From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-qk1-f172.google.com (mail-qk1-f172.google.com [209.85.222.172]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5EBB64307AB for ; Wed, 12 Aug 2026 16:42:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.222.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786552969; cv=none; b=Fhs8WHH1PnL7tzIIMTjhpwkw5Kk56mV7GEpOhTqA9kz5h/igvLe9AMFOQ/d4H0CUl6mJAc1VnbL4LpJ2kqjmqszoGt9yK03sxoL293rCMdjsNCDy1/ZqYow1prcjHLZl2Zb0bjONCVOejLztDo9QUmyj1dfFvNmF8CLnXNY7YdQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786552969; c=relaxed/simple; bh=jTBEJcKkHaGMEfbgRuVHcAiD7osuTPlGQ86vK3hD5B0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nldnFpZRV8pVN0/Dx8dmNiro52uhzEsEfnrNrO53eU+h6q/AJMkNb6geNSEKckZTqgAHfvb09JFIcIxNZqZ33/MEwgSyF2dJeNK3Wadmw86Kqwf5Vb5JyT401wjyNYO9I84t6DUGzlqZyTUjP51hONxW7/RuGtHImleByGfkLLc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=cmpxchg.org; spf=pass smtp.mailfrom=cmpxchg.org; dkim=pass (2048-bit key) header.d=cmpxchg.org header.i=@cmpxchg.org header.b=NBuE6w0k; arc=none smtp.client-ip=209.85.222.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=cmpxchg.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=cmpxchg.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=cmpxchg.org header.i=@cmpxchg.org header.b="NBuE6w0k" Received: by mail-qk1-f172.google.com with SMTP id af79cd13be357-92e6c4a867cso80492385a.0 for ; Wed, 12 Aug 2026 09:42:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg.org; s=google; t=1786552966; x=1787157766; darn=vger.kernel.org; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:from:to:cc:subject :date:message-id:reply-to:content-type; bh=x/EWoL2S0wFVxxUH9CDFZoGkIehfFWNIp7hAQk9bjwQ=; b=NBuE6w0kghiBry4Y6NFnJctmBBYwOl3VhaM0a/v0ssd4cEI8nMiaTRYZDXzuxyU5hD Cpcwp+2K9mSYAveQDQWaNfP18oG7h/T0tPvmbEYh+PSE7XLN3lfijX6umftdhAajQiv4 FeOmh8D2dJkUkSey8bP93L/AHGliV3GQtx1Zp0v+jpzviHThhxq3cmJV4kQj8MpKfjEq BCDmUsaa0S6sO+LzrKi/ZV7hhugxZvV33xq0Htpw+yEAd1f8zX5KbWBJRWNXc89nrtlx l8tuuEKnUhApKyzSBkqPJUB2uR435MCq8shMoN0FH+ayPTKRXEbjiNi0ybtY1okJqfhM 3VBQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786552966; x=1787157766; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=x/EWoL2S0wFVxxUH9CDFZoGkIehfFWNIp7hAQk9bjwQ=; b=H7VmJH3XCazmnphPpHRdZbL59LxpNmvAeXdWVISNC4i5gl71Z1J9oWayDaXZlhaQPA 9GPHFXH/LpkId5EK3YkML1wCX6jiQtUOiCAeyIV30QSgF+6NWdJyyyR17YCxkNUMMnhV /nwKJ7FLS49DTVZx8RtjOaUHLW0td15mTNx5/sXbvL6jFaH+5andjDWXuKAEYgwbVIl3 /ECehPreVaVZR6GwUw5lcaPbC0RTajBCtAC4scAVpltlMDN2A+ZZhmIvF3fnhAdXxpyP YgXJpeyP5yCcFPeGoQxMNKwQCwRHTW59Gc4LlfyrKiPoiSv1YuIe8IbfEMmEhmm3PTHT d9fA== X-Forwarded-Encrypted: i=1; AHgh+RqoW4aRkz9E9o3x16IB8PBDFO+dSYKVh2/Hp8MUu4DqT47Bb7af/Y446wpUbQZgAMX5zwTd3XuF5ATrB3k=@vger.kernel.org X-Gm-Message-State: AOJu0YxbCFbR2kJ6yzwPOl7pq6aj7ahqLEWCRk/wNT2NBiyQnuNOGrrI 8MTneSk8JEI5cTTJ5kYpMbbQdhxJq59f2qCvpaGKbadzxJFYc12dEjV99vz3hraHyERqPhxFwwZ 6ZLIK X-Gm-Gg: AR+sD11viOLRmvtZDmSfRk5N0HyXW/u5VSAoaQUrN2w/V2TsZ9Hq/Ka0XNUas0JxjQn mp4bSVLrQr+qXgReNE/GQEwKT5lN1VLUbjamu717F26Q5yQ0Fbfyc30/ToZ+EWhgHkUdxHD0A5Z 571jGz/3pECKyjdC3yxJfXKcsRzrYTEkXrJedEj7ZcSAbgTcAbp82saLij3sDv2qHD8ZVd3yB1u +JpUstA2LgzEfj4PUTZZL0RZUtQyi6mZDmF2Axdzq3UUUNx6fPII6N98xmue5NOYcxIl3aXC4JQ B8FmYv6kkTQbMJOHgpH3VYiBOH4yVCqtkdnnzEZ131tzwfUN9QudCQxAx5yodINKwylQ6+NkGjk nw2SM0f/zu6sLmcd8lJ4/u8mGe2oO4DCXK/CeRhWIL+8DIoRvWxF73zU61wVeF63WDY3P2TFC8/ E/j8VcRjMw7LPfZqMXynIwIbOhqwdQ9oQ/Ue7vGGUbys97fNB0AoRZkqkzf/U= X-Received: by 2002:a05:620a:1b8c:b0:915:f96b:8f5a with SMTP id af79cd13be357-936b3d9dee5mr568196385a.36.1786552965979; Wed, 12 Aug 2026 09:42:45 -0700 (PDT) Received: from localhost ([2603:7001:f100:500:365a:60ff:fe62:ff29]) by smtp.gmail.com with ESMTPSA id af79cd13be357-936b5dd3f5fsm167905385a.8.2026.08.12.09.42.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Aug 2026 09:42:45 -0700 (PDT) Date: Wed, 12 Aug 2026 12:42:37 -0400 From: Johannes Weiner To: Ridong Cc: Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton , Muchun Song , Tejun Heo , Michal =?iso-8859-1?Q?Koutn=FD?= , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cui.tao@linux.dev, Ridong Chen Subject: Re: [PATCH v2 1/2] memcg: acquire peaks_lock when reading memory.peak Message-ID: References: <20260807090000.1532495-1-ridong.chen@linux.dev> <20260807090000.1532495-2-ridong.chen@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: <20260807090000.1532495-2-ridong.chen@linux.dev> On Fri, Aug 07, 2026 at 04:59:59PM +0800, Ridong wrote: > From: Ridong Chen > > Sashiko reported that a reader can transiently observe a lower peak > within a race window [1]. peak_show() returns > max(local_watermark, ofp->value), but peak_write() updates those two > under peaks_lock while the reader takes no lock. The interleaving is: > > writer (reset on fd A) reader (fd B) > ---------------------- ------------- > usage = page_counter_read(pc) > WRITE_ONCE(local_watermark, usage) > // watermark lowered to usage > lw = READ_ONCE(local_watermark) > // sees the lowered usage > val = READ_ONCE(ofp->value) > // B's value not updated yet > return max(lw, val) > // both low -> low peak > WRITE_ONCE(peer_ctx->value, usage) > // B updated, but too late > > Fix it by acquiring peaks_lock when reading the peak, so the reader sees > a consistent snapshot of local_watermark and the per-fd values. The same > race applies to memory.swap.peak, which shares peaks_lock and the > peak_write() path, so take the lock there as well. > > [1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@linux.dev?part=1 > Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Ridong Chen Acked-by: Johannes Weiner