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b=ap2sqBiOyzNEg5d9WoWmhZjrYXZKC7JT2oPedkW5M1KgX6ee0qONqMx2Dylwh9xRG wWYD+NG4lLN8GsAC7SCUic669xXeIO/lo3u4OyQWc7zP73Pt9uxAQN8GJWbqAcrjoc LQ59Q0Y0Hh47txaeYLCuvXLNyk/Bflt4G8HFMj8dpnfNZRWXPt1OF6FLbriAu7k1px wN4cDDNb1Sw1JvfNfRDPiuvO85CSG2EEhQi9VaPccss3mXRD0/FbLkWZKSB/1SMLbo i5ecanrLK0dblbNLrCwiwZGWhGA6FUwN1T1YMcSs86zqODxcjv3NbyX59VgsdfY4SU v7i8ntllz1UNA== Message-ID: Date: Wed, 11 Mar 2026 08:38:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: vbabka@kernel.org Subject: Re: [PATCH 0/2] fix kmem over-charging for embedded obj_exts array To: Hao Li , Harry Yoo Cc: ranxiaokai627@163.com, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, akpm@linux-foundation.org, cl@gentwo.org, rientjes@google.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ran.xiaokai@zte.com.cn References: <20260310113804.245647-1-ranxiaokai627@163.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: D68501C0003 X-Stat-Signature: t7y15zyy8m4fas8e89nfyr5zqnko3bj5 X-Rspam-User: X-HE-Tag: 1773214711-5014 X-HE-Meta: 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 Q8C8zD4u J1Ajf Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/11/26 04:06, Hao Li wrote: > On Wed, Mar 11, 2026 at 11:13:18AM +0900, Harry Yoo wrote: >> On Tue, Mar 10, 2026 at 11:38:02AM +0000, ranxiaokai627@163.com wrote: >> > From: Ran Xiaokai >> > >> > Since commit a77d6d338685 ("mm/slab: place slabobj_ext metadata >> > in unused space within s->size"), the struct slabobj_ext array can >> > use slab leftover space or be embedded into the slub object to save >> > memory. In these cases, no extra kmalloc space is allocated for the >> > obj_exts array. >> > >> > However, obj_full_size() always returns extra sizeof(struct obj_cgroup *) >> > bytes for every object, which leads to over-charging for slabs with >> > embedded obj_exts. >> > >> > This series optimizes obj_full_size() to check whether obj_exts uses >> > slab leftover space or is embedded in the object. If so, only the object >> > size is charged. Otherwise, the extra obj_cgroup pointer space is also >> > charged. >> >> Hi Ran, >> >> At first look, I'm not sure if it's a good idea - although it's >> allocated from wasted space, it's still memory that's needed to >> charge objects. > > Yes, I've been thinking about this as well. > > For slabobj_ext that lives at the end of the whole slab, it seems reasonable to > charge it to the cgroup. > >> >> But for "embedded into the slub object" case, yeah, >> the metadata is charged twice, as it's already included in s->size. > > While reading patch 2, I was also thinking about whether it would make sense to > call obj_exts_in_slab() on every object allocation and free for this case... > > I wonder if we could use objext_flags to carry a bit of information about where > slabobj_ext is located. Uh maybe let's not complicate it and just drop obj_full_size() altogether and just charge s->size? Just accept that allocating slab objects has some overhead, we're not accounting all of it fully anyway. It shouldn't cause a big difference in practice.