From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 62BED2673AA for ; Thu, 24 Sep 2026 14:48:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790261330; cv=none; b=RwwWbeNbK2x7TY2ZKAC9LmzJPgcbg70vwb6FV9CFEYoG9V8MfQ7S4ihl7me8lQ8ttJWZBVaOgjdKO1Pvht6wBK+P5p3YphUCl5k06SUBhwl2MKLNG/7pOKkvy/SWOHUpmSLIL2POqUxh4yvglpUfk1xNE0uYxmN1gKhPAH2lwnU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790261330; c=relaxed/simple; bh=arRcFbM2J6+AtpVKaoki3Ylxn5Q76Hl2oJubtqT/foE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=njCR3HCDMcGJOk+c57bq/dYivm5yqtpYvAkb3X6Np2eq3/40oRVDsDVgPkTuVkVNJceSDhRNFzrCQT/Cev3kF/f3DgokX2B7QX0oVMTQSkmJNx0yY/fLoR5S11sEI2qIhgesp8Zv8kyyR36fkLIom6FmPyw7T1KJpZ+eKyXdNkQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZSBkP/4L; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZSBkP/4L" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1897F1F000FF; Thu, 24 Sep 2026 14:48:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790261323; bh=3Jt4EEpI7zfY01he2CxmwKfXEUhv3Y/sSQk7vJfi7HA=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ZSBkP/4LvrZ78metoR/UbWOjYRCWgY14tvosTodt4mMPTUUi0UKQ0Xbh/m82yIw9n /Nj6LJjb09A2xPuccqoLkxMyn9qkxZkdQzc6J8lsX1TbL69bHNDWXEO4W8yCoEjSC+ lXcvukPxCjIx8iVBWZPIMhyWVjYS2FsJDunqIZJHCB904P5dXatE60a65R3EJXM/PK JvAWJU3Z+xFXnJ0xsPKd2nQBXuBe4G1bxi/H1zdyfF456T+mU1yBRaUwkNYL6NBujn RYATiQu0zXhtiCaToJapXCXZdMTcnyWdeHnyH4Vqq5URr0M/XD8Fs3/0FicNNdGCXk 16sZVqZWwQSOA== Date: Thu, 24 Sep 2026 15:48:41 +0100 From: Harry Yoo To: "Vlastimil Babka (SUSE)" Cc: Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Geert Uytterhoeven , Conor Dooley , Damien Le Moal , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kees Cook Subject: Re: [PATCH RFC 2/8] mm, slab: introduce slab_debug=N Message-ID: References: <20260923-slub_tiny_rework-v1-0-a0e66d536eb5@kernel.org> <20260923-slub_tiny_rework-v1-2-a0e66d536eb5@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260923-slub_tiny_rework-v1-2-a0e66d536eb5@kernel.org> On Wed, Sep 23, 2026 at 05:45:07PM +0200, Vlastimil Babka (SUSE) wrote: > Enabling slab_debug has the side-effect of disabling all percpu caching > of objects (now via sheaves), which can be sometimes useful for saving > memory for e.g. kdump kernels. To make this possible without the > overhead (CPU or memory) of actual debugging options such as poisoning, > introduce a No-op debug option that only forces the debugging slow paths > and zero sheaf capacity. Just to clarify, the intended behavior of slab_debug=- still does not change, right? (the name "No-op" could be confusing here I guess) And My LLM flagged an issue here (that I reviewed) Implementing it as debug flag has a few side effects: - pointer hashing will be disabled unless hash_pointers=always is specified. [+Cc: Kees] - Disable slab merging. Since this does not affect object metadata layout, this could be excluded. -- Cheers, Harry / Hyeonggon > This will also allow reimplementing CONFIG_SLUB_TINY as a boot-time > option. > > While documenting the new N option, remove obsolete SLAB references from > the F option description. > > Signed-off-by: Vlastimil Babka (SUSE) > --- > Documentation/admin-guide/mm/slab.rst | 13 +++++++++++-- > include/linux/slab.h | 5 ++++- > mm/slab.h | 2 +- > mm/slub.c | 3 +++ > 4 files changed, 19 insertions(+), 4 deletions(-) > > diff --git a/Documentation/admin-guide/mm/slab.rst b/Documentation/admin-guide/mm/slab.rst > index 14429ab90611..0beeae71d472 100644 > --- a/Documentation/admin-guide/mm/slab.rst > +++ b/Documentation/admin-guide/mm/slab.rst > @@ -45,12 +45,12 @@ of the first "select slabs" blocks that matches the slab's name are applied. > > Possible debug options are:: > > - F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS > - Sorry SLAB legacy issues) > + F Sanity (slab consistency) checks on alloc and free > Z Red zoning > P Poisoning (object and padding) > U User tracking (free and alloc) > T Trace (please only use on single slabs) > + N No-op (force debugging slowpaths without any checks) > A Enable failslab filter mark for the cache > O Switch debugging off for caches that would have > caused higher minimum slab orders > @@ -85,6 +85,15 @@ in low memory situations or if there's high fragmentation of memory. To > > slab_debug=O > > +The No-op option can be useful to minimize slab memory overhead by forcing slab > +debugging slowpaths, which disables percpu object caching. This reduces SMP > +scalability significantly, but does not impose the extra cpu or memory overhead > +of debugging options that actually perform checks. This can be useful for e.g. > +kdump kernels where scalability is not a concern, but memory has to be > +pre-reserved from the production kernel. So for a kdump kernel you can use:: > + > + slab_debug=N > + > You can apply different options to different list of slab names, using blocks > of options. This will enable red zoning for dentry and user tracking for > kmalloc. All other slabs will not get any debugging enabled:: > diff --git a/include/linux/slab.h b/include/linux/slab.h > index cda126def67a..ed949e8522be 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -35,6 +35,7 @@ enum _slab_flag_bits { > _SLAB_PANIC, > _SLAB_TYPESAFE_BY_RCU, > _SLAB_TRACE, > + _SLAB_DEBUG_NOOP, > #ifdef CONFIG_DEBUG_OBJECTS > _SLAB_DEBUG_OBJECTS, > #endif > @@ -166,8 +167,10 @@ enum _slab_flag_bits { > * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU. > */ > #define SLAB_TYPESAFE_BY_RCU __SLAB_FLAG_BIT(_SLAB_TYPESAFE_BY_RCU) > -/* Trace allocations and frees */ > +/* DEBUG: Trace allocations and frees */ > #define SLAB_TRACE __SLAB_FLAG_BIT(_SLAB_TRACE) > +/* DEBUG: Force the debug slowpaths without actually doing anything */ > +#define SLAB_DEBUG_NOOP __SLAB_FLAG_BIT(_SLAB_DEBUG_NOOP) > > /* Flag to prevent checks on free */ > #ifdef CONFIG_DEBUG_OBJECTS > diff --git a/mm/slab.h b/mm/slab.h > index 8fd6835e4235..77fcbf99b7b4 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -490,7 +490,7 @@ void flush_rcu_sheaves_on_cache(struct kmem_cache *s); > SLAB_NO_USER_FLAGS | SLAB_KMALLOC | SLAB_NO_MERGE) > > #define SLAB_DEBUG_FLAGS (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \ > - SLAB_TRACE | SLAB_CONSISTENCY_CHECKS) > + SLAB_TRACE | SLAB_DEBUG_NOOP | SLAB_CONSISTENCY_CHECKS) > > #define SLAB_FLAGS_PERMITTED (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS) > > diff --git a/mm/slub.c b/mm/slub.c > index a107a111c75e..bc593f0078c0 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -1897,6 +1897,9 @@ parse_slub_debug_flags(const char *str, slab_flags_t *flags, const char **slabs, > case 't': > *flags |= SLAB_TRACE; > break; > + case 'n': > + *flags |= SLAB_DEBUG_NOOP; > + break; > case 'a': > *flags |= SLAB_FAILSLAB; > break; > > -- > 2.55.0