From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B86BA193 for ; Mon, 9 Sep 2024 01:56:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725847012; cv=none; b=ujKlXw/YnJwh9J4EOWcpkas+snNJuB6jZYmNiFlPaNHFaloPV/1lpioEHFFGcLoHmqDZlMn+0MUR6ntVLUnHErNQENJstRRk7FdqvH2J+dJxMcFCXv3VGvUuDPV3HHf3EhSpacPY18l44Oa+uwWJdx2RRLBp1Pcql7ZAlAnt3hE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725847012; c=relaxed/simple; bh=D2jk+6RJ4ARoiWSwO5zNzdViMCvGO6AEnBmhXrgAPmY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DzCIGzrK3SU1X327q158jS0klW7M1SfIqqyQXxS8B9MkoespCee7/cvAiAqX2Xaug9hKuRqlDYjaTut4CupteT4qX6C9pC/tWVlNZMVTCxaNCOacZDKWNtLjUnLZPW2GeEZFsa3kFvapHebqZfbtPXAaMPOb2Dtu7fefkndxSLM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PzPWr9kz; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PzPWr9kz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1EDF2C4CEC3; Mon, 9 Sep 2024 01:56:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1725847012; bh=D2jk+6RJ4ARoiWSwO5zNzdViMCvGO6AEnBmhXrgAPmY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=PzPWr9kzZfHiQ+rSqbBPpWJpWCfh/H1kmkRE3NiqJedR0PVpYGFvwbwNU3Ld8enL2 9HAodFauQRol51K3r8AdD0reDvdSb718qw1FmWAl2HfhdBySArRTNINwSRN5+SkEMI L/Dvnh6ukLD0r3khByEuyV7E4uGwzQkKlWJUarqUMbTe1gqQ6uY22ztCPaYZOSFGc6 OLafw46xJ5Sb3uf/Nb1DZkeynsXgX49sBJ3WSKsnfCdIccjJSoiDn1DVh3KiQcsc6M f9phs08kxRKMNsSPUNVa4jWYglT1E3hRKlszQWtrJBLbllxy67p1A0yo1QkgIzPSS0 QKYke5jeqk3ew== Message-ID: <7ab499df-f888-4d17-8a29-3f64f17ff71c@kernel.org> Date: Mon, 9 Sep 2024 10:56:49 +0900 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] block: elevator: avoid to load iosched module from this disk To: Ming Lei , Jens Axboe Cc: "Richard W.M. Jones" , linux-block@vger.kernel.org, Jeff Moyer , Jiri Jaburek , Christoph Hellwig , Bart Van Assche , Hannes Reinecke , Chaitanya Kulkarni References: <20240907014331.176152-1-ming.lei@redhat.com> <20240907073522.GW1450@redhat.com> <4d7280eb-7f26-4652-a1d4-4f82c4d99a4c@kernel.org> From: Damien Le Moal Content-Language: en-US Organization: Western Digital Research In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/9/24 10:24, Ming Lei wrote: > On Sat, Sep 07, 2024 at 07:50:32AM -0600, Jens Axboe wrote: >> On 9/7/24 3:04 AM, Damien Le Moal wrote: >>> On 9/7/24 16:58, Ming Lei wrote: >>>> On Sat, Sep 07, 2024 at 08:35:22AM +0100, Richard W.M. Jones wrote: >>>>> On Sat, Sep 07, 2024 at 09:43:31AM +0800, Ming Lei wrote: >>>>>> When switching io scheduler via sysfs, 'request_module' may be called >>>>>> if the specified scheduler doesn't exist. >>>>>> >>>>>> This was has deadlock risk because the module may be stored on FS behind >>>>>> our disk since request queue is frozen before switching its elevator. >>>>>> >>>>>> Fix it by returning -EDEADLK in case that the disk is claimed, which >>>>>> can be thought as one signal that the disk is mounted. >>>>>> >>>>>> Some distributions(Fedora) simulates the original kernel command line of >>>>>> 'elevator=foo' via 'echo foo > /sys/block/$DISK/queue/scheduler', and boot >>>>>> hang is triggered. >>>>>> >>>>>> Cc: Richard Jones >>>>>> Cc: Jeff Moyer >>>>>> Cc: Jiri Jaburek >>>>>> Signed-off-by: Ming Lei >>>>> >>>>> I'd suggest also: >>>>> >>>>> Bug: https://bugzilla.kernel.org/show_bug.cgi?id=219166 >>>>> Reported-by: Richard W.M. Jones >>>>> Reported-by: Jiri Jaburek >>>>> Tested-by: Richard W.M. Jones >>>>> >>>>> So I have tested this patch and it does fix the issue, at the possible >>>>> cost that now setting the scheduler can fail: >>>>> >>>>> + for f in /sys/block/{h,s,ub,v}d*/queue/scheduler >>>>> + echo noop >>>>> /init: line 109: echo: write error: Resource deadlock avoided >>>>> >>>>> (I know I'm setting it to an impossible value here, but this could >>>>> also happen when setting it to a valid one.) >>>> >>>> Actually in most of dist, io-schedulers are built-in, so request_module >>>> is just a nop, but meta IO must be started. >>>> >>>>> >>>>> Since almost no one checks the result of 'echo foo > /sys/...' that >>>>> would probably mean that sometimes a desired setting is silently not >>>>> set. >>>> >>>> As I mentioned, io-schedulers are built-in for most of dist, so >>>> request_module isn't called in case of one valid io-sched. >>>> >>>>> >>>>> Also I bisected this bug yesterday and found it was caused by (or, >>>>> more likely, exposed by): >>>>> >>>>> commit af2814149883e2c1851866ea2afcd8eadc040f79 >>>>> Author: Christoph Hellwig >>>>> Date: Mon Jun 17 08:04:38 2024 +0200 >>>>> >>>>> block: freeze the queue in queue_attr_store >>>>> >>>>> queue_attr_store updates attributes used to control generating I/O, and >>>>> can cause malformed bios if changed with I/O in flight. Freeze the queue >>>>> in common code instead of adding it to almost every attribute. >>>>> >>>>> Reverting this commit on top of git head also fixes the problem. >>>>> >>>>> Why did this commit expose the problem? >>>> >>>> That is really the 1st bad commit which moves queue freezing before >>>> calling request_module(), originally we won't freeze queue until >>>> we have to do it. >>>> >>>> Another candidate fix is to revert it, or at least not do it >>>> for storing elevator attribute. >>> >>> I do not think that reverting is acceptable. Rather, a proper fix would simply >>> be to do the request_module() before freezing the queue. >>> Something like below should work (totally untested and that may be overkill). >> >> I like this approach, but let's please call it something descriptive >> like "load_module" or something like that. > > But 'load_module' is too specific as interface, and we just only have > one case which need to load module exactly. > > I guess there may be same risk in queue_wb_lat_store() which calls into > GFP_KERNEL allocation which implies direct reclaim & IO. That needs to be changed to GFP_NOIO. > > Thanks, > Ming > -- Damien Le Moal Western Digital Research