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authorKirill A. Shutemov <kirill.shutemov@linux.intel.com>2015-11-07 03:29:54 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2015-11-07 04:50:42 +0300
commit1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch)
tree4b70d32439fb18ef699175413e4b82c4af206f81 /include/linux/mm.h
parentf1e61557f0230d51a3df8d825f2c156e75563bff (diff)
downloadlinux-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.tar.xz
mm: make compound_head() robust
Hugh has pointed that compound_head() call can be unsafe in some context. There's one example: CPU0 CPU1 isolate_migratepages_block() page_count() compound_head() !!PageTail() == true put_page() tail->first_page = NULL head = tail->first_page alloc_pages(__GFP_COMP) prep_compound_page() tail->first_page = head __SetPageTail(p); !!PageTail() == true <head == NULL dereferencing> The race is pure theoretical. I don't it's possible to trigger it in practice. But who knows. We can fix the race by changing how encode PageTail() and compound_head() within struct page to be able to update them in one shot. The patch introduces page->compound_head into third double word block in front of compound_dtor and compound_order. Bit 0 encodes PageTail() and the rest bits are pointer to head page if bit zero is set. The patch moves page->pmd_huge_pte out of word, just in case if an architecture defines pgtable_t into something what can have the bit 0 set. hugetlb_cgroup uses page->lru.next in the second tail page to store pointer struct hugetlb_cgroup. The patch switch it to use page->private in the second tail page instead. The space is free since ->first_page is removed from the union. The patch also opens possibility to remove HUGETLB_CGROUP_MIN_ORDER limitation, since there's now space in first tail page to store struct hugetlb_cgroup pointer. But that's out of scope of the patch. That means page->compound_head shares storage space with: - page->lru.next; - page->next; - page->rcu_head.next; That's too long list to be absolutely sure, but looks like nobody uses bit 0 of the word. page->rcu_head.next guaranteed[1] to have bit 0 clean as long as we use call_rcu(), call_rcu_bh(), call_rcu_sched(), or call_srcu(). But future call_rcu_lazy() is not allowed as it makes use of the bit and we can get false positive PageTail(). [1] http://lkml.kernel.org/g/20150827163634.GD4029@linux.vnet.ibm.com Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Michal Hocko <mhocko@suse.com> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: David Rientjes <rientjes@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Christoph Lameter <cl@linux.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/mm.h')
-rw-r--r--include/linux/mm.h53
1 files changed, 3 insertions, 50 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6581c21320cb..9671b6f23eda 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -430,46 +430,6 @@ static inline void compound_unlock_irqrestore(struct page *page,
#endif
}
-static inline struct page *compound_head_by_tail(struct page *tail)
-{
- struct page *head = tail->first_page;
-
- /*
- * page->first_page may be a dangling pointer to an old
- * compound page, so recheck that it is still a tail
- * page before returning.
- */
- smp_rmb();
- if (likely(PageTail(tail)))
- return head;
- return tail;
-}
-
-/*
- * Since either compound page could be dismantled asynchronously in THP
- * or we access asynchronously arbitrary positioned struct page, there
- * would be tail flag race. To handle this race, we should call
- * smp_rmb() before checking tail flag. compound_head_by_tail() did it.
- */
-static inline struct page *compound_head(struct page *page)
-{
- if (unlikely(PageTail(page)))
- return compound_head_by_tail(page);
- return page;
-}
-
-/*
- * If we access compound page synchronously such as access to
- * allocated page, there is no need to handle tail flag race, so we can
- * check tail flag directly without any synchronization primitive.
- */
-static inline struct page *compound_head_fast(struct page *page)
-{
- if (unlikely(PageTail(page)))
- return page->first_page;
- return page;
-}
-
/*
* The atomic page->_mapcount, starts from -1: so that transitions
* both from it and to it can be tracked, using atomic_inc_and_test
@@ -518,7 +478,7 @@ static inline void get_huge_page_tail(struct page *page)
VM_BUG_ON_PAGE(!PageTail(page), page);
VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
VM_BUG_ON_PAGE(atomic_read(&page->_count) != 0, page);
- if (compound_tail_refcounted(page->first_page))
+ if (compound_tail_refcounted(compound_head(page)))
atomic_inc(&page->_mapcount);
}
@@ -541,13 +501,7 @@ static inline struct page *virt_to_head_page(const void *x)
{
struct page *page = virt_to_page(x);
- /*
- * We don't need to worry about synchronization of tail flag
- * when we call virt_to_head_page() since it is only called for
- * already allocated page and this page won't be freed until
- * this virt_to_head_page() is finished. So use _fast variant.
- */
- return compound_head_fast(page);
+ return compound_head(page);
}
/*
@@ -1586,8 +1540,7 @@ static inline bool ptlock_init(struct page *page)
* with 0. Make sure nobody took it in use in between.
*
* It can happen if arch try to use slab for page table allocation:
- * slab code uses page->slab_cache and page->first_page (for tail
- * pages), which share storage with page->ptl.
+ * slab code uses page->slab_cache, which share storage with page->ptl.
*/
VM_BUG_ON_PAGE(*(unsigned long *)&page->ptl, page);
if (!ptlock_alloc(page))