[ofa-general] Re: [PATCH 01 of 11] mmu-notifier-core

Jack Steiner
2008-05-05T17:25:06Z

On Mon, May 05, 2008 at 07:14:34PM +0200, Andrea Arcangeli wrote:
> On Mon, May 05, 2008 at 11:21:13AM -0500, Jack Steiner wrote:
> > The GRU does the registration/deregistration of mmu notifiers from mmap/munmap.
> > At this point, the mmap_sem is already held writeable. I hit a deadlock
> > in mm_lock.
>
> It'd been better to know about this detail earlier,

Agree. My apologies... I should have caught it.

> but frankly this
> is a minor problem, the important thing is we all agree together on
> the more difficult parts ;).
>
> > A quick fix would be to do one of the following:
> >
> > - move the mmap_sem locking to the caller of the [de]registration routines.
> > Since the first/last thing done in mm_lock/mm_unlock is to
> > acquire/release mmap_sem, this change does not cause major changes.
>
> I don't like this solution very much. Nor GRU nor KVM will call
> mmu_notifier_register inside the mmap_sem protected sections, so I
> think the default mmu_notifier_register should be smp safe by itself
> without requiring additional locks to be artificially taken externally
> (especially because the need for mmap_sem in write mode is a very
> mmu_notifier internal detail).
>
> > - add a flag to mmu_notifier_[un]register routines to indicate
> > if mmap_sem is already locked.
>
> The interface would change like this:
>
> #define MMU_NOTIFIER_REGISTER_MMAP_SEM (1<<0)
> void mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm,
> unsigned long mmu_notifier_flags);

That works...

>
> A third solution is to add:
>
> /*
> * This must can be called instead of mmu_notifier_register after
> * taking the mmap_sem in write mode (read mode isn't enough).
> */
> void __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm);
>
> Do you still prefer the bitflag or you prefer
> __mmu_notifier_register. It's ok either ways, except
> __mmu_notifier_reigster could be removed in a backwards compatible
> way, the bitflag can't.
>
> > I've temporarily deleted the mm_lock locking of mmap_sem and am continuing to
> > test. More later....

__mmu_notifier_register/__mmu_notifier_unregister seems like a better way to
go, although either is ok.

>
> Sure! In the meantime go ahead this way.
>
> Another very minor change I've been thinking about is to make
> ->release not mandatory. It happens that with KVM ->release isn't
> strictly required because after mm_users reaches 0, no guest could
> possibly run anymore. So I'm using ->release only for debugging by
> placing -1UL in the root shadow pagetable, to be sure ;). So because
> at least one user won't strictly require ->release being consistent in
> having all method optional may be nicer. Alternatively we could make
> them all mandatory and if somebody doesn't need one of the methods it
> should implement it as a dummy function. Both ways have pros and cons,
> but they don't make any difference to us in practice. If I've to
> change the patch for the mmap_sem taken during registration I may as
> well cleanup this minor bit.

Let me finish my testing. At one time, I did not use ->release but
with all the locking & teardown changes, I need to do some reverification.

--- jack