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

From: Mike Kravetz
Date: Fri Jul 07 2017 - 13:16:13 EST

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.

Mike Kravetz