linux-kernel - Re: [PATCH v4] mm, pkey: treat pkey-0 special

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Message-ID: <CAKTCnzmSCT+VecdSRpyY2Rb_AW2ngCi3UTZfLE3VOLNSQn6vsA@mail.gmail.com>
Date:   Fri, 16 Mar 2018 22:02:22 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>, linux-mm <linux-mm@...ck.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
        "Haren Myneni/Beaverton/IBM" <hbabu@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>, fweimer@...hat.com,
        msuchanek@...e.com, Thomas Gleixner <tglx@...utronix.de>,
        Ulrich.Weigand@...ibm.com, Ram Pai <ram.n.pai@...il.com>
Subject: Re: [PATCH v4] mm, pkey: treat pkey-0 special

On Fri, Mar 16, 2018 at 9:33 PM, Ram Pai <linuxram@...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.