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

From: Kirill A. Shutemov
Date: Fri Jul 07 2017 - 06:23:32 EST

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