Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non_movable()

[Tang Chen <tangchen@cn.fujitsu.com>]

Hi Mel,

On 02/06/2013 05:56 PM, Mel Gorman wrote:
>
> There is the possibility that callbacks could be introduced for
> migrate_unpin() and migrate_pin() that takes a list of PFN pairs
> (old,new). The unpin callback should release the old PFNs and barrier
> against any operations until the migrate_pfn() callback is called with
> the updated pfns to be repinned. Again it would fully depend on subsystems
> implementing it properly.
>
> The callback interface would be more robust but puts a lot more work on
> the driver side where your milage will vary.
>

I'm very interested in the "callback" way you said.

For memory hot-remove case, the aio pages are pined in memory and making
the pages cannot be offlined, furthermore, the pages cannot be removed.

IIUC, you mean implement migrate_unpin() and migrate_pin() callbacks in aio
subsystem, and call them when hot-remove code tries to offline pages, 
right ?

If so, I'm wondering where should we put this callback pointers ?
In struct page ?


It has been a long time since this topic was discussed. But to solve this
problem cleanly for hotplug guys and CMA guys, please give some more 
comments.

Thanks. :)

>
> To guarantee CMA can migrate pages pinned by drivers I think you need
> migrate-related callsbacks to unpin, barrier the driver until migration
> completes and repin.
>
> I do not know, or at least have no heard, of anyone working on such a
> scheme.
>

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