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From:   Johannes Berg <johannes@sipsolutions.net>
To:     linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Berg <johannes.berg@intel.com>
Subject: [PATCH v2] mm/slub: disable user tracing for kmemleak caches by default
Date:   Wed, 13 Jan 2021 21:51:14 +0100
Message-Id: <20210113215114.d94efa13ba30.I117b6764e725b3192318bbcf4269b13b709539ae@changeid>
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From: Johannes Berg <johannes.berg@intel.com>

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.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
Perhaps instead it should go the other way around, and kmemleak
could even use/access the stack trace that's already in there ...
But I don't really care too much, I can just turn off slub debug
for the kmemleak caches via the command line anyway :-)

v2:
 - strip SLAB_STORE_USER only coming from slub_debug so that the
   command line args always take effect

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

diff --git a/mm/slub.c b/mm/slub.c
index 34dcc09e2ec9..a66c9948c529 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1412,6 +1412,15 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
 	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;
@@ -1446,7 +1455,7 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
 		}
 	}
 
-	return flags | slub_debug;
+	return flags | slub_debug_local;
 }
 #else /* !CONFIG_SLUB_DEBUG */
 static inline void setup_object_debug(struct kmem_cache *s,
-- 
2.26.2