From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 549771C84AB for ; Wed, 25 Mar 2026 01:44:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774403041; cv=none; b=Dch89u3dUXeXpl1KhN2yx+ph1cSHvifFDtF6xOgJv32QX3LomDDNh/5FdzDTi1E5CjXhXzZatj0nGOfjGWwNjp3YduBCEvF+RkyU5mBOYndOnyD3afCKZjNVKx2fJ0WPgTdTnfJLX+cAmD9ZLH7GpisagIlXLxVxzraJw1QhFoc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774403041; c=relaxed/simple; bh=I7dzjVKoQcTtp7Dkp6x2GsVxUVnjvFtpsLUpGsILp4A=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=hefz77KhxR/91CnCkskzefOgnxYfmqIfYzBVN43z1vJA6o9TtczvVcSIcrJbnakhpBvVlgoDZ8PF+MWUu/O0C1WDWVAUSakBqpCQlzKa1DnBXOd2huUK0kBixjVfsb9H94DaQ3dywi+IyH9jg5MelBLU/sOaCQkQwOWcTfy26Fg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mogDwU+Y; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mogDwU+Y" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7D121C19424; Wed, 25 Mar 2026 01:44:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774403040; bh=I7dzjVKoQcTtp7Dkp6x2GsVxUVnjvFtpsLUpGsILp4A=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=mogDwU+Y57SQzOhbOXGTVmrfdwli6fzxvt8tt22MnMvv4l8eJNRvEptsjwjtMxRjR 07/BrTX5+jqzNvSXnfvVwg3xrnjAHK8Q6LVaqGuGZW/0OoN4iQlJMuM6557KhFOEPy Grxsi/0Oi5C7pTnZ0eASklN/E96llYC4CzV1GznCkmKU7bwfN/RPZLvXRwn3rjv6x1 7IvByYSIHgow6pXfrJ9XVkOxPSPVO3W2G583GacNR94IH+VZ23Kl4+0sb3unuCBuZS JSbMRrc6H/txaiKtGeGFIrb2DIEtjWc6pKMgclLqumgx/h5ToaGShSBdx3ZeSYDKAa PspuOY4/ZX5lQ== Date: Wed, 25 Mar 2026 10:43:58 +0900 From: "Harry Yoo (Oracle)" To: Qi Zheng Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, yosry.ahmed@linux.dev, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chenridong@huaweicloud.com, mkoutny@suse.com, akpm@linux-foundation.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, lance.yang@linux.dev, bhe@redhat.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qi Zheng Subject: Re: [PATCH 2/3] mm: memcontrol: correct the parameter type of __mod_memcg{_lruvec}_state() Message-ID: References: <90524ca3806e24105ab5f2d69435f57c2ae034cb.1774342371.git.zhengqi.arch@bytedance.com> 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-Disposition: inline In-Reply-To: <90524ca3806e24105ab5f2d69435f57c2ae034cb.1774342371.git.zhengqi.arch@bytedance.com> On Tue, Mar 24, 2026 at 07:31:28PM +0800, Qi Zheng wrote: > From: Qi Zheng > > The __mod_memcg_state() and __mod_memcg_lruvec_state() were used to > reparent non-hierarchical stats, the values passed to them might exceed > the upper limit of the type int, so correct the val parameter type of them > to long. > > Signed-off-by: Qi Zheng > --- > include/trace/events/memcg.h | 10 +++++----- > mm/memcontrol.c | 8 ++++---- > 2 files changed, 9 insertions(+), 9 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 7fb9cbc10dfbb..4a78550f6174e 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -527,7 +527,7 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec, > > #ifdef CONFIG_MEMCG_V1 > static void __mod_memcg_lruvec_state(struct mem_cgroup_per_node *pn, > - enum node_stat_item idx, int val); > + enum node_stat_item idx, long val); > > void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg, > struct mem_cgroup *parent, int idx) > @@ -784,7 +784,7 @@ static int memcg_page_state_unit(int item); > * Normalize the value passed into memcg_rstat_updated() to be in pages. Round > * up non-zero sub-page updates to 1 page as zero page updates are ignored. > */ > -static int memcg_state_val_in_pages(int idx, int val) > +static long memcg_state_val_in_pages(int idx, long val) > { > int unit = memcg_page_state_unit(idx); Sashiko AI made an interesting argument [1] that this could lead to incorrectly returning a very large positive number. Let me verify that. [1] https://sashiko.dev/#/patchset/cover.1774342371.git.zhengqi.arch%40bytedance.com Sashiko wrote: > Does this change inadvertently break the handling of negative byte-sized > updates? > Looking at the rest of the function: > if (!val || unit == PAGE_SIZE) > return val; > else > return max(val * unit / PAGE_SIZE, 1UL); > PAGE_SIZE is defined as an unsigned long. Right, it's defined as 1UL << PAGE_SHIFT. > When val is negative, such as during uncharging of byte-sized stats like > MEMCG_ZSWAP_B, the expression val * unit is a negative long. Right. > Dividing a signed long by an unsigned long causes the signed long to be > promoted to unsigned before division, Right. > resulting in a massive positive > number instead of a small negative one. Let's look at an example (assuming unit is 1). val = val * unit = -16384 (-16 KiB) val * unit / PAGE_SIZE = 0xFFFFFFFFFFFFC000 / PAGE_SIZE = 0x3FFFFFFFFFFFFF max(0x3FFFFFFFFFFFFF, 1UL) = 0x3FFFFFFFFFF Yeah, that's a massive positive number. Hmm but how did it work when it was int? val = val * unit = -16384 (-16KiB) val * unit / PAGE_SIZE = 0xFFFFFFFFFFFFC000 / PAGE_SIZE = 0x3FFFFFFFFFFFFF max(val * unit / PAGE_SIZE, 1UL) = 0x3FFFFFFFFFFFFF (int)0x3FFFFFFFFFFFFF = 0xFFFFFFFF = (-1) That's incorrect. It should have been -4? > Before this change, the function returned an int, which implicitly truncated > the massive unsigned 64-bit result to a 32-bit int, accidentally yielding the > correct negative arithmetic value. So... "accidentally yielding the correct negative arithemetic value" is wrong. Sounds like it's been subtly broken even before this patch and nobody noticed. > By changing the return type to long, this implicit truncation is removed, > and the huge positive value is returned unaltered. That's true. > Could this corrupt tracepoint logs when passed to trace_mod_memcg_state? I'm not sure if that's critical but yeah that's true. > Also, would passing this huge positive value to memcg_rstat_updated instantly > exceed the charge batch threshold and trigger endless, expensive global > cgroup_rstat flushing, severely degrading system performance? It would lead to more frequent flushes, at least. -- Cheers, Harry / Hyeonggon