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t=1784625617; bh=RXbi65g0B2x3zSEY3/jiZ4q6NSDtzWd4WeQahT18Nm8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bw5OEAe2QR/KdmtZ71poTClbJsrEiBVJ/goxjbCH9efpvu50jRYD8zq8hbUOwOYpA a3To6pkAdT8nhq3oTSkBuChpYOURbkETb6xN1sXW+hUy8Oqrfya5K5tdwkc5p4x4DY 7C6Zk4NAUQJLSaI7MuvFQrK79x1JkZG5wk7soKjmyS/D6J6zaHfMpCDvCOEoignLiR x7ivuy/MsauqvrCuiCF1oGpDXoe/dOch98Ul8FuxUR3ebVEzEXjopke5FYmLw3jV/Z A4EFiXkvrqGM37n+66VmBl8OLX90+n8hzRaNfVAFps/cSf6U0Yxw/hv+2XklT1q12G veSMpIW1blnNg== Message-ID: <96468090-6b31-4f78-a813-c5729b53aa3d@kernel.org> Date: Tue, 21 Jul 2026 11:20:12 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/13] mm/slab: skip kfence objects in allocation profiling Content-Language: en-US To: Harry Yoo , Suren Baghdasaryan Cc: Hao Li , 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 References: <20260720-b4-objext_split-v2-0-2fa7c6f60dbe@kernel.org> <20260720-b4-objext_split-v2-1-2fa7c6f60dbe@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: oi43ezdedst5jk6fz57qkf5b9odrjmhw X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 03980180003 X-Rspam-User: X-HE-Tag: 1784625617-59315 X-HE-Meta: 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 VjTXVoQC 59JQlS1mg9o7W4SHLZ/z98g3Ov1f2Yd7tyR0mgPW401vYAxx7eYZQ6BWUTxfVcJ5EJ73b2VWIsoYNd1o6N0c6NjInWlL4ioaw5gAtFYzvH87mdshDjqc0uDp42uU2fTiEKatbob3EP9bL696zsp6fqlhQ186sdX2GqcyEtolebdz+lWsUfLhpR3zFIxutKhQLVf+06TC5sVHNm+x8kstulGOm/wy5QtgwqBKmBiRqVx3fa2X2OLlDCPz0UjrFQaRRzrOmwPhUbakgiIP7fiKga6rCNsH5P/NPbEGc Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/21/26 07:43, Harry Yoo wrote: > On 7/20/26 11:16 PM, 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. > > For a similar reason I wonder if we can drop obj_exts completely. To make it clear, you mean kfence objects wouldn't have objcg accounting and would essentially behave as if obj_exts for them could not be allocated. > The idea that kfence objects might have different slabobj_ext layout > isn't really worth the extra complexity if we have very limited amount > of kfence objects. The amount of objects that escape memcg accounting > will be quite limited. Indeed. I think the main point wasn't to have everything accounted, but to avoid specially cased kfence paths. But I think this would probably just require adding is_kfence checks to objext allocation paths to fail them, so that's manageable. Should perhaps allow moving some of the checks added by this patch to more generic shared places. > Any thoughts from KFENCE & MEMCG folks? > >> 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; >> + } > > I think we should check this before get_slab_obj_exts()? > > Checking if the object is from kfence after slab_obj_ext_codetag_ref() > looks bit weird. Hm indeed. > Otherwise LGTM. Thanks! >> + >> 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]); >> >> + 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); >> >