Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Aug 9, 2023 at 11:04 AM David Hildenbrand <david@redhat.com> wrote:
>
> >>>> Which ends up being
> >>>>
> >>>> VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
> >>>>
> >>>> I did not check if this is also the case on mainline, and if this series is responsible.
> >>>
> >>> Thanks for reporting! I'm checking it now.
> >>
> >> Hmm. From the code it's not obvious how lock_mm_and_find_vma() ends up
> >> calling find_vma() without mmap_lock after successfully completing
> >> get_mmap_lock_carefully(). lock_mm_and_find_vma+0x3f/0x270 points to
> >> the first invocation of find_vma(), so this is not even the lock
> >> upgrade path... I'll try to reproduce this issue and dig up more but
> >> from the information I have so far this issue does not seem to be
> >> related to this series.
>
> I just checked on mainline and it does not fail there.
>
> >
> > This is really weird. I added mmap_assert_locked(mm) calls into
> > get_mmap_lock_carefully() right after we acquire mmap_lock read lock
> > and one of them triggers right after successful
> > mmap_read_lock_killable(). Here is my modified version of
> > get_mmap_lock_carefully():
> >
> > static inline bool get_mmap_lock_carefully(struct mm_struct *mm,
> > struct pt_regs *regs) {
> >       /* Even if this succeeds, make it clear we might have slept */
> >       if (likely(mmap_read_trylock(mm))) {
> >           might_sleep();
> >           mmap_assert_locked(mm);
> >           return true;
> >       }
> >       if (regs && !user_mode(regs)) {
> >           unsigned long ip = instruction_pointer(regs);
> >           if (!search_exception_tables(ip))
> >               return false;
> >       }
> >       if (!mmap_read_lock_killable(mm)) {
> >           mmap_assert_locked(mm);                     <---- generates a BUG
> >           return true;
> >       }
> >       return false;
> > }
>
> Ehm, that's indeed weird.
>
> >
> > AFAIKT conditions for mmap_read_trylock() and
> > mmap_read_lock_killable() are checked correctly. Am I missing
> > something?
>
> Weirdly enough, it only triggers during that specific uffd test, right?

Yes, uffd-unit-tests. I even ran it separately to ensure it's not some
fallback from a previous test and I'm able to reproduce this
consistently.

>
> --
> Cheers,
>
> David / dhildenb
>