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

[Suren Baghdasaryan]

On Wed, Aug 9, 2023 at 11:31 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Wed, Aug 9, 2023 at 11:08 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Wed, Aug 9, 2023 at 11:04 AM David Hildenbrand <david@xxxxxxxxxx> 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.
>
> Thanks. Just to eliminate the possibility, I'll try reverting my
> patchset in mm-unstable and will try the test again. Will do that in
> the evening once I'm home.
>
> > >
> > > >
> > > > 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.

Yeah, it is somehow related to per-vma locking. Unfortunately I can't
reproduce the issue on my VM, so I have to use my host and bisection
is slow. I think I'll get to the bottom of this tomorrow.

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