diff options
| author |   Kirill A. Shutemov <kirill.shutemov@linux.intel.com> | 2015-11-06 16:29:54 -0800 |
|---|---|---|
| committer |   Linus Torvalds <torvalds@linux-foundation.org> | 2015-11-06 17:50:42 -0800 |
| commit | 1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch) | |
| tree | 4b70d32439fb18ef699175413e4b82c4af206f81 /include/linux/mm.h | |
| parent | mm: pack compound_dtor and compound_order into one word in struct page (diff) | |
| download | linux-dev-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.tar.xz linux-dev-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.zip | |
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.h | 53 |
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)) |