Re: [PATCH v3] mm: add mremap flag for preserving the old mapping

[Andy Lutomirski]

On Mon, Sep 29, 2014 at 9:55 PM, Daniel Micay <danielmicay@xxxxxxxxx> wrote:
> This introduces the MREMAP_RETAIN flag for preserving the source mapping
> when MREMAP_MAYMOVE moves the pages to a new destination. Accesses to
> the source location will fault and cause fresh pages to be mapped in.
>
> For consistency, the old_len >= new_len case could decommit the pages
> instead of unmapping. However, userspace can accomplish the same thing
> via madvise and a coherent definition of the flag is possible without
> the extra complexity.

IMO this needs very clear documentation of exactly what it does.

Does it preserve the contents of the source pages?  (If so, why?
Aren't you wasting a bunch of time on page faults and possibly
unnecessary COWs?)

Does it work on file mappings?  Can it extend file mappings while it moves them?

If you MREMAP_RETAIN a partially COWed private mapping, what happens?

Does it work on special mappings?  If so, please prevent it from doing
so.  mremapping x86's vdso is a thing, and duplicating x86's vdso
should not become a thing, because x86_32 in particular will become
extremely confused.

--Andy
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