Re: [PATCH 5/6] powerpc/mmu: drop mmap_sem now that locked_vm is atomic

[Davidlohr Bueso]

On Tue, 23 Apr 2019, Bueso wrote:

> On Wed, 03 Apr 2019, Daniel Jordan wrote:
>
> > On Wed, Apr 03, 2019 at 06:58:45AM +0200, Christophe Leroy wrote:
> > > Le 02/04/2019 à 22:41, Daniel Jordan a écrit :
> > > > With locked_vm now an atomic, there is no need to take mmap_sem as
> > > > writer.  Delete and refactor accordingly.
> > >
> > > Could you please detail the change ?
> >
> > Ok, I'll be more specific in the next version, using some of your language in
> > fact.  :)
> >
> > > It looks like this is not the only
> > > change. I'm wondering what the consequences are.
> > >
> > > Before we did:
> > > - lock
> > > - calculate future value
> > > - check the future value is acceptable
> > > - update value if future value acceptable
> > > - return error if future value non acceptable
> > > - unlock
> > >
> > > Now we do:
> > > - atomic update with future (possibly too high) value
> > > - check the new value is acceptable
> > > - atomic update back with older value if new value not acceptable and return
> > > error
> > >
> > > So if a concurrent action wants to increase locked_vm with an acceptable
> > > step while another one has temporarily set it too high, it will now fail.
> > >
> > > I think we should keep the previous approach and do a cmpxchg after
> > > validating the new value.
>
> Wouldn't the cmpxchg alternative also be exposed the locked_vm changing between
> validating the new value and the cmpxchg() and we'd bogusly fail even when there
> is still just because the value changed (I'm assuming we don't hold any locks,
> otherwise all this is pointless).
>
>  current_locked = atomic_read(&mm->locked_vm);
>  new_locked = current_locked + npages;
>  if (new_locked < lock_limit)
>     if (cmpxchg(&mm->locked_vm, current_locked, new_locked) == current_locked)

Err, this being != of course.