Re: Trying to make use of hotplug memory for xen balloon driver

[Jeremy Fitzhardinge]

Dave Hansen wrote:
> Yeah, this is your problem.  You've only allocated the iomem *resource*
> for the memory area, which means that you've basically claimed the
> physical addresses.
>
> But, you don't have any 'struct page's there.
>
> We really screwed up the memory hotplug code and ended up with some
> incredibly arcane function names.  You might want to look at
> add_memory().  It is hidden away in mm/memory_hotplug.c :)
>   

Sorry, I should have been clearer.  add_memory_resource() is a function 
I added; it's effectively add_memory() with the resource-allocating part 
factored out:

--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -171,7 +171,10 @@

#endif /* ! CONFIG_MEMORY_HOTPLUG */

+struct resource;
+
extern int add_memory(int nid, u64 start, u64 size);
+extern int add_memory_resource(int nid, struct resource *res);
extern int arch_add_memory(int nid, u64 start, u64 size);
extern int remove_memory(u64 start, u64 size);
extern int sparse_add_one_section(struct zone *zone, unsigned long start_pfn,
===================================================================
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -278,14 +278,28 @@

int add_memory(int nid, u64 start, u64 size)
{
-	pg_data_t *pgdat = NULL;
-	int new_pgdat = 0;
	struct resource *res;
	int ret;

	res = register_memory_resource(start, size);
	if (!res)
		return -EEXIST;
+
+	ret = add_memory_resource(nid, res);
+
+	if (ret)
+		release_memory_resource(res);
+
+	return ret;
+}
+
+int add_memory_resource(int nid, struct resource *res)
+{
+	pg_data_t *pgdat = NULL;
+	int new_pgdat = 0;
+	int ret;
+	u64 start = res->start;
+	u64 size = res->end - res->start + 1;

	if (!node_online(nid)) {
		pgdat = hotadd_new_pgdat(nid, start);
@@ -320,8 +334,6 @@
	/* rollback pgdat allocation and others */
	if (new_pgdat)
		rollback_node_hotadd(nid, pgdat);
-	if (res)
-		release_memory_resource(res);

	return ret;
}


> You might also note that most of the ppc64 memory hotplug is driven by
> userspace.  The hypervisor actually contacts a daemon on the guest to
> tell it where its new memory is.  That daemon does the addition
> through /sys/devices/system/memory/probe.  
>   

X86 Xen does it with a combination of hypervisor and userspace.  Mostly 
it comes down to asking the hypervisor to provide a machine page to put 
under a guest pseudo-physical page.

   J
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