linux-kernel - Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality

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Message-Id: <37f275bb-57c2-1485-02f2-dc71021f612a@linux.vnet.ibm.com>
Date:   Sun, 9 Jul 2017 12:53:30 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-mm@...ck.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Aaron Lu <aaron.lu@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing
 mirroring functionality

On 07/07/2017 10:44 PM, Mike Kravetz wrote:
> On 07/07/2017 01:45 AM, Anshuman Khandual wrote:
>> 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.
> 
> No real reason.  I just wanted to clearly separate the new interface from
> the old.  On second thought, it would be better to require old_size == 0
> as in the legacy interface.

That would be redundant. Mirroring will just happen because old_size is
0 whether we mention the MREMAP_MIRROR flag or not. IMHO it should just
mirror if the flag is specified irrespective of the old_size value.