On Tue, 20 Nov 2012 22:56:11 +0800
Jiang Liu [off-list ref] wrote:

On 11/20/2012 07:38 AM, Andrew Morton wrote:

quoted

On Mon, 19 Nov 2012 00:07:26 +0800
Jiang Liu [off-list ref] wrote:

...

quoted

Also, the existing comment tells us that spanned_pages and
present_pages are protected by span_seqlock but has not been updated to
describe the locking (if any) for managed_pages.

How about this?

...

Looks nice.
 

quoted

quoted

+	for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
+		if (!is_highmem(z))

Needs a comment explaining why we skip the highmem zone, please.

How about this?

...

Ditto.

quoted

quoted

@@ -106,6 +106,7 @@ static void get_page_bootmem(unsigned long info,  struct page *page,
 void __ref put_page_bootmem(struct page *page)
 {
 	unsigned long type;
+	static DEFINE_MUTEX(ppb_lock);
 
 	type = (unsigned long) page->lru.next;
 	BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||

@@ -115,7 +116,9 @@ void __ref put_page_bootmem(struct page *page)
 		ClearPagePrivate(page);
 		set_page_private(page, 0);
 		INIT_LIST_HEAD(&page->lru);
+		mutex_lock(&ppb_lock);
 		__free_pages_bootmem(page, 0);
+		mutex_unlock(&ppb_lock);

The mutex is odd.  Nothing in the changelog, no code comment. 
__free_pages_bootmem() is called from a lot of places but only this one
has locking.  I'm madly guessing that the lock is here to handle two or
more concurrent memory hotpluggings, but I shouldn't need to guess!!

Actually I'm a little hesitate whether we should add a lock here.

All callers of __free_pages_bootmem() other than put_page_bootmem() should
only be used at startup time. And currently the only caller of put_page_bootmem()
has already been protected by pgdat_resize_lock(pgdat, &flags). So there's
no real need for lock, just defensive.

I'm not sure which is the best solution here.
1) add a comments into __free_pages_bootmem() to state that the caller should
   serialize themselves.
2) Use a dedicated lock to serialize updates to zone->managed_pages, this need
   modifications to page_alloc.c and memory_hotplug.c.
3) The above solution to serialize in put_page_bootmem().
What's your suggestions here?

Firstly, let's be clear about what *data* we're protecting here.  I
think it's only ->managed_pages?

I agree that no locking is needed during the init-time code.

So afaict we only need be concerned about concurrent updates to
->managed_pages via memory hotplug, and lock_memory_hotplug() is
sufficient there.  We don't need to be concerned about readers of
managed_pages because it is an unsigned long (a u64 on 32-bit machines
would be a problem).

All correct?  If so, the code is OK as-is and this can all be
described/formalised in code comments.  If one wants to be really
confident, we could do something along the lines of

void mod_zone_managed_pages(struct zone *zone, signed long delta)
{
	WARN_ON(system_state != SYSTEM_BOOTING &&
		!is_locked_memory_hotplug());
	zone->managed_pages += delta;
}

And yes, is_locked_memory_hotplug() is a dopey name. 
[un]lock_memory_hotplug() should have been called
memory_hotplug_[un]lock()!


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