From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta0.migadu.com (out-172.mta0.migadu.com [91.218.175.172]) (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 2F03632B116 for ; Thu, 30 Jul 2026 15:17:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785424626; cv=none; b=ABV+UZVvSrvSd4vyY5xHp/6dLZXWntEn743JhMytqP8he5wW3r9yiqBfSi3wvHy1dyp+TqxHyHtr3Bi8rMYiABgAARRibdSyY8CrX4KxnaBoPJ3RJPgxtmov3mwH3+iFynS5cjTLgjUB+PQiQvoAnY5r3FzpWkxgwPcIz4jl8Ww= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785424626; c=relaxed/simple; bh=GgPPsgIRnv9URRgte3OSnVt0MpqQtun2qB6DMh6ziBA=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=fFH7IpvpyHel8YwZw167SkDAbByVWvBghnsijqSzLnlzoChll7wnJOGltn+8j/Q/MBdLH2FIJqCghKhgJuNI+PClWxTQpno3DNFD1+zHdgdPAIlg0hBScRGeIoHOXtTib+MyFyCUjYxXTS8eWQYdb9gkGJw1HBVfBCfzRb7PF0c= 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=CxdIPYz5; arc=none smtp.client-ip=91.218.175.172 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="CxdIPYz5" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785424619; 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=be+vywYOO52uR7pywIjLWsbFNIwfm83WliUEbwXuaao=; b=CxdIPYz5ftz4x6wxsBPJhs8StXdeOE+gJICfYkQ1rZldApSqmYTBYP+6q4JQg26JUDN3Ct UkhnIK6+syDBgl9WsaMRT0Nh3toU2fkIz/rZUuFTtjaZz8RYuWH/c/kefkBeCFIZZsqs2a q3t8er1OwU9ZjsRTVR9xW4A3RSbmACA= Date: Thu, 30 Jul 2026 23:16:49 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Cc: cui.tao@linux.dev, Muchun Song , David Finkel , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Tejun Heo , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ridong Chen Subject: Re: [PATCH] mm, memcg: fix memory.peak reset clobbering other fds' watermark To: Ridong , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton References: <20260730115314.1069089-1-ridong.chen@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Cui In-Reply-To: <20260730115314.1069089-1-ridong.chen@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2026/7/30 19:53, Ridong 写道: > From: Ridong Chen > > Writing to memory.peak resets the peak for that fd only. Each fd is a > watcher and reads back max(its own value, the shared local_watermark). > > peak_write() resets by lowering local_watermark to the current usage. > To keep the other watchers' peaks it then walks the watcher list, but it > stores the current usage into them instead of the old watermark. So once > usage has dropped from a peak, a reset on one fd wrongly drags every > other fd's peak down too, even fds that never reset. > > Reproduced on 7.2.0-rc5-next under QEMU, two fds A and B on one cgroup: > B sees the peak (410624 KB), usage drops, then A resets -- and B's peak > collapses to 1060 KB although B never reset. With this patch B keeps > reading 410624 KB. > > Fix: save the old watermark before lowering it and use that to floor the > other watchers, so a reset only affects the fd that issued it. > > Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Ridong Chen > --- > mm/memcontrol.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 60145aadfc5e..881e7c459c64 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4692,7 +4692,7 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, > loff_t off, struct page_counter *pc, > struct list_head *watchers) > { > - unsigned long usage; > + unsigned long usage, old_watermark; > struct cgroup_of_peak *peer_ctx; > struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of)); > struct cgroup_of_peak *ofp = of_peak(of); > @@ -4700,11 +4700,12 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, > spin_lock(&memcg->peaks_lock); > > usage = page_counter_read(pc); > + old_watermark = READ_ONCE(pc->local_watermark); > WRITE_ONCE(pc->local_watermark, usage); > > list_for_each_entry(peer_ctx, watchers, list) > - if (usage > peer_ctx->value) > - WRITE_ONCE(peer_ctx->value, usage); > + if (peer_ctx != ofp && old_watermark > peer_ctx->value) > + WRITE_ONCE(peer_ctx->value, old_watermark); > > /* initial write, register watcher */ > if (ofp->value == OFP_PEAK_UNSET) Acked-by: Tao Cui