From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) (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 99D521DF256 for ; Thu, 18 Dec 2025 06:32:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766039540; cv=none; b=eHjaX/0jnDCysYnxmfCiRanuNwBO6PwEsGzQM6IpQ3FUfk8Y7t39BC32GoHn7vETozLY9ujyt+E8ysr2iNM3z4YJcCW1zFqXDFiUgHRL8JOgWEu7aZpj4oUtgv9v+twJpUYreS7zz+pvadJlb9+0sbaC9VHZsKh+1v3bIhhGHTQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766039540; c=relaxed/simple; bh=IFVb0VYPAm/lxtp0/AMZ5EIN8RU8AHWjPFxrAK0q8YI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=CsbxpUVpOGX2/F3ywiPv53oHJ1CjpYCLzpYUL6fksBDo7GCZ+L27X1dQgS6DZ5TOZEfbCLZOHsJ7xI2E9yd7BoLOy728d5nPaUtBds97ofXJin4BR0vDjDI5zOfNkbUGYbfrrbFO/+YLxIt+icUBJ2CprPPlghh+diCm1raqNOk= 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=KFBee5nT; arc=none smtp.client-ip=95.215.58.179 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="KFBee5nT" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766039526; 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=GuMAIMAxghdct3aJXfk8ti9XZUSoxcVspsnX7srcGqs=; b=KFBee5nTIhzoI9YEZNoVUIkAkpAX+95cyo5ZbyznMbVg80cFiduvqe3iOLSyfH1F7yrANX BC9xI2JhDTlys1R/SarHKY2E12ETA0eO9YYcKBcmlJ6QozfDzcVEvwLaxCSkXX31/2xxTL gfQN/eSKa8XVV4ZxKZwfH7DlY53ijd8= Date: Thu, 18 Dec 2025 14:31:51 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio() To: Johannes Weiner Cc: 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, harry.yoo@oracle.com, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng References: <29e5c116de15e55be082a544e3f24d8ddb6b3476.1765956025.git.zhengqi.arch@bytedance.com> 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 12/18/25 5:45 AM, Johannes Weiner wrote: > On Wed, Dec 17, 2025 at 03:27:32PM +0800, Qi Zheng wrote: >> From: Muchun Song >> >> In the near future, a folio will no longer pin its corresponding >> memory cgroup. To ensure safety, it will only be appropriate to >> hold the rcu read lock or acquire a reference to the memory cgroup >> returned by folio_memcg(), thereby preventing it from being released. >> >> In the current patch, the rcu read lock is employed to safeguard >> against the release of the memory cgroup in get_mem_cgroup_from_folio(). >> >> This serves as a preparatory measure for the reparenting of the >> LRU pages. >> >> Signed-off-by: Muchun Song >> Signed-off-by: Qi Zheng >> Reviewed-by: Harry Yoo >> --- >> mm/memcontrol.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 21b5aad34cae7..431b3154c70c5 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void) >> */ >> struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio) >> { >> - struct mem_cgroup *memcg = folio_memcg(folio); >> + struct mem_cgroup *memcg; >> >> if (mem_cgroup_disabled()) >> return NULL; >> >> + if (!folio_memcg_charged(folio)) >> + return root_mem_cgroup; >> + >> rcu_read_lock(); >> - if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css))) >> - memcg = root_mem_cgroup; >> +retry: >> + memcg = folio_memcg(folio); >> + if (unlikely(!css_tryget(&memcg->css))) >> + goto retry; > > So starting in patch 27, the tryget can fail if the memcg is offlined, > and the folio's objcg is reparented concurrently. We'll retry until we > find a memcg that isn't dead yet. There's always root_mem_cgroup. > > It makes sense, but a loop like this begs the question of how it is > bounded. I pieced it together looking ahead. Since this is a small > diff, it would be nicer to fold it into 27. I didn't see anything in > between depending on it, but correct me if I'm wrong. Right, will fold it into #27 in the next version. > > Minor style preference: > > /* Comment explaining the above */ > do { > memcg = folio_memcg(folio); > } while (!css_tryget(&memcg->css)); OK, will do. Thanks, Qi