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 5B5F819D8AC for ; Wed, 25 Mar 2026 05:17:42 +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=1774415862; cv=none; b=tbtC2c4h//+bbFaMiQwDuaCdvs0bAsnPaiKxRv7DN0aHlHummG2+cXLTv6UUA19CfHqBeobn/4cuSpmCyEnxG5B2ALhTOhj15lKiuFWCtW0pqb0LCaGGUVCdp3VLXCGzfmZe8+sbQjDUFtNM2sCnVdJcerPZEqWE4P55NRnaYxc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774415862; c=relaxed/simple; bh=pjCTKc2xw7ZjdrFHVDgRIXNLOjgy/TRByZz3N7OKr24=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=K98W6U61t2tBRghl4CIgCC1jCYvvIabTURvbQgRu/hQXn8fPOokOmFjG14E7IeOkX9WF009ktVs/HLycp/OgU2W1tmeGBfNY5Hq9z8Sg2RrjZnQ53O0ZUK25y0Q3ecnln71vF6tackXNVzCTMOsuc0/h6bC+Bm+Ggqw9N1GKnfc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=sbyTnvQX; 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="sbyTnvQX" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8A087C4CEF7; Wed, 25 Mar 2026 05:17:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774415861; bh=pjCTKc2xw7ZjdrFHVDgRIXNLOjgy/TRByZz3N7OKr24=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=sbyTnvQXzYmouz0eYNVcPEX6pMhS8V6URGzQpnmCRb0P7u4qw+MybOEB1rtSyBcxl epwRgUITlUDkRsv9POXJEM5oXbLtv9PKoRgy3xmeJ1lTBKf/cm0QKBbGl6bF84Swjn Zr1ZBKZAwcAbKGF/uqLZvr1xPgaxD1vrVWK1WRnRKz571ns0ycn1JgKd8NL/aX+ntJ ThVYCz5bxT2XHtCFCMTSFWCf3hQ9O6gyU0KjF4ni2Px8pC9h0XnDtx4XxiD6V8qDIq zQdYqBz/9mnUARWpOVUPbkGF7twb9qM4Z8jcm2Q4TbDe7O2/iNizT+TgwMtroCbdXG QMZNdBkqxQCMg== Date: Wed, 25 Mar 2026 14:17:39 +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> <5a9d9152-8e23-4438-a322-ec524fd159a6@linux.dev> 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: <5a9d9152-8e23-4438-a322-ec524fd159a6@linux.dev> On Wed, Mar 25, 2026 at 11:25:06AM +0800, Qi Zheng wrote: > On 3/25/26 9:43 AM, Harry Yoo (Oracle) wrote: > > 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. > > Thank you for such a detailed analysis! And I think you are right. No problem ;) > The memcg_state_val_in_pages() is only to make @val to be in pages, so > perhaps we can avoid the above problem by taking the absolute value > first? That would work for memcg_rstat_updated(), but not for trace_mod_memcg_state()? -- Cheers, Harry / Hyeonggon