path: root/arch/xtensa/configs
diff options
authorKirill A. Shutemov <>2015-11-06 16:29:54 -0800
committerLinus Torvalds <>2015-11-06 17:50:42 -0800
commit1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch)
tree4b70d32439fb18ef699175413e4b82c4af206f81 /arch/xtensa/configs
parentf1e61557f0230d51a3df8d825f2c156e75563bff (diff)
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-> 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->; - page->next; - page->; That's too long list to be absolutely sure, but looks like nobody uses bit 0 of the word. page-> 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] Signed-off-by: Kirill A. Shutemov <> Acked-by: Michal Hocko <> Reviewed-by: Andrea Arcangeli <> Cc: Hugh Dickins <> Cc: David Rientjes <> Cc: Vlastimil Babka <> Acked-by: Paul E. McKenney <> Cc: Aneesh Kumar K.V <> Cc: Andi Kleen <> Cc: Christoph Lameter <> Cc: Joonsoo Kim <> Cc: Sergey Senozhatsky <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'arch/xtensa/configs')
1 files changed, 0 insertions, 1 deletions
diff --git a/arch/xtensa/configs/iss_defconfig b/arch/xtensa/configs/iss_defconfig
index f3dfe0d921c2..44c6764d9146 100644
--- a/arch/xtensa/configs/iss_defconfig
+++ b/arch/xtensa/configs/iss_defconfig
@@ -169,7 +169,6 @@ CONFIG_FLATMEM_MANUAL=y
# CONFIG_PHYS_ADDR_T_64BIT is not set