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

From: Anshuman Khandual
Date: Fri Jul 07 2017 - 04:20:07 EST

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.

> 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.

> 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 ?