From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (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 E493317A2EA for ; Mon, 22 Dec 2025 03:42:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766374982; cv=none; b=klY/EzLHPL4J/58OBfok6XYi5knerablwdpjBDyz4ULx66qVj2AaYq0nkVeVrMrAd+l244ulwVvFBxQjn1ZhhnlW1yv73QL7T/6e0MXh96GR5uv/DPOAIfaPTRPO982LgxMsbBV07DSB9K7owLnIPpktk+DOR3ZReeryh+X0k4s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766374982; c=relaxed/simple; bh=6OXjcEb1oFmKkCUODzH32erhSHCVKpgyCsrweISc5iU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VU++hJ52Mj6zRVSjAeWAdtonERGs/wafQSJrKSZCi42jeNP/oIbOlFoZRqOgeGxpEnRGJ6XPZH0PyfIy8afqzQN4OCJBVmqB3JSVIqGnBc/Xrvqp0j3qxdZvDyNo4J1qUAWjwd13oTyUoMPdE9vd1IepGxZQ90zcRfEphKlZszs= 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=hVmiX6uT; arc=none smtp.client-ip=91.218.175.178 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="hVmiX6uT" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766374973; 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=bz1xuPsgdNZC4b636x1LrK4fMi9S/EGh27EdWq41RBA=; b=hVmiX6uTnWbBzcdQMwstWbBHDPWFlAHlSeClZfctafz8wzOcVHzyKB8XX1SNWtyuAQP6kv TuQWLWUGs84KFl8rDmsFxUq27C45eAxgWZGQm3bHTLBNKiLJJqEAwM6hh4jpBBslKOnFtW KCrikMCmmHQwn2YykjCeCfScSa6UH14= Date: Mon, 22 Dec 2025 11:42:42 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping() To: Johannes Weiner , "David Hildenbrand (Red Hat)" Cc: hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, 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: <1554459c705a46324b83799ede617b670b9e22fb.1765956025.git.zhengqi.arch@bytedance.com> <3a6ab69e-a2cc-4c61-9de1-9b0958c72dda@kernel.org> 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 10:26 PM, Johannes Weiner wrote: > On Thu, Dec 18, 2025 at 10:09:21AM +0100, David Hildenbrand (Red Hat) wrote: >> On 12/17/25 08:27, 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 folio_migrate_mapping(). >> >> We usually avoid talking about "patches". >> >> In __folio_migrate_mapping(), the rcu read lock ... >> >>> >>> 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/migrate.c | 2 ++ >>> 1 file changed, 2 insertions(+) >>> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index 5169f9717f606..8bcd588c083ca 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct address_space *mapping, >>> struct lruvec *old_lruvec, *new_lruvec; >>> struct mem_cgroup *memcg; >>> >>> + rcu_read_lock(); >>> memcg = folio_memcg(folio); >> >> In general, LGTM >> >> I wonder, though, whether we should embed that in the ABI. >> >> Like "lock RCU and get the memcg" in one operation, to the "return memcg >> and unock rcu" in another operation. >> >> Something like "start / end" semantics. > > The advantage of open-coding this particular one is that 1) > rcu_read_lock() is something the caller could already be > holding/using, implicitly or explicitly; and 2) it's immediately > obvious that this is an atomic section (which was already useful in > spotting a bug in the workingset patch of this series). > > "start/end" terminology hides this. "lock" we can't use because it > would suggest binding stability. The only other idea I'd have would be > to spell it all out: > > memcg = folio_memcg_rcu_read_lock(folio); > stuff(memcg); > otherstuff(); > rcu_read_unlock(); > > But that might not be worth it. Maybe somebody can think of a better > name. But I'd be hesitant to trade off the obviousness of what's going > on given how simple the locking + access scheme is. Agree. I also prefer to keep the open-coding method for now, and if a better helper is available later, a cleanup patch can be added to accomplish this. Thanks, Qi