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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 01/13] mm/slab: skip kfence objects in allocation profiling Message-ID: References: <20260720-b4-objext_split-v2-0-2fa7c6f60dbe@kernel.org> <20260720-b4-objext_split-v2-1-2fa7c6f60dbe@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260720-b4-objext_split-v2-1-2fa7c6f60dbe@kernel.org> X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 622681C000C X-Rspam-User: X-Stat-Signature: pw171gtyar6s7zarx7yc4zdycgbeigks X-Rspamd-Server: rspam04 X-HE-Tag: 1784724530-822113 X-HE-Meta: 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 miIlql7p 2wGHt04xaQd40Esx2uQFoG+oEWz72L91etzbCUOdQfVXQFPk5LmKI2gYLy+v01OBWNc3zA8tlstPAvGp3qEfTcS6Z6F8u0DE4CUX1A0/pWwiGuU5mrffDPUiRL7grtQ2VwkYUx+tOgcx2Kc/Gp6D0ZEOnopl/mZR4+90X32UH0gTfJ9qI0wWGKngSdIdvcEmH1/i3b2M5NAP2fLjLw4qOJWTfXSEqiQ/Jz3UtwH0+GT07NO4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jul 20, 2026 at 04:16:15PM +0200, Vlastimil Babka (SUSE) wrote: > struct kfence_metadata only contains struct slabobj_ext with > CONFIG_MEMCG, which is then used for the "fake" slab's obj_exts field. > If CONFIG_MEMCG is enabled, the struct can also end up used for memory > allocation profiling. If CONFIG_MEMCG is disabled but profiling is > enabled, it will end up allocating its obj_exts via > prepare_slab_obj_exts_hook() and assigning them to the fake struct slab. > These will probably then never be freed. > > So things sorta work, but not always in the intended and optimal way. > The upcoming changes to slabobj_ext layout would additionally need a > proper refactoring to keep working. > > However, there's little benefit in accounting KFENCE objects. KFENCE > allocations are rare and there can be only CONFIG_KFENCE_NUM_OBJECTS > (default to 255) outstanding ones at any time. For any callsite > prominent enough in the memory allocation profiling stats, allocations > served from KFENCE will be lost in the noise. > > Thus let's not complicate things and simply stop accounting KFENCE > objects in allocation profiling and skip them in the related slab hooks. > > We also need to skip kfence objects in mark_obj_codetag_empty() in case > a sheaf is allocated from kfence, per earlier sashiko review. > > Signed-off-by: Vlastimil Babka (SUSE) > --- > Documentation/mm/allocation-profiling.rst | 7 +++++++ > mm/slub.c | 11 +++++++++++ > 2 files changed, 18 insertions(+) > > diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst > index 5389d241176a..d02eb54ee8f2 100644 > --- a/Documentation/mm/allocation-profiling.rst > +++ b/Documentation/mm/allocation-profiling.rst > @@ -112,3 +112,10 @@ break it out by rhashtable type. > > - Then, use the following form for your allocations: > alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...)) > + > +Notes > +===== > + > +- When a slab object is allocated from KFENCE, its accounting is skipped. > + KFENCE allocations are rare and limited to a small number, so this omission > + is negligible. > diff --git a/mm/slub.c b/mm/slub.c > index 0337e60db5ac..76acb78f2655 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2076,6 +2076,11 @@ static inline void mark_obj_codetag_empty(const void *obj) > struct slabobj_ext *ext = slab_obj_ext(obj_slab, > slab_exts, offs); > > + if (is_kfence_address(obj)) { > + put_slab_obj_exts(slab_exts); > + return; > + } > + > if (unlikely(is_codetag_empty(&ext->ref))) { > put_slab_obj_exts(slab_exts); > return; > @@ -2352,6 +2357,9 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags, > if (alloc_flags & SLAB_ALLOC_NO_RECURSE) > return; > > + if (is_kfence_address(object)) > + return; > + > slab = virt_to_slab(object); > obj_exts = prepare_slab_obj_exts_hook(s, slab, flags, alloc_flags, object); > /* > @@ -2399,6 +2407,9 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p > for (i = 0; i < objects; i++) { > unsigned int off = obj_to_index(s, slab, p[i]); I noticed obj_to_index also does the `if (is_kfence_address(obj))` check, so maybe we could use: if (is_kfence_address(p[i])) continue; off = __obj_to_index(cache, slab_address(slab), obj); > > + if (is_kfence_address(p[i])) > + continue; > + > alloc_tag_sub(&slab_obj_ext(slab, obj_exts, off)->ref, s->size); > } > put_slab_obj_exts(obj_exts); > > -- > 2.55.0 > -- Thanks, Hao