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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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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-Spam-Level: X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; TAGGED_RCPT(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWELVE(0.00)[21]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_DN_SOME(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[linux-foundation.org,gmail.com,linux.com,google.com,cmpxchg.org,kvack.org,kernel.org,linux.dev,arm.com,mit.edu,dilger.ca,vger.kernel.org]; MID_RHS_MATCH_FROM(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:mid,imap1.dmz-prg2.suse.org:helo] X-Spam-Flag: NO X-Spam-Score: -2.80 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 >