linux-kernel - Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

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Date:   Tue, 19 Dec 2017 21:50:24 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Dave Hansen <dave.hansen@...el.com>, Ram Pai <linuxram@...ibm.com>,
        mingo@...hat.com, akpm@...ux-foundation.org, corbet@....net,
        arnd@...db.de
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...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
Subject: Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

Dave Hansen <dave.hansen@...el.com> writes:

> On 11/06/2017 12:57 AM, Ram Pai wrote:
>> Expose useful information for programs using memory protection keys.
>> Provide implementation for powerpc and x86.
>> 
>> On a powerpc system with pkeys support, here is what is shown:
>> 
>> $ head /sys/kernel/mm/protection_keys/*
>> ==> /sys/kernel/mm/protection_keys/disable_access_supported <==
>> true
>
> This is cute, but I don't think it should be part of the ABI.  Put it in
> debugfs if you want it for cute tests.  The stuff that this tells you
> can and should come from pkey_alloc() for the ABI.

Yeah I agree this is not sysfs material.

In particular the total/usable numbers are completely useless vs other
threads allocating pkeys out from under you.

> http://man7.org/linux/man-pages/man7/pkeys.7.html
>
>>        Any application wanting to use protection keys needs to be able to
>>        function without them.  They might be unavailable because the
>>        hardware that the application runs on does not support them, the
>>        kernel code does not contain support, the kernel support has been
>>        disabled, or because the keys have all been allocated, perhaps by a
>>        library the application is using.  It is recommended that
>>        applications wanting to use protection keys should simply call
>>        pkey_alloc(2) and test whether the call succeeds, instead of
>>        attempting to detect support for the feature in any other way.
>
> Do you really not have standard way on ppc to say whether hardware
> features are supported by the kernel?  For instance, how do you know if
> a given set of registers are known to and are being context-switched by
> the kernel?

Yes we do, we emit feature bits in the AT_HWCAP entry of the aux vector,
same as some other architectures.

But I don't see the need to use a feature bit for pkeys. If they're not
supported then pkey_alloc() will just always fail. Apps have to handle
that anyway because keys are a finite resource.

cheers