Re: Multiple potential races on vma->vm_flags

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Oct 14, 2015 at 12:33 AM, Hugh Dickins <hughd@google.com> wrote:
> 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?

Yes, you're correct, there was a bug in KTSAN annotations.
Thank you for finding this.

Fixing that bug fixes the bad page reports.
Sorry for troubling you with this.

The race reports are still there though.

>
>>
>> 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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