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    SubjectRe: Multiple potential races on vma->vm_flags
    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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