From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) (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 C34D02586CE for ; Thu, 20 Nov 2025 13:32:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763645548; cv=none; b=V83NNivPWfwDAiVclp/hHlFenrAYNfmjoP9KcIjnIH3Y1eti59+gV7Xpi3LO9jPsBF3pV9koShXFL7sayyW51A4j+uIQzdeJLqMbcyVuf4AU2dWKJxr6+2MtgjT2mYoLy6Z62SsxWC/CICk8ulLbuzvo+cDvPnxOUNCP4PJVFPU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763645548; c=relaxed/simple; bh=IJxh2M3vKGK5A0CnshNuqGDiwNQWoK5AMGJhGneIs6w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=V8IxTaibDLJWtksI66/o5CP3dU9Z37Y8AZq9NKrzYYBhZE09A+JlVqUsCnpEB6ekf7NFl46Uy6nbViiuxf/TLOmpgJHkVFAIHVyyhYIebLoy1x8yUr9hPMwVFVNz/W670C9G/DbMBcTZhaaWnYZ342hL9u+vOc3stYmK3bFCy9s= 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=MrvtAjUo; arc=none smtp.client-ip=95.215.58.174 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="MrvtAjUo" Message-ID: <6d372282-d443-4fe4-942b-998aa742eb0a@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1763645543; 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=ybURcIVrz7mysPIWzBzQOyxvKWboFgst4pbQzAb0oQs=; b=MrvtAjUo07c82+yzmSH3h/kWt7CxnHcNiFmUBnzEiScilBeS3UAknDBBayZG72RaQ8b49Q WsAmvYaMaleu27E7PklGE0FVra3y9mswrhSRUNrvF0HZmdUAEY8r2jRI/jkyLrgiQttdNI kc5UbGFFwApLOHJkmNaZMpZXo4OaYmY= Date: Thu, 20 Nov 2025 21:32:09 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio() To: Harry Yoo Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng References: <9eca65dec044d4352bee84511fb58960a1402ddf.1761658310.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 11/19/25 4:06 PM, Harry Yoo wrote: > On Tue, Oct 28, 2025 at 09:58:20PM +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 >> --- > > This patch looks correct to me, so: > Reviewed-by: Harry Yoo > > with a few side question on current implementation rather than > this change... > >> mm/memcontrol.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index d484b632c790f..1da3ad77054d3 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))) > > I have no idea why we're already calling rcu_read_lock() when a folio pins > the memcg :) +1, this was introduced by https://github.com/torvalds/linux/commit/f77f0c7514789577125c1b2df145703161736359, may be some defensive programming. > >> - memcg = root_mem_cgroup; >> +retry: >> + memcg = folio_memcg(folio); >> + if (unlikely(!css_tryget(&memcg->css))) >> + goto retry; >> rcu_read_unlock(); >> return memcg; >> } >> -- >> 2.20.1 >