path: root/mm
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authorDavid Rientjes <rientjes@google.com>2014-03-03 15:38:18 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-03-04 07:55:47 -0800
commit668f9abbd4334e6c29fa8acd71635c4f9101caa7 (patch)
tree7060fc64f0164c4936b92f60569e63fcddfdfc51 /mm
parentaa15aa0e869cb218f68faff8f229fa6b6b5cdf08 (diff)
mm: close PageTail race
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 <rientjes@google.com> Cc: Holger Kiehl <Holger.Kiehl@dwd.de> Cc: Christoph Lameter <cl@linux.com> Cc: Rafael Aquini <aquini@redhat.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Michal Hocko <mhocko@suse.cz> Cc: Mel Gorman <mgorman@suse.de> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
4 files changed, 7 insertions, 5 deletions
diff --git a/mm/ksm.c b/mm/ksm.c
index aa4c7c7250c..68710e80994 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 2f2f34a4e77..90002ea4363 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1651,7 +1651,7 @@ 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 e3758a09a00..3d1bf889465 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
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 b31ba67d440..0092097b3f4 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -98,7 +98,7 @@ static void put_compound_page(struct page *page)
/* __split_huge_page_refcount can run under us */
- page_head = compound_trans_head(page);
+ page_head = compound_head(page);
* THP can not break up slab pages so avoid taking
@@ -253,7 +253,7 @@ bool __get_page_tail(struct page *page)
unsigned long flags;
bool got;
- struct page *page_head = compound_trans_head(page);
+ struct page *page_head = compound_head(page);
/* Ref to put_compound_page() comment. */
if (!__compound_tail_refcounted(page_head)) {