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b=S1kKAbt43t6hwTScs3DwkHJJCMlc7M7Ahq7fHaDVhxBEhEvHNLosu2UR1yXTrueFL 7CYT6F6LwZ4HsykK3FexnQN55RsVnCXzo0j3TTUr35kykQY03pIh4bIclOjim6FmQJ X1qqD3+n2MJuioOA6Hm124D8c8bt8TpNy4BBI2JRwnteGGeH2c0GgNB8DmzJbAFw+V nJNTCzD11VXk+k3chuvzXEhTx8Ztzd8H0BpkohlKoKKugqYXEJU+o9hFAyUPf6sSic 56UA8arTMiPOyvCey3yuknsaYM2vajh+PXaCI6L5wMOMcMYwKh+lvtJxSLiXXE6AKo xv7oC/UfH7PKA== Date: Sun, 16 Aug 2026 10:25:22 +0100 From: Harry Yoo To: Guopeng Zhang Cc: Johannes Weiner , Roman Gushchin , Vlastimil Babka , Andrew Morton , Michal Hocko , Shakeel Butt , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Guopeng Zhang , Ran Xiaokai Subject: Re: [PATCH] mm: memcg: fix slab over-accounting for in-object objcg metadata Message-ID: References: <20260812124858.73487-1-guopeng.zhang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260812124858.73487-1-guopeng.zhang@linux.dev> X-Stat-Signature: dxgti3brxucoietxasqn77fxizt7p7i6 X-Rspamd-Queue-Id: 15461100004 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1786874379-103251 X-HE-Meta: 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 NliyZWwZ A61f2YvSu7C9GkP3Vx1KoUi8Zmu8WnUf+LDw+uZB+FpwZzYEl297M7dsqOmqLQADQ8GlJWQsfZGmBaljYTAzIYQywjcP494iXIUWit+FbGXAoGt7mfGtJELKRMngyu2zbXbzyvMkGCVtziwSbw8oG8z7Xv6My2q3lTUDM/GAUjnC6DWXrR9Csm6mFzhLnBaimDCa9u+Gw8TYYKop86HuhXSEl5xHC8ySOF8HeJmg+l9ZChcBqpzfhduVWRrcVNz/Zz97cIaF5U1oKhJ77miaNni+tPtSzNf/VatM6xyVZ3idkiDwCki9cNh+kGeXrweOAgyEMFnmpjTIodDREoxJNsr5P0yDkM2dtS8I7md3KC5OFbgA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Aug 12, 2026 at 08:48:58PM +0800, Guopeng Zhang wrote: > From: Guopeng Zhang > > obj_full_size() adds the size of an obj_cgroup pointer to s->size to > account for slabobj_ext storage, which normally resides outside the > object. > > Since commit a77d6d338685 ("mm/slab: place slabobj_ext metadata in > unused space within s->size"), slabobj_ext can reside in object padding > already covered by s->size. The extra charge then accounts for the same > memory twice. > > The per-slab obj_exts_in_object bit introduced by commit b5bc35ace2c5 > ("mm/slab: replace slab.stride with obj_exts_in_object") identifies this > layout. Use it to omit the extra charge only for in-object metadata. > Unlike in-object metadata, slab-leftover and separately allocated metadata > are not covered by the per-object s->size charge, so retain the existing > charge for those cases. > > Reported-by: Ran Xiaokai > Link: https://lore.kernel.org/all/20260310113804.245647-1-ranxiaokai627@163.com/ > Fixes: a77d6d338685 ("mm/slab: place slabobj_ext metadata in unused space within s->size") > Signed-off-by: Guopeng Zhang > --- > mm/memcontrol.c | 14 ++++++++++---- > 1 file changed, 10 insertions(+), 4 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index d1312441a02b..aef480f46891 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3526,12 +3526,16 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size) > refill_obj_stock(objcg, size, true); > } > > -static inline size_t obj_full_size(struct kmem_cache *s) > +static inline size_t obj_full_size(struct kmem_cache *s, struct slab *slab) > { > /* > * For each accounted object there is an extra space which is used > - * to store obj_cgroup membership. Charge it too. > + * to store obj_cgroup membership. Charge it too, unless it is stored > + * in object padding already covered by s->size. > */ > + if (obj_exts_in_object(slab)) > + return s->size; > + > return s->size + sizeof(struct obj_cgroup *); >From correctness terms of view, even this is not 100% correct because it doesn't account for the metadata for memory allocation profiling. But as Vlastimil questioned 5 months ago [1]: is it worth the complexity? Will simply charging s->size always make difference in practice? [1] https://lore.kernel.org/linux-mm/e7ccb3b0-791e-4e01-86c3-65b613e3cc5a@kernel.org