Re: [RFC 2.6.11-rc2-mm2 0/7] mm: manual page migration -- overview II

[Andi Kleen]

On Mon, Feb 21, 2005 at 02:42:16AM -0600, Ray Bryant wrote:
> All,
> 
> Just an update on the idea of migrating a process without suspending
> it.
> 
> The hard part of the problem here is to make sure that the page_migrate()
> system call sees all of the pages to migrate.  If the process that is
> being migrated can still allocate pages, then the page_migrate() call
> may miss some of the pages.

I would do an easy 95% solution:

When process has default process policy set temporarily a prefered policy
with the new node

[this won't work with multiple nodes though, so you have to decide on one
or stop the process if that is unacceptable] 

> 
> One way to solve this problem is to force the process to start allocating
> pages on the new nodes before calling page_migrate().  There are a couple
> of subcases:
> 
> (1)  For memory mapped files with a non-DEFAULT associated memory policy,
>      one can use mbind() to fixup the memory policy.  (This assumes the
>      Steve Longerbeam patches are applied, as I understand things).

I would just ignore them.  If user space wants it can handle it,
but it's probably not worth it.

> (1) could be handled as part of the page_migrate() system call --
> make one pass through the address space searching for mempolicy()
> data structures, and updating them as necessary.  Then make a second
> pass through and do the migrations.  Any new allocations will then
> be done under the new mempolicy, so they won't be missed.  But this
> still gets us into trouble if the old and new node lists are not
> disjoint.

I wouldn't bother fixing up VMA policies. 

> This doesn't handle anonymous memory or mapped files associated with
> the DEFAULT policy.  A way around that would be to add a target cpu_id

[...]

I would set temporarily a prefered policy as mentioned above.

That only handles a single node, but you solution is not better.

-Andi
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