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

From: Anshuman Khandual
Date: Sun Jul 09 2017 - 03:23:58 EST

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.