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

[Daniel Jordan]

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.

That's a good idea, and especially worth doing considering that an arbitrary
number of threads that charge a low amount of locked_vm can fail just because
one thread charges lots of it.

pinned_vm appears to be broken the same way, so I can fix it too unless someone
beats me to it.