mm: kmemleak: use the memory pool for early allocations

Currently kmemleak uses a static early_log buffer to trace all memory
allocation/freeing before the slab allocator is initialised.  Such early
log is replayed during kmemleak_init() to properly initialise the kmemleak
metadata for objects allocated up that point.  With a memory pool that
does not rely on the slab allocator, it is possible to skip this early log
entirely.

In order to remove the early logging, consider kmemleak_enabled == 1 by
default while the kmem_cache availability is checked directly on the
object_cache and scan_area_cache variables.  The RCU callback is only
invoked after object_cache has been initialised as we wouldn't have any
concurrent list traversal before this.

In order to reduce the number of callbacks before kmemleak is fully
initialised, move the kmemleak_init() call to mm_init().

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: remove WARN_ON(), per Catalin]
Link: http://lkml.kernel.org/r/20190812160642.52134-4-catalin.marinas@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.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>

init/main.c

@@ -556,6 +556,7 @@ static void __init mm_init(void)
 	report_meminit();
 	mem_init();
 	kmem_cache_init();
+	kmemleak_init();
 	pgtable_init();
 	debug_objects_mem_init();
 	vmalloc_init();
@@ -740,7 +741,6 @@ asmlinkage __visible void __init start_kernel(void)
 		initrd_start = 0;
 	}
 #endif
-	kmemleak_init();
 	setup_per_cpu_pageset();
 	numa_policy_init();
 	acpi_early_init();

lib/Kconfig.debug

@@ -576,17 +576,18 @@ config DEBUG_KMEMLEAK
 	  In order to access the kmemleak file, debugfs needs to be
 	  mounted (usually at /sys/kernel/debug).
 
-config DEBUG_KMEMLEAK_EARLY_LOG_SIZE
-	int "Maximum kmemleak early log entries"
+config DEBUG_KMEMLEAK_MEM_POOL_SIZE
+	int "Kmemleak memory pool size"
 	depends on DEBUG_KMEMLEAK
 	range 200 40000
 	default 16000
 	help
 	  Kmemleak must track all the memory allocations to avoid
 	  reporting false positives. Since memory may be allocated or
-	  freed before kmemleak is initialised, an early log buffer is
-	  used to store these actions. If kmemleak reports "early log
-	  buffer exceeded", please increase this value.
+	  freed before kmemleak is fully initialised, use a static pool
+	  of metadata objects to track such callbacks. After kmemleak is
+	  fully initialised, this memory pool acts as an emergency one
+	  if slab allocations fail.
 
 config DEBUG_KMEMLEAK_TEST
 	tristate "Simple test for the kernel memory leak detector"