mm, kmemleak: little optimization while scanning

kmemleak_scan() goes through all online nodes and tries to scan all used
pages.

We can do better and use pfn_to_online_page(), so in case we have
CONFIG_MEMORY_HOTPLUG, offlined pages will be skiped automatically.  For
boxes where CONFIG_MEMORY_HOTPLUG is not present, pfn_to_online_page()
will fallback to pfn_valid().

Another little optimization is to check if the page belongs to the node we
are currently checking, so in case we have nodes interleaved we will not
check the same pfn multiple times.

I ran some tests:

Add some memory to node1 and node2 making it interleaved:

(qemu) object_add memory-backend-ram,id=ram0,size=1G
(qemu) device_add pc-dimm,id=dimm0,memdev=ram0,node=1
(qemu) object_add memory-backend-ram,id=ram1,size=1G
(qemu) device_add pc-dimm,id=dimm1,memdev=ram1,node=2
(qemu) object_add memory-backend-ram,id=ram2,size=1G
(qemu) device_add pc-dimm,id=dimm2,memdev=ram2,node=1

Then, we offline that memory:
 # for i in {32..39} ; do echo "offline" > /sys/devices/system/node/node1/memory$i/state;done
 # for i in {48..55} ; do echo "offline" > /sys/devices/system/node/node1/memory$i/state;don
 # for i in {40..47} ; do echo "offline" > /sys/devices/system/node/node2/memory$i/state;done

And we run kmemleak_scan:

 # echo "scan" > /sys/kernel/debug/kmemleak

before the patch:

kmemleak: time spend: 41596 us

after the patch:

kmemleak: time spend: 34899 us

[[email protected]: remove stray newline, per Oscar]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Oscar Salvador <[email protected]>
Reviewed-by: Wei Yang <[email protected]>
Suggested-by: Michal Hocko <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kmemleak.c

@@ -1547,11 +1547,14 @@ static void kmemleak_scan(void)
 		unsigned long pfn;
 
 		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
-			struct page *page;
+			struct page *page = pfn_to_online_page(pfn);
 
-			if (!pfn_valid(pfn))
+			if (!page)
+				continue;
+
+			/* only scan pages belonging to this node */
+			if (page_to_nid(page) != i)
 				continue;
-			page = pfn_to_page(pfn);
 			/* only scan if page is in use */
 			if (page_count(page) == 0)
 				continue;