linux-kernel - Re: [PATCH v13 3/3] mm, powerpc, x86: introduce an additional vma bit for powerpc pkey

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Date:   Fri, 4 May 2018 18:12:43 -0700
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
Subject: Re: [PATCH v13 3/3] mm, powerpc, x86: introduce an additional vma
 bit for powerpc pkey

On Fri, May 04, 2018 at 03:57:33PM -0700, Dave Hansen wrote:
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 0c9e392..3ddddc7 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -679,6 +679,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> >  		[ilog2(VM_PKEY_BIT1)]	= "",
> >  		[ilog2(VM_PKEY_BIT2)]	= "",
> >  		[ilog2(VM_PKEY_BIT3)]	= "",
> > +		[ilog2(VM_PKEY_BIT4)]	= "",
> >  #endif /* CONFIG_ARCH_HAS_PKEYS */
> ...
> > +#if defined(CONFIG_PPC)
> > +# define VM_PKEY_BIT4	VM_HIGH_ARCH_4
> > +#else 
> > +# define VM_PKEY_BIT4	0
> > +#endif
> >  #endif /* CONFIG_ARCH_HAS_PKEYS */
> 
> That new line boils down to:
> 
> 		[ilog2(0)]	= "",
> 
> on x86.  It wasn't *obvious* to me that it is OK to do that.  The other
> possibly undefined bits (VM_SOFTDIRTY for instance) #ifdef themselves
> out of this array.
> 
> I would just be a wee bit worried that this would overwrite the 0 entry
> ("??") with "".

Yes it would :-( and could potentially break anything that depends on
0th entry being "??"

Is the following fix acceptable?

#if VM_PKEY_BIT4
                [ilog2(VM_PKEY_BIT4)]   = "",
#endif

-- 
Ram Pai