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From: Shakeel Butt To: Qi Zheng Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , 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 2/4] memcg: uint16_t for nr_bytes in obj_stock_pcp Message-ID: References: <20260522011908.1669332-1-shakeel.butt@linux.dev> <20260522011908.1669332-3-shakeel.butt@linux.dev> <3eaa3522-b41f-4e69-a260-ebfd94fad722@linux.dev> 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-Disposition: inline In-Reply-To: <3eaa3522-b41f-4e69-a260-ebfd94fad722@linux.dev> X-Migadu-Flow: FLOW_OUT On Fri, May 22, 2026 at 10:23:31AM +0800, Qi Zheng wrote: > > > On 5/22/26 9:19 AM, 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. > > > > 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 v1: > > - Collected tags > > - Rearrange fields of obj_stock_pcp (David Laight) > > - Fix comparison operator (Harry) > > > > mm/memcontrol.c | 33 +++++++++++++++++++++++++++------ > > 1 file changed, 27 insertions(+), 6 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index d7c162946719..e4f00a8159d5 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -2019,8 +2019,8 @@ static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = { > > struct obj_stock_pcp { > > local_trylock_t lock; > > - unsigned int nr_bytes; > > struct obj_cgroup *cached_objcg; > > + uint16_t nr_bytes; > > int16_t node_id; > > int nr_slab_reclaimable_b; > > int nr_slab_unreclaimable_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 */ > > ^ > > should also be changed to: don't refill > U16_MAX ? > > Otherwise: > > Acked-by: Qi Zheng Thanks. If I send a new version, I will fix this otherwise I will ask Andrew to fix this inplace.