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Date: Wed, 22 Oct 2014 12:22:23 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Dominik Dingel <dingel@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, Mel Gorman <mgorman@suse.de>,
        Michal Hocko <mhocko@suse.cz>, Paolo Bonzini <pbonzini@redhat.com>,
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        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: [PATCH 2/4] mm: introduce mm_forbids_zeropage function
Message-Id: <20141022122223.f3bef0f497941fa8e0805dbf@linux-foundation.org>
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On Wed, 22 Oct 2014 13:09:28 +0200 Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:

> Add a new function stub to allow architectures to disable for
> an mm_structthe backing of non-present, anonymous pages with
> read-only empty zero pages.
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -56,6 +56,10 @@ extern int sysctl_legacy_va_layout;
>  #define __pa_symbol(x)  __pa(RELOC_HIDE((unsigned long)(x), 0))
>  #endif
>  
> +#ifndef mm_forbids_zeropage
> +#define mm_forbids_zeropage(X)  (0)
> +#endif

Can we document this please?  What it does, why it does it.  We should
also specify precisely which arch header file is responsible for
defining mm_forbids_zeropage.