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Mon, 21 Sep 2026 09:27:14 +0000 (GMT) Received: from [9.61.138.180] (unknown [9.61.138.180]) by smtpav01.wdc07v.mail.ibm.com (Postfix) with ESMTP; Mon, 21 Sep 2026 09:27:13 +0000 (GMT) Message-ID: <37de70f6-67f8-466f-a267-03584973dd3a@linux.ibm.com> Date: Mon, 21 Sep 2026 14:57:12 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/3] blk-cgroup: store blkcg in bio instead of blkg To: Yu Kuai , Jens Axboe , Josef Bacik , Tejun Heo , Johannes Weiner , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Jonathan Corbet , Shuah Khan , Randy Dunlap , Coly Li , Kent Overstreet , Alasdair Kergon , Mike Snitzer , Mikulas Patocka , Benjamin Marzinski , Song Liu , Li Nan , Xiao Ni , Andreas Gruenbacher , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Youngjun Park , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt Cc: Yu Kuai , Christoph Hellwig , Tao Cui , linux-block@vger.kernel.org, cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-bcache@vger.kernel.org, dm-devel@lists.linux.dev, linux-raid@vger.kernel.org, gfs2@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, llvm@lists.linux.dev References: <20260920092351.2434283-1-yukuai@kernel.org> <20260920092351.2434283-3-yukuai@kernel.org> Content-Language: en-US From: Nilay Shroff In-Reply-To: <20260920092351.2434283-3-yukuai@kernel.org> Content-Type: text/plain; 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This forces bio > association and remap paths to look up or create a blkg even when no > blkcg policy will use the bio, and ties the stored state to the current > block device. > > Store and reference the queue-independent blkcg in the bio instead. Add > helpers that lazily look up or create the queue-local blkg when a policy > needs it, and pin the result until the bio changes devices or releases > its cgroup state. Look up and pin existing blkgs under RCU, taking the > queue lock only when a blkg may need to be created. > > Keep completion and accounting users lookup-only when they do not create > a blkg themselves. In particular, blk_cgroup_bio_start() only accounts a > bio when an online blkg already exists. This avoids creating a blkg > from the unconditional accounting hook when no policy is enabled. > > Protect every rhashtable lookup with an RCU read-side critical section. > Annotate the lookup helpers and recursive wrappers with > __must_hold_shared(RCU) so Clang can verify that contract. > > Before entering a bio-based driver's ->submit_bio(), drop the pinned blkg > after blk_cgroup_bio_start() has completed bio accounting. blk-throttle is > the only policy supported by bio-based queues, so no queue-local blkg > state needs to enter stacking drivers. The queue-independent blkcg > association remains attached to the bio. > > If blkg creation fails while walking down the hierarchy, use the closest > available ancestor and update the bio's blkcg association before > recording the blkg reference. This keeps later CSS ID hash lookups > matched with the pinned blkg. > > Signed-off-by: Yu Kuai Looks good to me. Reviewed-by: Nilay Shroff