From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta1.migadu.com (out-189.mta1.migadu.com [95.215.58.189]) (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 1CD01470EA9 for ; Thu, 23 Jul 2026 03:06:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784775982; cv=none; b=JvzPA5IvG9kP3E+q4R45Qu1b7p2XVmart+eFXPB72gjuXFAkenfbOSrRd5JDVXWDeqVEoBEbVe7A/5AmO3vxRFJF9Gv989qS4h1/Rn03p2soHDxlaYiRdVrZ9Y4rd4s7qNKSIzZGSJAcL2wV8ySyo3yGOvo8ydL3UyGua8G22lQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784775982; c=relaxed/simple; bh=NOp2u7MtIeO6/+FYbDqAadAeTocT+0MH08QZBM7/8Cc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=p7zevy9CxnJSuKulk7X6wfiN0epJ/2l259Yg5CqSGgFtFgLeG8Nu7l28Pq/XAm1jFeOREZhSgJOAdshrYjb//cuf8UwmWURh7LnFjk3h4urCigXpBzPX9O8S6bHuWEYTmV7Eay2V61vIHCNfN/QaxIRZaaJjSFPZjVij5gmLhY8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=lXXIzlBL; arc=none smtp.client-ip=95.215.58.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="lXXIzlBL" Date: Thu, 23 Jul 2026 11:05:49 +0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784775979; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=gR/NCSV33JYMmMHnqisAYHsCi1Pk0aOBobkpC/mIj4s=; b=lXXIzlBL0OKuwlMWJsTKQuMa2q+crmjGo44URbyRWMUlTPNhqPVhtZrUA0L2TKuQQyWOgG 0elMWbeC1NYn9eGNqg8amndzB9var5ZeoJU1yFaBzSrQTVrOv0Qb8yj5t9he7maWp9YwKR QePOPn3ymEPUhjh8BLFjJ6+Xs5PgAaM= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Li To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Suren Baghdasaryan , Shakeel Butt , Alexander Potapenko , Marco Elver , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH v2 09/13] mm/slab: change struct slabobj_ext to a union Message-ID: References: <20260720-b4-objext_split-v2-0-2fa7c6f60dbe@kernel.org> <20260720-b4-objext_split-v2-9-2fa7c6f60dbe@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: <20260720-b4-objext_split-v2-9-2fa7c6f60dbe@kernel.org> X-Migadu-Flow: FLOW_OUT On Mon, Jul 20, 2026 at 04:16:23PM +0200, Vlastimil Babka (SUSE) wrote: > Currently, struct slabobj_ext can hold both objcg pointer and > codetag_ref (when both are compile-enabled) and there is an array of as > many slabobj_ext instances as there are objects in a slab. > > This makes the layout fixed so even if codetag_ref is unused (because > memory allocation profiling is disabled), the space for them is > allocated and wasted. Similarly, some caches (currently kmalloc_normal) > do not ever need objcg pointers, leading to wasted memory with memory > allocation profiling enabled. > > To make this more flexible, change the layout so that struct slabobj_ext > becomes a union of objcg pointer and codetag_ref (to ensure uniform > size; in practice both are the same size anyway). The slabobj_ext array > then can have twice as many elements as before. For cache locality > purposes, the effective memory layout is unchanged, so objcg and codetag > ref for a given object are still adjacent. > > cache_obj_ext_size() returns the effective size of (0-2) struct > slabobj_ext's for a cache, slab_obj_ext_size() for a slab. Currently > both return a constant value derived from the config options, but will > be made dynamic later. Replace all sizeof(slabobj_ext) usage with these. > > No functional change intended, the layout is still effectively static. > > Reviewed-by: Suren Baghdasaryan > Signed-off-by: Vlastimil Babka (SUSE) > --- > mm/slab.h | 49 +++++++++++++++++++++++++++++++++++++++---------- > mm/slub.c | 19 +++++++++++-------- > 2 files changed, 50 insertions(+), 18 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index e586798e4f16..f8446167e175 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -550,18 +550,42 @@ static inline bool need_kmalloc_no_objext(void) > } > > /* > - * Extended information for slab objects stored as an array in page->memcg_data > - * if MEMCG_DATA_OBJEXTS is set. > + * Extended information for slab objects stored as a pointer to an array in > + * slab->obj_exts (aliasing page->memcg_data) if MEMCG_DATA_OBJEXTS is set. > */ > struct slabobj_ext { > + /* > + * All elements of the union should be pointer-sized to avoid memory > + * waste > + */ > + union { > #ifdef CONFIG_MEMCG > - struct obj_cgroup *_objcg; > + struct obj_cgroup *_objcg; > #endif > #ifdef CONFIG_MEM_ALLOC_PROFILING > - union codetag_ref _ctref; > + union codetag_ref _ctref; > #endif > + }; > } __aligned(8); slabobj_ext is now acting as a generic struct, serving as either _objcg or _ctref. However, because it's set to __aligned(8), I'm wondering if this might waste half the memory on 32-bit? > > +static inline size_t cache_obj_ext_size(struct kmem_cache *s) > +{ > + size_t sz = 0; > + > + if (IS_ENABLED(CONFIG_MEMCG)) > + sz += 1; > + > + if (IS_ENABLED(CONFIG_MEM_ALLOC_PROFILING)) > + sz += 1; > + > + return sizeof(struct slabobj_ext) * sz; > +} > + > +static inline size_t slab_obj_ext_size(struct slab *slab) > +{ > + return cache_obj_ext_size(slab->slab_cache); > +} > + > #ifdef CONFIG_SLAB_OBJ_EXT > > /* > @@ -651,18 +675,18 @@ slab_obj_ext(struct kmem_cache *s, struct slab *slab, unsigned long obj_exts, > { > struct slabobj_ext *obj_ext; > unsigned int index; > + unsigned int stride; > > VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab)); > > index = obj_to_index(s, slab, obj); > > - if (!obj_exts_in_object(slab)) { > - obj_ext = ((struct slabobj_ext *)obj_exts) + index; > - } else { > - unsigned int stride = s->size; > + if (!obj_exts_in_object(slab)) > + stride = slab_obj_ext_size(slab); > + else > + stride = s->size; > > - obj_ext = (struct slabobj_ext *)(obj_exts + index * stride); > - } > + obj_ext = (struct slabobj_ext *)(obj_exts + index * stride); > > return kasan_reset_tag(obj_ext); > } > @@ -670,12 +694,14 @@ slab_obj_ext(struct kmem_cache *s, struct slab *slab, unsigned long obj_exts, > #ifdef CONFIG_MEMCG > static inline struct obj_cgroup *slab_obj_ext_objcg(struct slabobj_ext *obj_ext) > { > + /* if objcg exists, it comes first, so we don't need to do anything */ > return obj_ext->_objcg; > } > > static inline void slab_obj_ext_set_objcg(struct slabobj_ext *obj_ext, > struct obj_cgroup *objcg) > { > + /* if objcg exists, it comes first, so we don't need to do anything */ > obj_ext->_objcg = objcg; > } > #endif > @@ -684,6 +710,9 @@ static inline void slab_obj_ext_set_objcg(struct slabobj_ext *obj_ext, > static inline union codetag_ref * > slab_obj_ext_codetag_ref(struct slab *slab, struct slabobj_ext *obj_ext) > { > + if (IS_ENABLED(CONFIG_MEMCG)) > + obj_ext += 1; > + > return &obj_ext->_ctref; > } > #endif > diff --git a/mm/slub.c b/mm/slub.c > index f5b3eb44cb8a..287cbf3eddcc 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -803,7 +803,7 @@ static inline bool need_slab_obj_exts(struct kmem_cache *s) > > static inline unsigned int obj_exts_size_in_slab(struct slab *slab) > { > - return sizeof(struct slabobj_ext) * slab->objects; > + return slab_obj_ext_size(slab) * slab->objects; > } > > static inline unsigned long obj_exts_offset_in_slab(struct kmem_cache *s, > @@ -1199,7 +1199,7 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p) > off += kasan_metadata_size(s, false); > > if (obj_exts_in_object(slab)) > - off += sizeof(struct slabobj_ext); > + off += slab_obj_ext_size(slab); > > if (off != size_from_object(s)) > /* Beginning of the filler is the free pointer */ > @@ -1404,7 +1404,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p) > off += kasan_metadata_size(s, false); > > if (obj_exts_in_object(slab)) > - off += sizeof(struct slabobj_ext); > + off += slab_obj_ext_size(slab); > > if (size_from_object(s) == off) > return 1; > @@ -2094,6 +2094,8 @@ static inline bool mark_failed_objexts_alloc(struct slab *slab) > static inline void handle_failed_objexts_alloc(struct slab *slab, > unsigned long obj_exts, struct slabobj_ext *vec) > { > + unsigned int stride; > + > if (!mem_alloc_profiling_enabled()) > return; > > @@ -2105,11 +2107,13 @@ static inline void handle_failed_objexts_alloc(struct slab *slab, > if (obj_exts != OBJEXTS_ALLOC_FAIL) > return; > > + stride = slab_obj_ext_size(slab) / sizeof(*vec); > + > for (unsigned int i = 0; i < slab->objects; i++) { > union codetag_ref *ref = slab_obj_ext_codetag_ref(slab, vec); > > set_codetag_empty(ref); > - vec++; > + vec += stride; > } > } > > @@ -2135,7 +2139,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > unsigned long new_exts; > unsigned long old_exts; > struct slabobj_ext *vec; > - size_t sz = sizeof(struct slabobj_ext) * slab->objects; > + size_t sz = slab_obj_ext_size(slab) * slab->objects; > > gfp &= ~OBJCGS_CLEAR_MASK; > /* > @@ -2278,8 +2282,7 @@ static void alloc_slab_obj_exts_early(struct kmem_cache *s, struct slab *slab) > > get_slab_obj_exts(obj_exts); > for_each_object(addr, s, slab_address(slab), slab->objects) > - memset(kasan_reset_tag(addr) + offset, 0, > - sizeof(struct slabobj_ext)); > + memset(kasan_reset_tag(addr) + offset, 0, slab_obj_ext_size(slab)); > put_slab_obj_exts(obj_exts); > > #ifdef CONFIG_MEMCG > @@ -7938,7 +7941,7 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s) > aligned_size = ALIGN(size, s->align); > #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT) > if (slab_args_unmergeable(args, s->flags) && > - (aligned_size - size >= sizeof(struct slabobj_ext))) > + (aligned_size - size >= cache_obj_ext_size(s))) > s->flags |= SLAB_OBJ_EXT_IN_OBJ; > #endif > size = aligned_size; > > -- > 2.55.0 > -- Thanks, Hao