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From: Balbir Singh <bsingharora@gmail.com>
Date: Fri, 16 Mar 2018 22:02:22 +1100
Message-ID: <CAKTCnzmSCT+VecdSRpyY2Rb_AW2ngCi3UTZfLE3VOLNSQn6vsA@mail.gmail.com>
Subject: Re: [PATCH v4] mm, pkey: treat pkey-0 special
To: Ram Pai <linuxram@us.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>, Ingo Molnar <mingo@redhat.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)"
        <linuxppc-dev@lists.ozlabs.org>,
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        linux-arch <linux-arch@vger.kernel.org>,
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        Dave Hansen <dave.hansen@intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        Aneesh Kumar KV <aneesh.kumar@linux.vnet.ibm.com>,
        "Haren Myneni/Beaverton/IBM" <hbabu@us.ibm.com>,
        Michal Hocko <mhocko@kernel.org>,
        Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jonathan Corbet <corbet@lwn.net>, Arnd Bergmann <arnd@arndb.de>,
        fweimer@redhat.com, msuchanek@suse.com,
        Thomas Gleixner <tglx@linutronix.de>, Ulrich.Weigand@de.ibm.com,
        Ram Pai <ram.n.pai@gmail.com>
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On Fri, Mar 16, 2018 at 9:33 PM, Ram Pai <linuxram@us.ibm.com> wrote:
> Applications need the ability to associate an address-range with some
> key and latter revert to its initial default key. Pkey-0 comes close to
> providing this function but falls short, because the current
> implementation disallows applications to explicitly associate pkey-0 to
> the address range.
>
> Clarify the semantics of pkey-0 and provide the corresponding
> implementation.
>
> Pkey-0 is special with the following semantics.
> (a) it is implicitly allocated and can never be freed. It always exists.
> (b) it is the default key assigned to any address-range.
> (c) it can be explicitly associated with any address-range.
>
> Tested on powerpc only. Could not test on x86.


Ram,

I was wondering if we should check the AMOR values on the ppc side to make sure
that pkey0 is indeed available for use as default. I am still of the
opinion that we
should consider non-0 default pkey in the long run. I'm OK with the patches for
now, but really 0 is not special except for it being the default bit
values present
in the PTE.

The patches themselves look OK to me

Balbir Singh.