Re: [RFC v5 34/38] procfs: display the protection-key number associated with a vma

From: Ram Pai <linuxram@us.ibm.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
	benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
	khandual@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com,
	bsingharora@gmail.com, hbabu@us.ibm.com, arnd@arndb.de,
	akpm@linux-foundation.org, corbet@lwn.net, mingo@redhat.com
Subject: Re: [RFC v5 34/38] procfs: display the protection-key number associated with a vma
Date: Thu, 13 Jul 2017 10:04:11 -0700	[thread overview]
Message-ID: <20170713170411.GI5525@ram.oc3035372033.ibm.com> (raw)
In-Reply-To: <e3355a7a-8899-b69d-968a-6862c29633a2@intel.com>

On Thu, Jul 13, 2017 at 07:07:48AM -0700, Dave Hansen wrote:
> On 07/13/2017 01:03 AM, Ram Pai wrote:
> > On Tue, Jul 11, 2017 at 11:13:56AM -0700, Dave Hansen wrote:
> >> On 07/05/2017 02:22 PM, Ram Pai wrote:
> >>> +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> >>> +void arch_show_smap(struct seq_file *m, struct vm_area_struct *vma)
> >>> +{
> >>> +	seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> >>> +}
> >>> +#endif /* CONFIG_PPC64_MEMORY_PROTECTION_KEYS */
> >>
> >> This seems like kinda silly unnecessary duplication.  Could we just put
> >> this in the fs/proc/ code and #ifdef it on ARCH_HAS_PKEYS?
> > 
> > Well x86 predicates it based on availability of X86_FEATURE_OSPKE.
> > 
> > powerpc doesn't need that check or any similar check. So trying to
> > generalize the code does not save much IMHO.
> 
> I know all your hardware doesn't support it. :)

Wow! you bring a good point which I had not considered yet. I need some
runtime checks for RPT.

But regardless, my above statement is still partially true. x86
predicates it based on availability of X86_FEATURE_OSPKE, and powerpc
should predicate it based on HPT. So we have our own
customized checks. Hence a unified function won't suffice.

> 
> So, for instance, if you are running on a new POWER9 with radix page
> tables, you will just always output "ProtectionKey: 0" in every VMA,
> regardless?
> 
> > maybe have a seperate inline function that does
> > seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> > and is called from x86 and powerpc's arch_show_smap()?
> > At least will keep the string format captured in 
> > one single place.
> 
> Now that we have two architectures, is there a strong reason we can't
> just have an arch_pkeys_enabled(), and stick the seq_printf() back in
> generic code?

correct. that looks like the correct approach. Was trying to avoid
touching arch neutral code. But this approach will force me
do so. Will do.

-- 
Ram Pai

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