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

[Anshuman Khandual]

On 07/07/2017 11:39 PM, Mike Kravetz wrote:
> On 07/07/2017 10:45 AM, Kirill A. Shutemov wrote:
>> On Fri, Jul 07, 2017 at 10:29:52AM -0700, Mike Kravetz wrote:
>>> On 07/07/2017 03:23 AM, Kirill A. Shutemov wrote:
>>>> 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.
>>>
>>> Actually, the patch does not add any new functionality.  It only provides
>>> a new interface to existing functionality.
>>>
>>> Is it not possible to have an anon page mapped twice into the same process
>>> via system V shared memory?  shmget(anon), shmat(), shmat.  
>>> Of course, those are shared rather than private anon pages.
>>
>> By anon pages I mean, private anon or file pages. These are subject to CoW.
>>
>>>> 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.
>>>
>>> The code today works for anon shared mappings.  See simple program below.
>>>
>>> You are correct in that it makes little or no sense for private mappings.
>>> When looking closer at existing code, mremap() creates a new private
>>> mapping in this case.  This is most likely a bug.
>>
>> IIRC, existing code doesn't create mirrors of private pages as it requires
>> old_len to be zero. There's no way to get private pages mapped twice this
>> way.
> 
> Correct.
> As mentioned above, mremap does 'something' for private anon pages when
> old_len == 0.  However, this may be considered a bug.  In this case, mremap
> creates a new private anon mapping of length new_size.  Since old_len == 0,
> it does not unmap any of the old mapping.  So, in this case mremap basically
> creates a new private mapping (unrealted to the original) and does not
> modify the old mapping.
> 

Yeah, in my experiment, after the mremap() exists we have two different VMAs
which can contain two different set of data. No page sharing is happening.