linux-kernel - mm, x86, powerpc: pkey semantics for key-0 ?

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Date:   Thu, 8 Mar 2018 10:25:17 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     mpe@...erman.id.au, mingo@...hat.com, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com, corbet@....net,
        arnd@...db.de, fweimer@...hat.com, msuchanek@...e.com,
        Ulrich.Weigand@...ibm.com
Subject: mm, x86, powerpc: pkey semantics for key-0 ?

Dave,

Is there a reason why the default key; key-0, is not allowed to be
explicitly associated with pages using pkey_mprotect()?

I see valid usecases where an application may initially want to
associate an address-range with some key and latter choose to revert to
its initial state, by associating key-0.  However our implementation
(both x86 and power) do not allow pkey_mprotect() to be called with
key-0.

I do not see a reason why it must be blocked.

Thoughts?
RP