Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing
 mirroring functionality
To: Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1499357846-7481-1-git-send-email-mike.kravetz@oracle.com>
 <1499357846-7481-2-git-send-email-mike.kravetz@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, Michal Hocko <mhocko@suse.com>,
        Aaron Lu <aaron.lu@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Date: Fri, 7 Jul 2017 14:15:58 +0530
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On 07/06/2017 09:47 PM, Mike Kravetz wrote:
> The mremap system call has the ability to 'mirror' parts of an existing
> mapping.  To do so, it creates a new mapping that maps the same pages as
> the original mapping, just at a different virtual address.  This
> functionality has existed since at least the 2.6 kernel.
> 
> This patch simply adds a new flag to mremap which will make this
> functionality part of the API.  It maintains backward compatibility with
> the existing way of requesting mirroring (old_size == 0).
> 
> If this new MREMAP_MIRROR flag is specified, then new_size must equal
> old_size.  In addition, the MREMAP_MAYMOVE flag must be specified.

Yeah it all looks good. But why is this requirement that if
MREMAP_MAYMOVE is specified then old_size and new_size must
be equal.