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Message-ID: <31b91946-af43-8795-0d4a-cb93899ccbce@suse.cz>
Date: Thu, 14 Jan 2021 12:07:54 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Berg <johannes@...solutions.net>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Catalin Marinas <catalin.marinas@....com>,
Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH v2] mm/slub: disable user tracing for kmemleak caches by
default
On 1/13/21 9:51 PM, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.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@...el.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> 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,
>
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