From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-114.mta1.migadu.com [95.215.58.114]) (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 C33DC353A95 for ; Wed, 12 Aug 2026 23:46:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.114 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786578379; cv=none; b=Q+D04328IWa6TwwDXfeFPj9AovvWZ4nC3kNYqkMz/kujqUdJdECFlYCEl5m+GypAOPkxtEB/udHTw80ts7ATuQLexrapB/h6Sl0tvfKxe/liyQc10U87lpYEnwnv2rIfMfeAJ8tBnEHQmrjRHGYgBjsu86IzZNvbSrYsifdgz/U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786578379; c=relaxed/simple; bh=Suug6ZlLMRpt7AO3wExksttzRhjDowxSc/U7ICXnYsU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=iCfT+raGkLMnzwy/8Q5ePJscyn/kOWdzrl3Sq1/0GoR3CP0sYBksH/3eEWnl2N270YEdFJZzC6vNKFNx41xxlHnRKv+yzVCAB3WpNev31UIG7P0gYzh99oG2C6e7JRKCGYFy6KFjdGuctFojs7pjHHdqUiwo3NTni9opZNNAX6E= 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=pjReRsjU; arc=none smtp.client-ip=95.215.58.114 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="pjReRsjU" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=Suug6ZlLMRpt7AO3wExksttzRhjDowxSc/U7ICXnYsU=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1786578375; v=1; x=1787183175; b=pjReRsjUK7CyUIS/VWoH+vfERRzp+sncklUeXxXGg039MQaOLBKWvmxHiWiIB9PpQk7B6M7o rUFReJpRTI1SB1NQnLvEAQTtd9cNR4zi3cXXj5vMCA5K96QLQc5l+UiL37kw+K544UKsO0vaCGW rlJereWzZsS0ManoIwk1Vya8= X-Envelope-To: linux-kernel@vger.kernel.org Received: from localhost (2a03:2880:10ff:a::) by smtp.migadu.com with ESMTPS id cf2368f0b44ab78b; Wed, 12 Aug 2026 23:46:15 +0000 X-Migadu-Flow: FLOW_OUT Date: Wed, 12 Aug 2026 16:46:09 -0700 From: Shakeel Butt To: Ridong Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Andrew Morton , Muchun Song , Tejun Heo , Michal =?utf-8?Q?Koutn=C3=BD?= , 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: Shakeel Butt