From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 22E961684A4 for ; Wed, 25 Mar 2026 03:25:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774409135; cv=none; b=l1remVrou7u5QTPFBEi0NjZY0cxIm+KepbJHGZF/5wtD+rKRAiN2tLS6mdsVzr5qLzdlcOvG+KJwwRbXtND0cZdMWFz+RnKPjRNjyP7x39uJiiuUeLW4Ij0VJuwkD6dMUBW5uB+aWDrX6qzPnMKvKFm2V7g8+XEXc27cAaSPjKk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774409135; c=relaxed/simple; bh=/zC2EqfG3jqc31w1EJDmtGTEkmehH4nIZq4+6T1jPxo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Jr08iAe/6Xoshf8La7e1GNRTFwadwdvsIVnkILi696+c8cuk6jbw9D+25Gms20pzFKlFWQ5TE2NoG4kRQyL1AZMdpdzUx2SJzTrlU/X86G9tRjLouQIEtjw6mJyBvIwJnBCH6jX4ZQh0MD5ZiZsZCgn9h0KNI4PwZzQ9xLEE/dg= 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=H1bG4E6X; arc=none smtp.client-ip=91.218.175.182 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="H1bG4E6X" Message-ID: <5a9d9152-8e23-4438-a322-ec524fd159a6@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774409130; 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=iyq37Tj+lXRbovU+PWFQtd1g+xZuehOm3R+HUURCGJ0=; b=H1bG4E6XhyAflt//Q000vYymltx98oPx9y//qq92TQKF+2l/ZlJ7pN+i3R+ZJ3RoWLtMLH jLwqUGOVYgSxkDO0ugDEfue5q7pNkbV/Pyc3Xc+ceYIY2OZqAql+HtTXB8vCN9KMkmDZeU 1iXbIdXnEc2WhRqZJJCKPzPG2oHfFAA= Date: Wed, 25 Mar 2026 11:25:06 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 2/3] mm: memcontrol: correct the parameter type of __mod_memcg{_lruvec}_state() To: "Harry Yoo (Oracle)" 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 References: <90524ca3806e24105ab5f2d69435f57c2ae034cb.1774342371.git.zhengqi.arch@bytedance.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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. 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? Thanks, Qi > >> 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. >