Re: [PATCH V4] mm: Extend memory hotplug API to allow memory hotplug in virtual machines

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 25 May 2011 00:27:33 +0200
Daniel Kiper <dkiper@net-space.pl> wrote:

> This patch applies to Linus' git tree, git commit 98b98d316349e9a028e632629fe813d07fa5afdd
> (Merge branch 'drm-core-next' of git://git.kernel.org/pub/scm/linux/kernel/git/airlied/drm-2.6)
> with a few prerequisite patches available at https://lkml.org/lkml/2011/5/2/296
> and https://lkml.org/lkml/2011/5/17/408 (all prerequisite patches were included in -mm tree).
> 
> This patch contains online_page_callback and apropriate functions for
> registering/unregistering online page callbacks. It allows to do some
> machine specific tasks during online page stage which is required
> to implement memory hotplug in virtual machines. Currently this patch
> is required by latest memory hotplug support for Xen balloon driver
> patch which will be posted soon.
> 
> Additionally, originial online_page() function was splited into
> following functions doing "atomic" operations:
>   - __online_page_set_limits() - set new limits for memory management code,
>   - __online_page_increment_counters() - increment totalram_pages and totalhigh_pages,
>   - __online_page_free() - free page to allocator.
> 
> It was done to:
>   - not duplicate existing code,
>   - ease hotplug code devolpment by usage of well defined interface,
>   - avoid stupid bugs which are unavoidable when the same code
>     (by design) is developed in many places.

I grabbed this and the xen patch.  I assume that all prerequisites
are now in mainline?

Please give some thought to making this extra code Kconfigurable, and
selected by Xen?  See if we can avoid a bit of bloat for other kernel
users.

What is missing from the patchset is an explanation of why we should
merge it ;) Why is this feature desirable?  What value does it provide
to our users?  Why should we bother?  Answering these questions in a
form which can be pasted into the changelog would be convenient,
thanks.

Is there any propsect that the other virtualisation schemes will use
this facility?  If not, why not?

>
> ...
>
> @@ -388,7 +450,7 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
>  	if (PageReserved(pfn_to_page(start_pfn)))
>  		for (i = 0; i < nr_pages; i++) {
>  			page = pfn_to_page(start_pfn + i);
> -			online_page(page);
> +			online_page_callback(page);

nit.  I'll change this to

			(*online_page_callback)(page);

because that syntax communicates some useful information to the reader.