From: Johannes Berg <redacted>
Subject: mm/slub: disable user tracing for kmemleak caches by default

If kmemleak is enabled, it uses a kmem cache for its own objects.  These
objects are used to hold information kmemleak uses, including a stack
trace.  If slub_debug is also turned on, each of them has *another* stack
trace, so the overhead adds up, and on my tests (on ARCH=um, admittedly)
2/3rds of the allocations end up being doing the stack tracing.

Turn off SLAB_STORE_USER if SLAB_NOLEAKTRACE was given, to avoid storing
the essentially same data twice.

Link: https://lkml.kernel.org/r/20210113215114.d94efa13ba30.I117b6764e725b3192318bbcf4269b13b709539ae@changeid
Signed-off-by: Johannes Berg <redacted>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Vlastimil Babka <redacted>
Cc: Christoph Lameter <redacted>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/slub.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/mm/slub.c~mm-slub-disable-user-tracing-for-kmemleak-caches-by-default
+++ a/mm/slub.c

@@ -1413,6 +1413,15 @@ slab_flags_t kmem_cache_flags(unsigned i
 	size_t len;
 	char *next_block;
 	slab_flags_t block_flags;
+	slab_flags_t slub_debug_local = slub_debug;
+
+	/*
+	 * If the slab cache is for debugging (e.g. kmemleak) then
+	 * don't store user (stack trace) information by default,
+	 * but let the user enable it via the command line below.
+	 */
+	if (flags & SLAB_NOLEAKTRACE)
+		slub_debug_local &= ~SLAB_STORE_USER;
 
 	len = strlen(name);
 	next_block = slub_debug_string;

@@ -1447,7 +1456,7 @@ slab_flags_t kmem_cache_flags(unsigned i
 		}
 	}
 
-	return flags | slub_debug;
+	return flags | slub_debug_local;
 }
 #else /* !CONFIG_SLUB_DEBUG */
 static inline void setup_object_debug(struct kmem_cache *s,

_