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Thu, 30 Oct 2025 16:33:30 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id X0eHOVqTA2msHQAAD6G6ig (envelope-from ); Thu, 30 Oct 2025 16:33:30 +0000 Message-ID: <48029aab-20ea-4d90-bfd1-255592b2018e@suse.cz> Date: Thu, 30 Oct 2025 17:33:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH V3 6/7] mm/slab: save memory by allocating slabobj_ext array from leftover Content-Language: en-US To: Harry Yoo , Suren Baghdasaryan Cc: akpm@linux-foundation.org, andreyknvl@gmail.com, cl@linux.com, dvyukov@google.com, glider@google.com, hannes@cmpxchg.org, linux-mm@kvack.org, mhocko@kernel.org, muchun.song@linux.dev, rientjes@google.com, roman.gushchin@linux.dev, ryabinin.a.a@gmail.com, shakeel.butt@linux.dev, vincenzo.frascino@arm.com, yeoreum.yun@arm.com, tytso@mit.edu, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org References: <20251027122847.320924-1-harry.yoo@oracle.com> <20251027122847.320924-7-harry.yoo@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: as1o8tq1skpnrp84g4e8we6amu9nhtz6 X-Rspamd-Queue-Id: 9E76D1C000A X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1761842012-705510 X-HE-Meta: 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 Al9udLLx xdvUY81l6wUGkEmT5ab4q996JFigEwc8xu86XGXXg2Zx4rsS7uf2BKQsRSfY78hy09tZgAZyg2vddon8mX+J2zQv7lcOu6Vv5yGNtjWbaP5zR7njIPNBVHj8JO61A4s7OvW1Su104xtQnZADJBz9MCPy0zNEBGAvVuLTG90S6ofL+ATXItejmStT1yYG8kc8lPXsH4cbj2i/9pPHJoR9MuNQcNZE4wFBYDoshBaxE7BuYb8Qf6noqPVZNqoaqDOmEvGcQhW3X5N5vbINvZcOmRBotBMcuFOj4VsE0rilzrSz6pxK4SU82+LYuhPqmPvDrODztuiGdp3n+65an3SnwjPUUgK0rBMvn2iFIdGnfn7dR7BZvHlvJYTyrhtQ8EeOEbSn/RqMQOGhXswoEUJDbhwPmKQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/30/25 01:40, Harry Yoo wrote: > On Wed, Oct 29, 2025 at 11:37:27AM -0700, Suren Baghdasaryan wrote: >> > > > mod_node_page_state(slab_pgdat(slab), cache_vmstat_idx(s), >> > > > @@ -3219,9 +3352,6 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node) >> > > > slab->objects = oo_objects(oo);slab_obj_exts >> > > > slab->inuse = 0; >> > > > slab->frozen = 0; >> > > > - init_slab_obj_exts(slab); >> > > > - >> > > > - account_slab(slab, oo_order(oo), s, flags); >> > > > >> > > > slab->slab_cache = s; >> > > > >> > > > @@ -3230,6 +3360,13 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node) >> > > > start = slab_address(slab); >> > > > >> > > > setup_slab_debug(s, slab, start); >> > > > + init_slab_obj_exts(slab); >> > > > + /* >> > > > + * Poison the slab before initializing the slabobj_ext array >> > > > + * to prevent the array from being overwritten. >> > > > + */ >> > > > + alloc_slab_obj_exts_early(s, slab); >> > > > + account_slab(slab, oo_order(oo), s, flags); >> > > >> > > alloc_slab_obj_exts() is called in 2 other places: >> > > 1. __memcg_slab_post_alloc_hook() >> > > 2. prepare_slab_obj_exts_hook() >> > > >> > > Don't you need alloc_slab_obj_exts_early() there as well? >> > >> > That's good point, and I thought it's difficult to address >> > concurrency problem without using a per-slab lock. >> > >> > Thread A Thread B >> > - sees slab->obj_exts == 0 >> > - sees slab->obj_exts == 0 >> > - allocates the vector from unused space >> > and initializes it. >> > - try cmpxchg() >> > - allocates the vector >> > from unused space and >> > initializes it. >> > (the vector is already >> > in use and it's overwritten!) >> > >> > - try cmpxchg() >> > >> > But since this is slowpath, using slab_{lock,unlock}() here is probably >> > fine. What do you think? >> >> Ok, was your original intent to leave these callers as is and allocate >> the vector like we do today even if obj_exts fit inside the slab? > > Yes that's what I intended, and maybe later we could allocate the vector > from the unused space even after the slab is allocated, as long as > it doesn't hurt performance. It would be nice. I guess what can happen is there's a cache without SLAB_ACCOUNT but then some allocations from that will use __GFP_ACCOUNT and we need to allocate obj_exts on-demand, right? >> > >> > -- >> > Cheers, >> > Harry / Hyeonggon >