Re: Multiple potential races on vma->vm_flags

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Sep 10, 2015 at 03:27:59PM +0200, Andrey Konovalov wrote:
> Can a vma be shared among a few mm's?

Define "shared".

vma can belong only to one process (mm_struct), but it can be accessed
from other process like in rmap case below.

rmap uses anon_vma_lock for anon vma and i_mmap_rwsem for file vma to make
sure that the vma will not disappear under it.

> If yes, then taking current->mm->mmap_sem to protect vma is not enough.

Depends on what protection you are talking about.
 
> In the first report below both T378 and T398 take
> current->mm->mmap_sem at mm/mlock.c:650, but they turn out to be
> different locks (the addresses are different).

See i_mmap_lock_read() in T398. It will guarantee that vma is there.

> In the second report T309 doesn't take any locks at all, since it
> assumes that after checking atomic_dec_and_test(&mm->mm_users) the mm
> has no other users, but then it does a write to vma.

This one is tricky. I *assume* the mm cannot be generally accessible after
mm_users drops to zero, but I'm not entirely sure about it.
procfs? ptrace?

The VMA is still accessible via rmap at this point. And I think it can be
a problem:

		CPU0					CPU1
exit_mmap()
  // mmap_sem is *not* taken
  munlock_vma_pages_all()
    munlock_vma_pages_range()
    						try_to_unmap_one()
						  down_read_trylock(&vma->vm_mm->mmap_sem))
						  !!(vma->vm_flags & VM_LOCKED) == true
      vma->vm_flags &= ~VM_LOCKED;
      <munlock the page>
      						  mlock_vma_page(page);
						  // mlocked pages is leaked.

The obvious solution is to take mmap_sem in exit path, but it would cause
performance regression.

Any comments?

> 
> ==================================================================
> ThreadSanitizer: data-race in munlock_vma_pages_range
> 
> Write of size 8 by thread T378 (K2633, CPU3):
>  [<ffffffff81212579>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
>  [<     inlined    >] SYSC_munlock mm/mlock.c:651
>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> Locks held by T378:
> #0 Lock 25710428 taken here:
>  [<     inlined    >] SYSC_munlock mm/mlock.c:650
>  [<ffffffff8121308c>] SyS_munlock+0x4c/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> Previous read of size 8 by thread T398 (K2623, CPU2):
>  [<ffffffff8121d198>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1540
>  [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559
>  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
>  [<     inlined    >] SYSC_munlock mm/mlock.c:651
>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> Locks held by T398:
> #0 Lock 21b00c68 taken here:
>  [<     inlined    >] SYSC_munlock mm/mlock.c:650
>  [<ffffffff8121308c>] SyS_munlock+0x4c/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> #1 Lock bac2d750 taken here:
>  [<     inlined    >] i_mmap_lock_read include/linux/fs.h:509
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1533
>  [<ffffffff8121e6e8>] rmap_walk+0x78/0x450 mm/rmap.c:1559
>  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
>  [<     inlined    >] SYSC_munlock mm/mlock.c:651
>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> #2 Lock 0895f570 taken here:
>  [<     inlined    >] spin_lock include/linux/spinlock.h:312
>  [<ffffffff8121c959>] __page_check_address+0xd9/0x210 mm/rmap.c:681
>  [<     inlined    >] page_check_address include/linux/rmap.h:204
>  [<ffffffff8121d173>] try_to_unmap_one+0x53/0x4f0 mm/rmap.c:1198
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1540
>  [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559
>  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
>  [<     inlined    >] SYSC_munlock mm/mlock.c:651
>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> DBG: addr: ffff880222610e10
> DBG: first offset: 0, second offset: 0
> DBG: T378 clock: {T378: 4486533, T398: 2405850}
> DBG: T398 clock: {T398: 2406009}
> ==================================================================
> 
> ==================================================================
> ThreadSanitizer: data-race in munlock_vma_pages_range
> 
> Write of size 8 by thread T309 (K2577, CPU0):
>  [<ffffffff81211fc9>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
>  [<     inlined    >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81216cc3>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085685>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inlined    >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c3a7>] do_exit+0x457/0x1420 kernel/exit.c:733
>  [<ffffffff8108f08f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<     inlined    >] SYSC_exit_group kernel/exit.c:885
>  [<ffffffff8108f170>] __wake_up_parent+0x0/0x50 kernel/exit.c:883
>  [<ffffffff81eadb2e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> Locks held by T309:
> 
> Previous read of size 8 by thread T293 (K2573, CPU3):
>  [<ffffffff8121cbe8>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1540
>  [<ffffffff8121e207>] rmap_walk+0x147/0x450 mm/rmap.c:1559
>  [<ffffffff8121e9c2>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211600>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81211ab6>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812122f0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<     inlined    >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81216cc3>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085685>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inlined    >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c3a7>] do_exit+0x457/0x1420 kernel/exit.c:733
>  [<ffffffff8108f08f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<     inlined    >] SYSC_exit_group kernel/exit.c:885
>  [<ffffffff8108f170>] __wake_up_parent+0x0/0x50 kernel/exit.c:883
>  [<ffffffff81eadb2e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> Locks held by T293:
> #0 Lock bb0dc710 taken here:
>  [<     inlined    >] i_mmap_lock_read include/linux/fs.h:509
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1533
>  [<ffffffff8121e138>] rmap_walk+0x78/0x450 mm/rmap.c:1559
>  [<ffffffff8121e9c2>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211600>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81211ab6>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812122f0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<     inlined    >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81216cc3>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085685>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inlined    >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c3a7>] do_exit+0x457/0x1420 kernel/exit.c:733
>  [<ffffffff8108f08f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<     inlined    >] SYSC_exit_group kernel/exit.c:885
>  [<ffffffff8108f170>] __wake_up_parent+0x0/0x50 kernel/exit.c:883
>  [<ffffffff81eadb2e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> #1 Lock 02e0f1b0 taken here:
>  [<     inlined    >] spin_lock include/linux/spinlock.h:312
>  [<ffffffff8121c3a9>] __page_check_address+0xd9/0x210 mm/rmap.c:681
>  [<     inlined    >] page_check_address include/linux/rmap.h:204
>  [<ffffffff8121cbc3>] try_to_unmap_one+0x53/0x4f0 mm/rmap.c:1198
>  [<     inlined    >] rmap_walk_file mm/rmap.c:1540
>  [<ffffffff8121e207>] rmap_walk+0x147/0x450 mm/rmap.c:1559
>  [<ffffffff8121e9c2>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
>  [<ffffffff81211600>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81211ab6>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff812122f0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<     inlined    >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81216cc3>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085685>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inlined    >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c3a7>] do_exit+0x457/0x1420 kernel/exit.c:733
>  [<ffffffff8108f08f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<     inlined    >] SYSC_exit_group kernel/exit.c:885
>  [<ffffffff8108f170>] __wake_up_parent+0x0/0x50 kernel/exit.c:883
>  [<ffffffff81eadb2e>] entry_SYSCALL_64_fastpath+0x12/0x71
> arch/x86/entry/entry_64.S:186
> 
> DBG: addr: ffff8800bb153a78
> DBG: first offset: 0, second offset: 0
> DBG: T309 clock: {T309: 1297809, T293: 747168}
> DBG: T293 clock: {T293: 747528}
> ==================================================================
> 
> On Thu, Sep 10, 2015 at 10:36 AM, Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
> > On Wed, Sep 09, 2015 at 08:58:26PM -0400, Sasha Levin wrote:
> >> On 09/07/2015 07:40 AM, Kirill A. Shutemov wrote:
> >> > On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote:
> >> >> > ==================================================================
> >> >> > ThreadSanitizer: data-race in munlock_vma_pages_range
> >> >> >
> >> >> > Write of size 8 by thread T378 (K2633, CPU3):
> >> >> >  [<ffffffff81212579>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
> >> >> >  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >> >> >  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >> >> >  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >> >> >  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >> >> >  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >> >> > arch/x86/entry/entry_64.S:186
> >> > ...
> >> >
> >> >> > Previous read of size 8 by thread T398 (K2623, CPU2):
> >> >> >  [<ffffffff8121d198>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208
> >> >> >  [<     inlined    >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540
> >> >> >  [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559
> >> >> >  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
> >> >> >  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
> >> >> >  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
> >> >> >  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
> >> >> >  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >> >> >  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >> >> >  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >> >> >  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >> >> >  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >> >> > arch/x86/entry/entry_64.S:186
> >> > Okay, the detected race is mlock/munlock vs. rmap.
> >> >
> >> > On rmap side we check vma->vm_flags in few places without taking
> >> > vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem
> >> > or anon_vma_lock, but nothing prevent vma->vm_flags from changing under
> >> > us.
> >> >
> >> > In this particular case, speculative check in beginning of
> >> > try_to_unmap_one() is fine, since we re-check it under mmap_sem later in
> >> > the function.
> >>
> >> So you're suggesting that this isn't the cause of the bad page flags
> >> error observed by Andrey and myself?
> >
> > I don't see it, but who knows.
> >
> > --
> >  Kirill A. Shutemov

-- 
 Kirill A. Shutemov

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