linux/Documentation/vm
Kirill A. Shutemov 1d798ca3f1 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>
2015-11-06 17:50:42 -08:00
..
.gitignore
00-INDEX mm: introduce idle page tracking 2015-09-10 13:29:01 -07:00
active_mm.txt
balance mm, page_alloc: distinguish between being unable to sleep, unwilling to sleep and avoiding waking kswapd 2015-11-06 17:50:42 -08:00
cleancache.txt cleancache: forbid overriding cleancache_ops 2015-04-14 16:49:03 -07:00
frontswap.txt
highmem.txt
hugetlbpage.txt Documentation: update libhugetlbfs location and use for testing 2015-09-08 15:35:28 -07:00
hwpoison.txt
idle_page_tracking.txt mm: introduce idle page tracking 2015-09-10 13:29:01 -07:00
ksm.txt
numa
numa_memory_policy.txt
overcommit-accounting
page_migration mm: page migration avoid touching newpage until no going back 2015-11-05 19:34:48 -08:00
page_owner.txt
pagemap.txt proc: export idle flag via kpageflags 2015-09-10 13:29:01 -07:00
remap_file_pages.txt mm: replace remap_file_pages() syscall with emulation 2015-02-10 14:30:30 -08:00
slub.txt Doc/slub: document slabinfo-gnuplot.sh script 2015-11-05 19:34:48 -08:00
soft-dirty.txt
split_page_table_lock mm: make compound_head() robust 2015-11-06 17:50:42 -08:00
transhuge.txt Documentation/vm/transhuge.txt: add information about max_ptes_swap 2015-11-05 19:34:48 -08:00
unevictable-lru.txt mm: rmap use pte lock not mmap_sem to set PageMlocked 2015-11-05 19:34:48 -08:00
userfaultfd.txt userfaultfd: change the read API to return a uffd_msg 2015-09-04 16:54:41 -07:00
zsmalloc.txt zsmalloc: zsmalloc documentation 2015-04-15 16:35:21 -07:00
zswap.txt zswap: update docs for runtime-changeable attributes 2015-09-10 13:29:01 -07:00