From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailgw.kylinos.cn (mailgw.kylinos.cn [124.126.103.232]) (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 AA0604E36CC for ; Tue, 13 Oct 2026 11:02:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=124.126.103.232 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791889372; cv=none; b=BrLJoPBY8sd1inhModC4vDC0I8r322G/rFDTZEw6yX/3bz7HVBhS+Gb91S6W8gM+f9pRkU/xiQ4uh6Ev9ullKFIG+6dg+X7lMaOr3Z90uvo0fohNk8NeLcak1DyWbWt+Fv4usDmGw7OGGDL9AkoJKvmaav2ZtJ9TPAIIzBtRFNE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791889372; c=relaxed/simple; bh=TQ1Y8utDeuENqeSwKfZ3I/riD6Qj6YmGzuUsxnLbTnQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=m7tvS/q0iQJqiRL7yiItREXJNwemWfhCLz/k5HWNz0vWDt5AN8YQ/WbjOkgWn2Vd216oD3f6hIvstZ1rXg5Lv8TaMGwAjlqitNYpY6NSO1vQS8N+th4oX58eXETySlRDuIFILEDj43IEbm0NAbnlTNZgkGwHfJYXqVAmcURbJY0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kylinos.cn; spf=pass smtp.mailfrom=kylinos.cn; arc=none smtp.client-ip=124.126.103.232 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kylinos.cn Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=kylinos.cn X-UUID: 9b23fc3ac6f511f19a56ed5b684f684d-20261013 X-CID-P-RULE: Release_Ham X-CID-O-INFO: VERSION:1.3.19,REQID:6db9e078-1538-4d08-8ef9-9329f447ff6b,IP:0,U RL:0,TC:0,Content:0,EDM:0,RT:0,SF:0,FILE:0,BULK:0,RULE:Release_Ham,ACTION: release,TS:0 X-CID-META: VersionHash:7db8b62,CLOUDID:597e9dc93505a2ad45b7ff19eaf9495b,BulkI D:nil,BulkQuantity:0,SF:80|81|82|83|102|136|850|865|898,TC:nil,Content:0|1 5|50|99,EDM:-3|-100,IP:nil,URL:1,File:nil,RT:nil,Bulk:nil,QS:nil,BEC:nil,C OL:0,OSI:0,OSA:0,AV:0,LES:1,SPR:NO,DKR:0,DKP:0,BRR:0,BRE:0,ARC:0 X-CID-BVR: 2,SSN|SDN X-CID-BAS: 2,SSN|SDN,0,_ X-CID-FACTOR: TF_CID_SPAM_ULS,TF_CID_SPAM_SNR X-CID-RHF: D41D8CD98F00B204E9800998ECF8427E X-UUID: 9b23fc3ac6f511f19a56ed5b684f684d-20261013 X-User: duanchenghao@kylinos.cn Received: from localhost [(10.44.16.150)] by mailgw.kylinos.cn (envelope-from ) (Generic MTA with TLSv1.3 TLS_AES_256_GCM_SHA384 256/256) with ESMTP id 675544311; Tue, 13 Oct 2026 19:02:39 +0800 Date: Tue, 13 Oct 2026 19:02:37 +0800 From: Chenghao Duan To: Luca Boccassi Cc: jianghaoran@kylinos.cn, avagin@gmail.com, linux-mm@kvack.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, systemd-devel@lists.freedesktop.org Subject: Re: Question about obtaining LUO session FD for non-systemd-managed service Message-ID: <20261013110237.GA836401@chenghao-pc> References: <6ac99eb9.6b935c14.1905ce.7c50SMTPIN_ADDED_MISSING@mx.google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Sat, Oct 10, 2026 at 03:05:40PM +0200, Luca Boccassi wrote: > On Sat, 10 Oct 2026 at 04:11, Chenghao Duan wrote: > > > > Hi Luca, > > > > I watched your excellent presentation from the LPC conference in Prague. > > > > I have a question I’d like to consult with you. > > > > I am currently working on the `memfd_luo` feature in CRIU. My goal is to > > leverage the LUO capability to store checkpoint images inside memfds. > > After a kexec reboot, the image data will be retrieved directly from > > memory instead of from disk. > > > > Here is my pull request on the CRIU GitHub repository: > > https://github.com/checkpoint-restore/criu/pull/3152 > > > > In the PR discussion, avagin suggested using systemd as the CRIU daemon > > to centrally manage LUO sessions and access to `/dev/liveupdate`. I have > > watched your talk and read parts of the systemd source code. > > > > From my understanding, systemd currently provides a unit‑based solution > > where child processes can inherit LUO session file descriptors. However, > > this approach requires the target service to run as a child of a systemd > > unit. As far as I know, CRIU does not run as a systemd‑managed service. > > > > So my question is: for services such as CRIU that are **not systemd‑managed > > child services**, how can they obtain LUO‑session‑related file descriptors > > managed by systemd? > > > > Thank you very much for reading this email. My knowledge of systemd is still > > limited, so please feel free to correct me if I have misunderstood anything. > > > > Thanks & best regards, > > Chenghao Duan > > Hi, > > At some point up in the stack, everything runs as a systemd unit, so > it's only a question of figuring out in your setup how to pass the > FD(s) up and down the stack. Hi Luca, Thanks for your reply. The `LUOSession=` configuration only takes effect for services managed by the system manager. If I run the CRIU command from an SSH login shell as I am doing now, the environment belongs to a transient unit under the user manager. In this case, we cannot use `LUOSession=` to obtain the LUO session fd via systemd. Does this mean the process needs to open `/dev/liveupdate` by itself to create the session fd? Below is the attached service of the process found via `systemctl status`. [root@localhost criu]# ps -ef |grep criu root 57589 7729 0 18:25 pts/2 00:00:00 ./criu/criu dump -D ../test -t 52922 -j --memfd-images -v4 root 57784 7571 0 18:27 pts/0 00:00:00 grep --color=auto criu [root@localhost criu]# systemctl status 57589 ● session-5.scope - Session 5 of User root Loaded: loaded (/run/systemd/transient/session-5.scope; transient) Transient: yes Active: active (running) since Tue 2026-10-13 10:03:55 CST; 8h ago Tasks: 4 Memory: 148.9M (peak: 1.3G) CPU: 47.000s CGroup: /user.slice/user-0.slice/session-5.scope ├─ 7712 "sshd: root [priv]" ├─ 7724 "sshd: root@pts/2" ├─ 7729 -bash └─57589 ./criu/criu dump -D ../test -t 52922 -j --memfd-images -v4 I'm not sure whether my understanding is correct. Could you please provide your feedback? BRs Chenghao