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

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 ?

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