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