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 15:57:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ram Pai <linuxram@...ibm.com>, mpe@...erman.id.au,
        mingo@...hat.com, akpm@...ux-foundation.org
Cc:     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

> 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 "".