From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-156.mta1.migadu.com [95.215.58.156]) (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 409D7343D9D for ; Fri, 14 Aug 2026 03:30:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.156 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786678235; cv=none; b=TWExipEdvGF4F/Tt5lnoCREVusgnJgBNMGOTb3DT67qXXOXLJjzydEcbzFZqmq97/K+bFW429FCbEhDQ+mMdiytVss4WvMK3pB2z0iJvOH+ED5+5PV8gXys+NoGsoa0buhunO46IvG+ZNxEpK3PJ5XW4PzeBd+1nKy82eBkpAx4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786678235; c=relaxed/simple; bh=M3M6KYCU8WrjOH8M0FSq9Y5uJuL5tl6xBP8M1lGPC2M=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=eByNm74yqZ+wH4K0H929mU922EAXi1zdydGR9nzItQy6bxk1tux3h3DDnzMvBrLNFewBAn2NPv1CEoBDz03/9NOeTrYcUWo/j21c4hEM/tftXZL5U9bqQ970ydQg77qFRpc1rpOV9T5Y8G16+4LO55ZkyLpI1HWRAtWlL46cwVo= 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=V943aBKj; arc=none smtp.client-ip=95.215.58.156 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="V943aBKj" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=M3M6KYCU8WrjOH8M0FSq9Y5uJuL5tl6xBP8M1lGPC2M=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1786678230; v=1; x=1787283030; b=V943aBKjkdKDyNGKceNY3ZfcfJ9m50izyAcyqCSA66icCX383muzykY0F6PuuM98+s8Qs3Kc ywJy1S/iodyIS/o5Dcdm4WmfCy/B919sNZoqOHuztg25wnK/ksEV5Cnw7GCs8vJxP20/6PWqLNe ys8jE5+qBhxtwLDyrXrFpfAM= X-Envelope-To: linux-kernel@vger.kernel.org Received: from mi-ThinkCentre-M760t.mioffice.cn (14.29.108.92) by smtp.migadu.com with ESMTPS id bff796e104338a1c; Fri, 14 Aug 2026 03:30:30 +0000 X-Migadu-Flow: FLOW_OUT From: Ridong Chen To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , David Finkel , Tejun Heo , cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen , Ridong Chen Subject: [PATCH v4 1/2] memcg: acquire peaks_lock when reading memory.peak Date: Fri, 14 Aug 2026 11:30:04 +0800 Message-Id: <20260814033005.2481920-2-ridong.chen@linux.dev> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260814033005.2481920-1-ridong.chen@linux.dev> References: <20260814033005.2481920-1-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-Transfer-Encoding: 8bit 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 Acked-by: Shakeel Butt --- mm/memcontrol.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 17da1f43b7d3..ceb43b855122 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4714,6 +4714,7 @@ static int memory_peak_show(struct seq_file *sf, void *v) { struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf)); + guard(spinlock)(&memcg->peaks_lock); return peak_show(sf, v, &memcg->memory); } @@ -5859,6 +5860,7 @@ static int swap_peak_show(struct seq_file *sf, void *v) { struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf)); + guard(spinlock)(&memcg->peaks_lock); return peak_show(sf, v, &memcg->swap); } -- 2.34.1