mm: kmemleak: make the tool tolerant to struct scan_area allocation failures

Patch series "mm: kmemleak: Use a memory pool for kmemleak object
allocations", v3.

Following the discussions on v2 of this patch(set) [1], this series takes
slightly different approach:

- it implements its own simple memory pool that does not rely on the
  slab allocator

- drops the early log buffer logic entirely since it can now allocate
  metadata from the memory pool directly before kmemleak is fully
  initialised

- CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option is renamed to
  CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE

- moves the kmemleak_init() call earlier (mm_init())

- to avoid a separate memory pool for struct scan_area, it makes the
  tool robust when such allocations fail as scan areas are rather an
  optimisation

[1] http://lkml.kernel.org/r/20190727132334.9184-1-catalin.marinas@arm.com

This patch (of 3):

Object scan areas are an optimisation aimed to decrease the false
positives and slightly improve the scanning time of large objects known to
only have a few specific pointers.  If a struct scan_area fails to
allocate, kmemleak can still function normally by scanning the full
object.

Introduce an OBJECT_FULL_SCAN flag and mark objects as such when scan_area
allocation fails.

Link: http://lkml.kernel.org/r/20190812160642.52134-2-catalin.marinas@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kmemleak.c

@@ -168,6 +168,8 @@ struct kmemleak_object {
 #define OBJECT_REPORTED		(1 << 1)
 /* flag set to not scan the object */
 #define OBJECT_NO_SCAN		(1 << 2)
+/* flag set to fully scan the object when scan_area allocation failed */
+#define OBJECT_FULL_SCAN	(1 << 3)
 
 #define HEX_PREFIX		"    "
 /* number of bytes to print per line; must be 16 or 32 */
@@ -773,12 +775,14 @@ static void add_scan_area(unsigned long ptr, size_t size, gfp_t gfp)
 	}
 
 	area = kmem_cache_alloc(scan_area_cache, gfp_kmemleak_mask(gfp));
-	if (!area) {
-		pr_warn("Cannot allocate a scan area\n");
-		goto out;
-	}
 
 	spin_lock_irqsave(&object->lock, flags);
+	if (!area) {
+		pr_warn_once("Cannot allocate a scan area, scanning the full object\n");
+		/* mark the object for full scan to avoid false positives */
+		object->flags |= OBJECT_FULL_SCAN;
+		goto out_unlock;
+	}
 	if (size == SIZE_MAX) {
 		size = object->pointer + object->size - ptr;
 	} else if (ptr + size > object->pointer + object->size) {
@@ -795,7 +799,6 @@ static void add_scan_area(unsigned long ptr, size_t size, gfp_t gfp)
 	hlist_add_head(&area->node, &object->area_list);
 out_unlock:
 	spin_unlock_irqrestore(&object->lock, flags);
-out:
 	put_object(object);
 }
 
@@ -1408,7 +1411,8 @@ static void scan_object(struct kmemleak_object *object)
 	if (!(object->flags & OBJECT_ALLOCATED))
 		/* already freed object */
 		goto out;
-	if (hlist_empty(&object->area_list)) {
+	if (hlist_empty(&object->area_list) ||
+	    object->flags & OBJECT_FULL_SCAN) {
 		void *start = (void *)object->pointer;
 		void *end = (void *)(object->pointer + object->size);
 		void *next;