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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45eb6d64eb1sm2686180f8f.32.2026.05.22.00.50.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 22 May 2026 00:50:54 -0700 (PDT) Date: Fri, 22 May 2026 08:50:51 +0100 From: David Laight To: Shakeel Butt Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Alexandre Ghiti , Joshua Hahn , Harry Yoo , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Subject: Re: [PATCH v2 3/4] memcg: int16_t for cached slab stats Message-ID: <20260522085051.0b4a3f9c@pumpkin> In-Reply-To: <20260522011908.1669332-4-shakeel.butt@linux.dev> References: <20260522011908.1669332-1-shakeel.butt@linux.dev> <20260522011908.1669332-4-shakeel.butt@linux.dev> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 21 May 2026 18:19:07 -0700 Shakeel Butt wrote: > Currently struct obj_stock_pcp stores cached slab stats in 'int' which > is 4 bytes per counter on 64-bit machines. Switch them to int16_t to > shrink the cached metadata. > > The existing PAGE_SIZE flush in __account_obj_stock() bounds *bytes at > PAGE_SIZE on 4KiB and 16KiB page archs, well within int16_t. On 64KiB > pages PAGE_SIZE is well above S16_MAX so that flush never fires, and a > sufficiently long run of accumulations would overflow the cache. Add > an explicit S16_MAX guard before each add: when the next add would > push abs(*bytes) past S16_MAX, fold the cached value into @nr and > flush directly via mod_objcg_mlstate() before the accumulation. > > Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type") > Tested-by: kernel test robot > Signed-off-by: Shakeel Butt > Reviewed-by: Harry Yoo (Oracle) > --- > > Changes since v2: > - Collected tags > > mm/memcontrol.c | 16 +++++++++++++--- > 1 file changed, 13 insertions(+), 3 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index e4f00a8159d5..78c02451312b 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2022,8 +2022,8 @@ struct obj_stock_pcp { > struct obj_cgroup *cached_objcg; > uint16_t nr_bytes; > int16_t node_id; > - int nr_slab_reclaimable_b; > - int nr_slab_unreclaimable_b; > + int16_t nr_slab_reclaimable_b; > + int16_t nr_slab_unreclaimable_b; > > struct work_struct work; > unsigned long flags; > @@ -3158,7 +3158,7 @@ static void __account_obj_stock(struct obj_cgroup *objcg, > struct obj_stock_pcp *stock, int nr, > struct pglist_data *pgdat, enum node_stat_item idx) > { > - int *bytes; > + int16_t *bytes; > > /* > * Though at the moment MAX_NUMNODES <= 1024 in all archs but let's make > @@ -3195,6 +3195,16 @@ static void __account_obj_stock(struct obj_cgroup *objcg, > > bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b > : &stock->nr_slab_unreclaimable_b; > + /* > + * To avoid overflow or underflow, flush directly if accumulating @nr > + * would push the cached value past S16_MAX. > + */ > + if (abs(nr + *bytes) > S16_MAX) { > + nr += *bytes; > + *bytes = 0; > + goto direct; > + } > + I think you should do the add first: nr += *bytes; if (abs(nr) < S16_MAX && (!*bytes || abs(nr) < PAGE_SIZE)) { *bytes = nr; } else { *bytes = 0; mod_objcg_mlstate(objcg, pgdat, idx, nr); } -- David > /* > * Even for large object >= PAGE_SIZE, the vmstat data will still be > * cached locally at least once before pushing it out.