From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) (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 8FFB92C2374 for ; Wed, 25 Mar 2026 07:27:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774423632; cv=none; b=o/IuTojbsveyh7hwCm/qThKnPDmVL0W8YXQtRmdiI0aZ8HmLSP8uxnSUkEdXPcLrpi/IqYI4me8bf/GcvBxsisd4isABPWXpRsRINx6mLWBT7HkWDy1NhFS+qzT8R2bvS+B3I4kMCQfO0LrGE0o1zL+CsDwFQmXUQtm8pdhKdX4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774423632; c=relaxed/simple; bh=CvoZVYjyrD/N/ncBjCNtTEWexCGHIv4BxTiRe5v+DFY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=L2R2/xOpcoz2LIZRpvgtVG6cm9mdjPVVdYs+5jQEIBj+kfNltfA4Iiv9qPkAc+e+VBiN1yO+WqGoM0ljIfpJPLkcj8TXuTe+IXddfK/0ct3CSNu29jSe2zbV/Z+nKSt0MfHXNXNx2RjqIQjrBKVSWMaBrE5TfXQQBqajwB6+QpQ= 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=guzOAPQq; arc=none smtp.client-ip=95.215.58.188 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="guzOAPQq" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774423628; 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=mxT8crAtAWHT1Y0DjEeladDi+Mk4USm8O2CuoWFzL3c=; b=guzOAPQqXXAYWBVhbYx6yBWSOGmKidsvRO6IJG2ETMZw6DEvalHDZEuBjyJIPjH3vHvrMn MT0V3231oMus0jf5KwCTgxZrgEEyKqdEyBw4P5bBfEFNIRe4mcJy2yPAv+0y7xZavCe01M CDV+K8VsUa1OKf3KuYzoMtdhuRPn0qg= Date: Wed, 25 Mar 2026 15:26:46 +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> <5a9d9152-8e23-4438-a322-ec524fd159a6@linux.dev> 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 1:17 PM, Harry Yoo (Oracle) wrote: > 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? For more clearly, here is the diff: diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6491ca74b3398..2b34805b01476 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -787,11 +787,17 @@ static int memcg_page_state_unit(int item); static long memcg_state_val_in_pages(int idx, long val) { int unit = memcg_page_state_unit(idx); + unsigned long uval; + long res; if (!val || unit == PAGE_SIZE) return val; - else - return max(val * unit / PAGE_SIZE, 1UL); + + uval = val < 0 ? -(unsigned long)val : (unsigned long)val; + + res = max(mult_frac(uval, unit, PAGE_SIZE), 1UL); + + return val < 0 ? -res : res; } #ifdef CONFIG_MEMCG_V1 > > That would work for memcg_rstat_updated(), > but not for trace_mod_memcg_state()? Why? The trace_mod_memcg_state() seems to accept 'long' type, am I missing something? Thanks, Qi >