path: root/virt
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authorDavid Rientjes <rientjes@google.com>2014-03-03 15:38:18 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-04-03 12:01:05 -0700
commitdef52acc90faab583b124f3177d55c15d125e2d1 (patch)
tree067db9dcb4ce2ebdbc0894763c02818bd1eceb6d /virt
parentd113edc6c7027a8290ddfb2f0c5ab8291a582945 (diff)
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 <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> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'virt')
1 files changed, 2 insertions, 2 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index eb99458f5b6..8cf1cd2fada 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -105,12 +105,12 @@ bool kvm_is_mmio_pfn(pfn_t pfn)
if (pfn_valid(pfn)) {
int reserved;
struct page *tail = pfn_to_page(pfn);
- struct page *head = compound_trans_head(tail);
+ struct page *head = compound_head(tail);
reserved = PageReserved(head);
if (head != tail) {
* "head" is not a dangling pointer
- * (compound_trans_head takes care of that)
+ * (compound_head takes care of that)
* but the hugepage may have been splitted
* from under us (and we may not hold a
* reference count on the head page so it can