Daniel Borkmann reported a VM_BUG_ON assertion failing:

  ------------[ cut here ]------------
  kernel BUG at mm/mlock.c:528!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: ccm arc4 iwldvm [...]
   video
  CPU: 3 PID: 2266 Comm: netsniff-ng Not tainted 3.14.0-rc2+ #8
  Hardware name: LENOVO 2429BP3/2429BP3, BIOS G4ET37WW (1.12 ) 05/29/2012
  task: ffff8801f87f9820 ti: ffff88002cb44000 task.ti: ffff88002cb44000
  RIP: 0010:[<ffffffff81171ad0>]  [<ffffffff81171ad0>] munlock_vma_pages_range+0x2e0/0x2f0
  Call Trace:
    do_munmap+0x18f/0x3b0
    vm_munmap+0x41/0x60
    SyS_munmap+0x22/0x30
    system_call_fastpath+0x1a/0x1f
  RIP   munlock_vma_pages_range+0x2e0/0x2f0
  ---[ end trace a0088dcf07ae10f2 ]---

because munlock_vma_pages_range() thinks it's unexpectedly in the middle
of a THP page.  This can be reproduced with default config since 3.11
kernels.  A reproducer can be found in the kernel's selftest directory
for networking by running ./psock_tpacket.

The problem is that an order=2 compound page (allocated by
alloc_one_pg_vec_page() is part of the munlocked VM_MIXEDMAP vma (mapped
by packet_mmap()) and mistaken for a THP page and assumed to be order=9.

The checks for THP in munlock came with commit ff6a6da6 ("mm:
accelerate munlock() treatment of THP pages"), i.e.  since 3.9, but did
not trigger a bug.  It just makes munlock_vma_pages_range() skip such
compound pages until the next 512-pages-aligned page, when it encounters
a head page.  This is however not a problem for vma's where mlocking has
no effect anyway, but it can distort the accounting.

Since commit 7225522b ("mm: munlock: batch non-THP page isolation
and munlock+putback using pagevec") this can trigger a VM_BUG_ON in
PageTransHuge() check.

This patch fixes the issue by adding VM_MIXEDMAP flag to VM_SPECIAL, a
list of flags that make vma's non-mlockable and non-mergeable.  The
reasoning is that VM_MIXEDMAP vma's are similar to VM_PFNMAP, which is
already on the VM_SPECIAL list, and both are intended for non-LRU pages
where mlocking makes no sense anyway.  Related Lkml discussion can be
found in [2].

 [1] tools/testing/selftests/net/psock_tpacket
 [2] https://lkml.org/lkml/2014/1/10/427



Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Reported-by: Daniel Borkmann <dborkman@redhat.com>
Tested-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: John David Anglin <dave.anglin@bell.net>
Cc: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Carsten Otte <cotte@de.ibm.com>
Cc: Jared Hulbert <jaredeh@gmail.com>
Tested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org> [3.11.x+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>