, 0); p->first_page = page; + /* Make sure p->first_page is always valid for PageTail() */ + smp_wmb(); + __SetPageTail(p); } } diff --git a/mm/swap.c b/mm/swap.c index ea58dbde788e..4e35f3ff0427 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -81,7 +81,7 @@ static void put_compound_page(struct page *page) { if (unlikely(PageTail(page))) { /* __split_huge_page_refcount can run under us */ - struct page *page_head = compound_trans_head(page); + struct page *page_head = compound_head(page); if (likely(page != page_head && get_page_unless_zero(page_head))) { @@ -219,7 +219,7 @@ bool __get_page_tail(struct page *page) */ unsigned long flags; bool got = false; - struct page *page_head = compound_trans_head(page); + struct page *page_head = compound_head(page); if (likely(page != page_head && get_page_unless_zero(page_head))) { /* Ref to put_compound_page() comment. */ -- cgit v1.2.3