From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-187.mta1.migadu.com (out-187.mta1.migadu.com [95.215.58.187]) (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 0A970378824 for ; Wed, 25 Mar 2026 07:40:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.187 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774424431; cv=none; b=eD92nGiXEkT7oXFSddwj/Q1Q1fzgW7h/jFoSmKBEwbp0gc9B1Sd0h+XLRb3y2xR8LRlTBvHFbbouk4gjsJJ4zAuSBGdwBBvQ34ZhpE45+xJebU06fu/T6QN/G09mm02Mi0/oZ9z+w7xCB9GDy/mGKJYU4nus1w8nZ52dgW+xEcs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774424431; c=relaxed/simple; bh=QqHosbiaenj7c2Hgv1RuYl3TBkFkG7j/rA7cOCEfa3o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Wl/fWExTk4qlolTw16kbAqFlk4ItD27y2yQlKB5CC1uEn/wQaalJhEoK15GMxxVAHnFEX9DIAseitIIq3EcGijzVLiA3VX/6cFF7eTkYMYbyCslDZx1VrXYddakz3btGeo29HLNhlhcwrSgLaA1tkC/Pqx5V5f4M9FZ6UpBXP8I= 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=gXtG5BnY; arc=none smtp.client-ip=95.215.58.187 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="gXtG5BnY" Message-ID: <5fed0611-434c-4fd4-956c-39f23e0459a1@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774424421; 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=B0CLR6YbY6BbPn5BbvUeaUgxF//HUlX85mhdlERM2EU=; b=gXtG5BnYmoxr+q9PJgyyvvbmG2Y7X3JRDmYJ6pwDIdQW+l2WQjQHq+YR4OeDKfJQvgrutJ rnQrPUSNZo0i//tEs4OKF2kK2m3UlvTKo1dLwCAxV228fWFN0LvtrwSC/HcVVujH/ndpSr itpv6UhpawYGqJhloWmooFazaaFOXMY= Date: Wed, 25 Mar 2026 15:39:56 +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 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 3:36 PM, Harry Yoo (Oracle) wrote: > On Wed, Mar 25, 2026 at 03:26:46PM +0800, Qi Zheng wrote: >> >> >> 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; > > We could use abs() macro in linux/math.h here :) > > val should not be LONG_MIN, but it should be fine > to assume it won't reach LONG_MIN. OK, will use abs() macro. > >> + >> + 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? > > Nevermind. Without the diff, I misunderstood what you meant. OK, will apply this diff and send the v2. Thanks, Qi > > Thanks! >