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From: Shakeel Butt To: Harry Yoo Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Alexandre Ghiti , Joshua Hahn , 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: References: <20260520053123.2709959-1-shakeel.butt@linux.dev> <20260520053123.2709959-3-shakeel.butt@linux.dev> <55d36f91-fcae-4506-9b76-4a7cad3d256d@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55d36f91-fcae-4506-9b76-4a7cad3d256d@kernel.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 7D03940011 X-Stat-Signature: s5i7kbj5m711agwwxj8qsoxbm4y9swrf X-Rspamd-Server: rspam06 X-HE-Tag: 1779325305-503609 X-HE-Meta: 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 B0veKY86 iMLys/WQ1RuaNZv+kMrHqHmiaK/oIHg1HKirwJL93bScfu18YwqqMJGFH26VZiI+uxv9l9ZWPWg2OX7xV9+84d4mxeJvfH91WHhUeTmQ4ueFp5t/GfHYqRM2X7z+o10sPepGWjFuN54wOz/RhC9uGpxz58d3/8VHTPt7dIS+vXmNuHe10WQXIqnLeQVcZxhAoyOb6nDjQh4nP+ROqwCxT7z1TZ3GkyfP1JhrKFYZM2HQ2b3kmLPZUOnYmpQO4cXF4oRU+kYLDDvfj+xr6VSjAKWlk2Tld3XgJFiZQ/X31z+KtP4nSR9G8G75LptTT7+CGVUB6Sgg8v11Lp0LeMOVkIHnREE+QOaKJwf7D Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, May 20, 2026 at 04:01:45PM +0900, Harry Yoo wrote: > > > On 5/20/26 2:31 PM, 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 > > @@ -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) { > > nit: This should be > U16_MAX? Ack.