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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48fe4dac000sm406738105e9.0.2026.05.20.06.20.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 May 2026 06:20:24 -0700 (PDT) Date: Wed, 20 May 2026 14:20:23 +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 2/4] memcg: uint16_t for nr_bytes in obj_stock_pcp Message-ID: <20260520142023.6eae5ec7@pumpkin> In-Reply-To: <20260520053123.2709959-3-shakeel.butt@linux.dev> References: <20260520053123.2709959-1-shakeel.butt@linux.dev> <20260520053123.2709959-3-shakeel.butt@linux.dev> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@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 Tue, 19 May 2026 22:31:20 -0700 Shakeel Butt wrote: > Currently struct obj_stock_pcp stores nr_bytes in an 'unsigned int' > which is 4 bytes on 64-bit machines. Switch the field to uint16_t to > shrink the per-CPU cache. > > The kernel supports PAGE_SIZE_4KB, _8KB, _16KB, _32KB, _64KB and > _256KB (see HAVE_PAGE_SIZE_* in arch/Kconfig). After the > PAGE_SIZE-aligned flush in __refill_obj_stock(), the sub-page > remainder fits in uint16_t up through 64KiB pages where PAGE_SIZE - 1 > == U16_MAX, but on 256KiB pages PAGE_SIZE - 1 == 0x3FFFF exceeds > U16_MAX. The accumulator also needs to stay within uint16_t between > page-aligned flushes on 64KiB pages where PAGE_SIZE itself is > U16_MAX + 1. > > Accumulate the new total in an 'unsigned int' local, then: > > 1. Flush whenever the accumulator would hit U16_MAX. Together with > the existing allow_uncharge flush at PAGE_SIZE, this keeps the > uint16_t safe on PAGE_SIZE <= 64KiB. > > 2. On configs with PAGE_SHIFT > 16 (PAGE_SIZE_256KB on hexagon and > powerpc 44x), push any sub-page remainder above U16_MAX into > objcg->nr_charged_bytes via atomic_add before storing back, so > the store cannot silently truncate. The PAGE_SHIFT > 16 guard > folds the branch out at compile time on smaller page sizes. > > Signed-off-by: Shakeel Butt > Tested-by: kernel test robot > --- > mm/memcontrol.c | 33 +++++++++++++++++++++++++++------ > 1 file changed, 27 insertions(+), 6 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index d7c162946719..b3d63d9f267c 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2019,7 +2019,7 @@ static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = { > > struct obj_stock_pcp { > local_trylock_t lock; > - unsigned int nr_bytes; > + uint16_t nr_bytes; > struct obj_cgroup *cached_objcg; > int16_t node_id; You might want to move it to this hole. The size of 'lock' depends on kernel build options. -- David > int nr_slab_reclaimable_b; > @@ -3331,6 +3331,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, > bool allow_uncharge) > { > unsigned int nr_pages = 0; > + unsigned int stock_nr_bytes; > > if (!stock) { > nr_pages = nr_bytes >> PAGE_SHIFT; > @@ -3339,21 +3340,41 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, > goto out; > } > > + stock_nr_bytes = stock->nr_bytes; > if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */ > drain_obj_stock(stock); > obj_cgroup_get(objcg); > - stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes) > + stock_nr_bytes = atomic_read(&objcg->nr_charged_bytes) > ? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0; > WRITE_ONCE(stock->cached_objcg, objcg); > > allow_uncharge = true; /* Allow uncharge when objcg changes */ > } > - stock->nr_bytes += nr_bytes; > + stock_nr_bytes += nr_bytes; > + > + /* Since stock->nr_bytes is uint16_t, don't refill >= U16_MAX */ > + if ((allow_uncharge && (stock_nr_bytes > PAGE_SIZE)) || > + stock_nr_bytes >= U16_MAX) { > + nr_pages = stock_nr_bytes >> PAGE_SHIFT; > + stock_nr_bytes &= (PAGE_SIZE - 1); > + > + /* > + * On configs with PAGE_SHIFT > 16 (PAGE_SIZE_256KB on > + * hexagon and powerpc 44x), the sub-page remainder can > + * still exceed U16_MAX. Push the excess back to > + * objcg->nr_charged_bytes so the store into uint16_t > + * cannot silently truncate; folded out at compile time > + * on smaller page sizes. > + */ > + if (PAGE_SHIFT > 16 && stock_nr_bytes > U16_MAX) { > + unsigned int kept = stock_nr_bytes & U16_MAX; > > - if (allow_uncharge && (stock->nr_bytes > PAGE_SIZE)) { > - nr_pages = stock->nr_bytes >> PAGE_SHIFT; > - stock->nr_bytes &= (PAGE_SIZE - 1); > + atomic_add(stock_nr_bytes - kept, > + &objcg->nr_charged_bytes); > + stock_nr_bytes = kept; > + } > } > + stock->nr_bytes = stock_nr_bytes; > > out: > if (nr_pages)