Subject: Re: [PATCH] powerpc/mm: Recompute hash value after a failed update

From: Michael Ellerman <[email-masked]>
Date: Wed, 16 Sep 2015 17:27:48 +1000

On Wed, 2015-09-16 at 11:27 +0530, Aneesh Kumar K.V wrote:
> Michael Ellerman <[email-masked]> writes:
> 
> > On Wed, 2015-09-16 at 08:53 +0530, Aneesh Kumar K.V wrote:
> >> "Aneesh Kumar K.V" <[email-masked]> writes:
> >> 
> >> > If we had secondary hash flag set, we ended up modifying hash value in
> >> > the updatepp code path. Hence with a failed updatepp we will be using
> >> > a wrong hash value for the following hash insert. Fix this by
> >> > recomputing hash before insert.
> >> 
> >> Without this patch we can end up with using wrong slot number in linux
> >> pte. That can result in us missing an hash pte update or invalidate
> >> which can cause memory corruption or even machine check ?
> >
> > Thanks. When did this break? Always? If so this should go to stable?
> >
> 
> IIUC we have this issue with initial support for THP (6d492ecc6489113968ec269be1cf88942d4a5d29)
> " powerpc/THP: Add code to handle HPTE faults for hugepages". So yes
> this should got to stable.

Thanks. And that went into 3.11.

You haven't actually seen any crashes that are definitely linked to this though
am I right? You just found it by code inspection?

cheers