From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta0.migadu.com (out-172.mta0.migadu.com [91.218.175.172]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D8BD125B09F for ; Fri, 22 May 2026 02:24:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779416651; cv=none; b=tnB/jljl2F3iWPGCD1BWhbSy9Fx8KEISfkT1RQaWjmGgxtxGumvzLIUkaQH6JG0bycw+dzwPBYrb9n0gABtYD+Y3d7LbFgQCdYOe6ar+AlPbba/n+Bqt/IQC+UQoZwekmibnuKGbLBg2ci/DpScBB/pltHuAppuhlks+8Hav3q4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779416651; c=relaxed/simple; bh=MgmG/UQzDO+rmrLnLu3XIzZ7/WCZ3CZ0K2U681MVv+8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=RgpgQRN98i3kM1pYR5RSSGvw5+AmjUThQIHJy1EENZ+lhC1fVwI0c312o1imIVlq28SB6eX2kDC3kl9ATtCBxw7q4alj0kFUkXmliovHGfmuVSPjRs4W9kvtBnINRJXGHUvRyVVNgQqV7r5R+T5vTS15mv14jMF5I6v+PHNg5b0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=uZMZU+Ao; arc=none smtp.client-ip=91.218.175.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="uZMZU+Ao" Message-ID: <3eaa3522-b41f-4e69-a260-ebfd94fad722@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779416646; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HbQIAvOJY9pLyi2McPglRZSeFKk8rXs49yW8pjAXz28=; b=uZMZU+Aoo7vlKannwgx46GlcQSL2F/RTH5pzZEsYEqWykgwRxE4gNZqt74RlDWAm6osy6F bLo2Ip7BZ0E7KU7cgSuPf7rmVGB0VzDpIQoEav/jRZDfpDPxj7XiRQPpMZuOGr6cheBaJ2 2l45vYcjqlEGnBLjQuuXjIcZl4rx7c0= Date: Fri, 22 May 2026 10:23:31 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 2/4] memcg: uint16_t for nr_bytes in obj_stock_pcp To: Shakeel Butt , Andrew Morton Cc: 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 References: <20260522011908.1669332-1-shakeel.butt@linux.dev> <20260522011908.1669332-3-shakeel.butt@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: <20260522011908.1669332-3-shakeel.butt@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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 ((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)