[PATCH v2] mm/kmemleak: Avoid scanning potential huge holes

[Lang Yu <lang.yu@xxxxxxx>]

When using devm_request_free_mem_region() and devm_memremap_pages()
to add ZONE_DEVICE memory, if requested free mem region's end pfn
were huge(e.g., 0x400000000), the node_end_pfn() will be also huge
(see move_pfn_range_to_zone()). Thus it creates a huge hole between
node_start_pfn() and node_end_pfn().

We found on some AMD APUs, amdkfd requested such a free mem region
and created a huge hole. In such a case, following code snippet was
just doing busy test_bit() looping on the huge hole.

for (pfn = start_pfn; pfn < end_pfn; pfn++) {
	struct page *page = pfn_to_online_page(pfn);
		if (!page)
			continue;
	...
}

So we got a soft lockup:

watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [bash:1221]
CPU: 6 PID: 1221 Comm: bash Not tainted 5.15.0-custom #1
RIP: 0010:pfn_to_online_page+0x5/0xd0
Call Trace:
  ? kmemleak_scan+0x16a/0x440
  kmemleak_write+0x306/0x3a0
  ? common_file_perm+0x72/0x170
  full_proxy_write+0x5c/0x90
  vfs_write+0xb9/0x260
  ksys_write+0x67/0xe0
  __x64_sys_write+0x1a/0x20
  do_syscall_64+0x3b/0xc0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

I did some tests with the patch.

(1) amdgpu module unloaded

before the patch:

real    0m0.976s
user    0m0.000s
sys     0m0.968s

after the patch:

real    0m0.981s
user    0m0.000s
sys     0m0.973s

(2) amdgpu module loaded

before the patch:

real    0m35.365s
user    0m0.000s
sys     0m35.354s

after the patch:

real    0m1.049s
user    0m0.000s
sys     0m1.042s

v2:
- Only scan pages belonging to the zone.(David Hildenbrand)
- Use __maybe_unused to make compilers happy.

Signed-off-by: Lang Yu <lang.yu@xxxxxxx>
---
 mm/kmemleak.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index b57383c17cf6..adbe5aa01184 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1403,7 +1403,8 @@ static void kmemleak_scan(void)
 {
 	unsigned long flags;
 	struct kmemleak_object *object;
-	int i;
+	struct zone *zone;
+	int __maybe_unused i;
 	int new_leaks = 0;
 
 	jiffies_last_scan = jiffies;
@@ -1443,9 +1444,9 @@ static void kmemleak_scan(void)
 	 * Struct page scanning for each node.
 	 */
 	get_online_mems();
-	for_each_online_node(i) {
-		unsigned long start_pfn = node_start_pfn(i);
-		unsigned long end_pfn = node_end_pfn(i);
+	for_each_populated_zone(zone) {
+		unsigned long start_pfn = zone->zone_start_pfn;
+		unsigned long end_pfn = zone_end_pfn(zone);
 		unsigned long pfn;
 
 		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
@@ -1454,8 +1455,8 @@ static void kmemleak_scan(void)
 			if (!page)
 				continue;
 
-			/* only scan pages belonging to this node */
-			if (page_to_nid(page) != i)
+			/* only scan pages belonging to this zone */
+			if (page_zone(page) != zone)
 				continue;
 			/* only scan if page is in use */
 			if (page_count(page) == 0)
-- 
2.25.1