Re: Multiple potential races on vma->vm_flags

[Hugh Dickins <hughd@google.com>]

On Wed, 23 Sep 2015, Andrey Konovalov wrote:
> On Wed, Sep 23, 2015 at 3:39 AM, Hugh Dickins <hughd@google.com> wrote:
> > This is totally untested, and one of you may quickly prove me wrong;
> > but I went in to fix your "Bad page state (mlocked)" by holding pte
> > lock across the down_read_trylock of mmap_sem in try_to_unmap_one(),
> > then couldn't see why it would need mmap_sem at all, given how mlock
> > and munlock first assert intention by setting or clearing VM_LOCKED
> > in vm_flags, then work their way up the vma, taking pte locks.
> >
> > Calling mlock_vma_page() under pte lock may look suspicious
> > at first: but what it does is similar to clear_page_mlock(),
> > which we regularly call under pte lock from page_remove_rmap().
> >
> > I'd rather wait to hear whether this appears to work in practice,
> > and whether you agree that it should work in theory, before writing
> > the proper description.  I'd love to lose that down_read_trylock.
> 
> No, unfortunately it doesn't work, I still see "Bad page state (mlocked)".

I think I've found the answer to that at last: we were indeed
all looking in the wrong direction.  Your ktsan tree shows

static __always_inline int atomic_add_negative(int i, atomic_t *v)
{
#ifndef CONFIG_KTSAN
	GEN_BINARY_RMWcc(LOCK_PREFIX "addl", v->counter, "er", i, "%0", "s");
#else
	return (ktsan_atomic32_fetch_add((void *)v, i,
			ktsan_memory_order_acq_rel) + i) < 0;
#endif
}

but ktsan_atomic32_fetch_add() returns u32: so it looks like
your implementation of atomic_add_negative() always returns 0,
and page_remove_file_rmap() never calls clear_page_mlock(), as
it ought when an Mlocked page has been truncated or punched out.

/proc/meminfo gives you crazy AnonPages and Mapped too, yes?

> 
> It seems that your patch doesn't fix the race from the report below, since pte
> lock is not taken when 'vma->vm_flags &= ~VM_LOCKED;' (mlock.c:425)
> is being executed. (Line numbers are from kernel with your patch applied.)

I was not trying to "fix" that with my patch, because I couldn't find
any problem with the way it reads vm_flags there; I can't even see any
need for READ_ONCE or more barriers, we have sufficient locking already.

Sure, try_to_unmap_one() may read vm_flags an instant before or after
a racing mlock() or munlock() or exit_mmap() sets or clears VM_LOCKED;
but the syscalls (or exit) then work their way up the address space to
establish the final state, no problem.

But I am glad you drew attention to the inadequacy of the
down_read_trylock(mmap_sem) in try_to_unmap_one(), and since posting
that patch (doing the mlock_vma_page under pt lock instead), I have
identifed one case that it would fix - though it clearly wasn't
involved in your stacktrace (it's a race with truncating COWed pages,
but your trace was holepunching, which leaves the COWs alone).

I'll go forward with that patch, but it rather falls into a series
I was preparing, must finish up all their comments before posting.

Hugh

> 
> ===
> ThreadSanitizer: data-race in munlock_vma_pages_range
> 
> Write at 0xffff880282a93290 of size 8 by thread 2546 on CPU 2:
>  [<ffffffff81211009>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
>  [<     inline     >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81215d03>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085635>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inline     >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c2a7>] do_exit+0x457/0x1400 kernel/exit.c:733
>  [<ffffffff8108ef3f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<ffffffff810a03a5>] get_signal+0x375/0xa70 kernel/signal.c:2353
>  [<ffffffff8100619c>] do_signal+0x2c/0xad0 arch/x86/kernel/signal.c:704
>  [<ffffffff81006cbd>] do_notify_resume+0x7d/0x80 arch/x86/kernel/signal.c:749
>  [<ffffffff81ea87a4>] int_signal+0x12/0x17 arch/x86/entry/entry_64.S:329
> 
> Previous read at 0xffff880282a93290 of size 8 by thread 2545 on CPU 1:
>  [<ffffffff8121bc1a>] try_to_unmap_one+0x6a/0x450 mm/rmap.c:1208
>  [<     inline     >] rmap_walk_file mm/rmap.c:1522
>  [<ffffffff8121d1a7>] rmap_walk+0x147/0x450 mm/rmap.c:1541
>  [<ffffffff8121d962>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1405
>  [<ffffffff81210640>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
>  [<ffffffff81210af6>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
>  [<ffffffff81211330>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
>  [<     inline     >] munlock_vma_pages_all mm/internal.h:252
>  [<ffffffff81215d03>] exit_mmap+0x163/0x190 mm/mmap.c:2824
>  [<ffffffff81085635>] mmput+0x65/0x190 kernel/fork.c:708
>  [<     inline     >] exit_mm kernel/exit.c:437
>  [<ffffffff8108c2a7>] do_exit+0x457/0x1400 kernel/exit.c:733
>  [<ffffffff8108ef3f>] do_group_exit+0x7f/0x140 kernel/exit.c:874
>  [<ffffffff810a03a5>] get_signal+0x375/0xa70 kernel/signal.c:2353
>  [<ffffffff8100619c>] do_signal+0x2c/0xad0 arch/x86/kernel/signal.c:704
>  [<ffffffff81006cbd>] do_notify_resume+0x7d/0x80 arch/x86/kernel/signal.c:749
>  [<ffffffff81ea87a4>] int_signal+0x12/0x17 arch/x86/entry/entry_64.S:329
> ===
> 

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