From patchwork Wed Aug 31 15:51:30 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ziyang Zhang X-Patchwork-Id: 12960986 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ED115ECAAD1 for ; Wed, 31 Aug 2022 15:52:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231958AbiHaPwq (ORCPT ); Wed, 31 Aug 2022 11:52:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36180 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231959AbiHaPwp (ORCPT ); Wed, 31 Aug 2022 11:52:45 -0400 Received: from out30-44.freemail.mail.aliyun.com (out30-44.freemail.mail.aliyun.com [115.124.30.44]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EE9C5AB405; Wed, 31 Aug 2022 08:52:33 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R521e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046051;MF=ziyangzhang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VNrdnpg_1661961144; Received: from localhost.localdomain(mailfrom:ZiyangZhang@linux.alibaba.com fp:SMTPD_---0VNrdnpg_1661961144) by smtp.aliyun-inc.com; Wed, 31 Aug 2022 23:52:29 +0800 From: ZiyangZhang To: ming.lei@redhat.com, axboe@kernel.dk Cc: xiaoguang.wang@linux.alibaba.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com, ZiyangZhang Subject: [RFC PATCH V2 0/6] ublk_drv: add USER_RECOVERY support Date: Wed, 31 Aug 2022 23:51:30 +0800 Message-Id: <20220831155136.23434-1-ZiyangZhang@linux.alibaba.com> X-Mailer: git-send-email 2.27.0 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org ublk_drv is a driver simply passes all blk-mq rqs to ublksrv[1] in userspace. For each ublk queue, there is one ubq_daemon(pthread). All ubq_daemons share the same process which opens /dev/ublkcX. The ubq_daemon code infinitely loops on io_uring_enter() to send/receive io_uring cmds which pass information of blk-mq rqs. Since the real IO handler(the process opening /dev/ublkcX) is in userspace, it could crash if: (1) the user kills -9 it because of IO hang on backend, system reboot, etc... (2) the process catches a exception(segfault, divisor error, oom...) Therefore, the kernel driver has to deal with a dying process. Now, if one ubq_daemon(pthread) or the process crashes, ublk_drv must abort the dying ubq, stop the device and release everything. This is not a good choice in practice because users do not expect aborted requests, I/O errors and a released device. They may want a recovery machenism so that no requests are aborted and no I/O error occurs. Anyway, users just want everything works as uaual. This RFC patchset implements USER_RECOVERY support. If the process crashes, we allow ublksrv to provide new process and ubq_daemons. We do not support single ubq_daemon(pthread) recovery because a pthread rarely crashes. Note: The responsibility of recovery belongs to the user who opens /dev/ublkcX. After a process crash, the kernel driver only switch the device's status to be ready for recovery or termination(STOP_DEV). This patchset does not provide how to detect such a process crash in userspace. A very straightfoward idea may be adding a watchdog. Recovery feature is quite useful for real products. In detail, we support this scenario: (1) The /dev/ublkc0 is opened by process 0; (2) Fio is running on /dev/ublkb0 exposed by ublk_drv and all rqs are handled by process 0. (3) Process 0 suddenly crashes(e.g. segfault); (4) Fio is still running and submit IOs(but these IOs cannot complete now) (5) User recovers with process 1 and attach it to /dev/ublkc0 (6) All rqs are handled by process 1 now and IOs can be completed now. Note: The backend must tolerate double-write because we re-issue a rq sent to the old(dying) process before. We allow users to choose whether re-issue these rqs or not, please see patch 7 for more detail. We provide a sample script here to simulate the above steps: ***************************script*************************** LOOPS=10 __ublk_get_pid() { pid=`./ublk list -n 0 | grep "pid" | awk '{print $7}'` echo $pid } ublk_recover_kill() { for CNT in `seq $LOOPS`; do dmesg -C pid=`__ublk_get_pid` echo -e "*** kill $pid now ***" kill -9 $pid sleep 6 echo -e "*** recover now ***" ./ublk recover -n 0 sleep 6 done } ublk_test() { dmesg -C echo -e "*** add ublk device ***" ./ublk add -t null -d 4 -i 1 sleep 2 echo -e "*** start fio ***" fio --bs=4k \ --filename=/dev/ublkb0 \ --runtime=140s \ --rw=read & sleep 4 ublk_recover_kill wait echo -e "*** delete ublk device ***" ./ublk del -n 0 } for CNT in `seq 4`; do modprobe -rv ublk_drv modprobe ublk_drv echo -e "************ round $CNT ************" ublk_test sleep 5 done ***************************script*************************** You may run it with our modified ublksrv[2] which supports recovey feature. No I/O error occurs and you can verify it by typing $ perf-tools/bin/tpoint block:block_rq_error The basic idea of USER_RECOVERY is quite straightfoward: (1) release/free everything belongs to the dying process. Note: Since ublk_drv does save information about user process, this work is important because we don't expect any resource lekage. Particularly, ioucmds from the dying ubq_daemons need to be completed(freed). (2) init ublk queues including requeuing/aborting rqs. (3) allow new ubq_daemons issue FETCH_REQ. Here is steps to reocver: (1) The monitor_work detects a crash, and it should requeue/abort inflight rqs, complete old ioucmds and quiesce request queue to ban any incoming ublk_queue_rq(). Then the ublk device is ready for a recovery/stop procedure. (2) For a user, after a process crash, he sends START_USER_RECOVERY ctrl-cmd to /dev/ublk-control with a dev_id X (such as 3 for /dev/ublkc3). (2) Then ublk_drv should perpare for a new process to attach /dev/ublkcX. All ublk_io structures are cleared and ubq_daemons are reset. (3) Then, user should start a new process and ubq_daemons(pthreads) and send FETCH_REQ by io_uring_enter() to make all ubqs be ready. The user must correctly setup queues, flags and so on(how to persist user's information is not related to this patchset). (4) The user sends END_USER_RECOVERY ctrl-cmd to /dev/ublk-control with a dev_id X. (5) ublk_drv waits for all ubq_daemons getting ready. Then it unquiesces request queue and new rqs are allowed. You should use ublksrv[2] and tests[3] provided by us. We add 2 additional tests to verify that recovery feature works. Our code will be PR-ed to Ming's repo soon. [1] https://github.com/ming1/ubdsrv [2] https://github.com/old-memories/ubdsrv/tree/recovery-v1 [3] https://github.com/old-memories/ubdsrv/tree/recovery-v1/tests/generic Since V1: (1) refactor cover letter. Add intruduction on "how to detect a crash" and "why we need recovery feature". (2) do not refactor task_work and ublk_queue_rq(). (3) allow users freely stop/recover the device. (4) add comment on ublk_cancel_queue(). (5) refactor monitor_work and aborting machenism since we add recovery machenism in monitor_work. ZiyangZhang (6): ublk_drv: check 'current' instead of 'ubq_daemon' ublk_drv: refactor ublk_cancel_queue() ublk_drv: define macros for recovery feature and check them ublk_drv: requeue rqs with recovery feature enabled ublk_drv: consider recovery feature in aborting mechanism ublk_drv: add START_USER_RECOVERY and END_USER_RECOVERY support drivers/block/ublk_drv.c | 439 +++++++++++++++++++++++++++++++--- include/uapi/linux/ublk_cmd.h | 7 + 2 files changed, 419 insertions(+), 27 deletions(-)