Re: [RFC PATCH 0/1] mm/mremap: add MREMAP_MIRROR flag

[Mike Kravetz]

On 07/07/2017 01:19 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 [1].  A comment
>> was added to the code to help preserve this feature.
> 
> 
> Is this the comment ? If yes, then its not very clear.
> 
> 	/*
> 	 * We allow a zero old-len as a special case
> 	 * for DOS-emu "duplicate shm area" thing. But
> 	 * a zero new-len is nonsensical.
> 	 */
> 

Yes, I believe that is the comment.

>>
>> The Oracle JVM team has discovered this feature and used it while
>> prototyping a new garbage collection model.  This new model shows promise,
>> and they are considering its use in a future release.  However, since
>> the only mention of this functionality is a single comment in the kernel,
>> they are concerned about its future.
>>
>> I propose the addition of a new MREMAP_MIRROR flag to explicitly request
>> this functionality.  The flag simply provides the same functionality as
>> the existing undocumented 'old_size == 0' interface.  As an alternative,
>> we could simply document the 'old_size == 0' interface in the man page.
>> In either case, man page modifications would be needed.
> 
> Right. Adding MREMAP_MIRROR sounds cleaner from application programming
> point of view. But it extends the interface.

Yes.  That is the reason for the RFC.  We currently have functionality
that is not clearly part of a programming interface.  Application programmers
do not like to depend on something that is not part of an interface.

>>
>> Future Direction
>>
>> After more formally adding this to the API (either new flag or documenting
>> existing interface), the mremap code could be enhanced to optimize this
>> case.  Currently, 'mirroring' only sets up the new mapping.  It does not
>> create page table entries for new mapping.  This could be added as an
>> enhancement.
> 
> Then how it achieves mirroring, both the pointers should see the same
> data, that can happen with page table entries pointing to same pages,
> right ?

Correct.

In the code today, page tables for the new (mirrored) mapping are created
as needed via faults.  The enhancement would be to create page table entries
for the new mapping.

-- 
Mike Kravetz