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

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Date:   Fri, 7 Jul 2017 13:23:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        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 Thu, Jul 06, 2017 at 09:17:26AM -0700, 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.

The patch breaks important invariant that anon page can be mapped into a
process only once.

What is going to happen to mirrored after CoW for instance?

In my opinion, it shouldn't be allowed for anon/private mappings at least.
And with this limitation, I don't see much sense in the new interface --
just create mirror by mmap()ing the file again.

-- 
 Kirill A. Shutemov