linux-kernel - Re: [GIT PULL 02/58] lightnvm: prevent bd removal if busy

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Message-Id: <A09D1991-FDAF-4098-A654-8B6A0750C98E@gmail.com>
Date:   Fri, 13 Oct 2017 17:58:59 +0200
From:   Javier González <javigon.napster@...il.com>
To:     Rakesh Pandit <rakesh@...era.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Matias Bjørling <m@...rling.me>, axboe@...com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL 02/58] lightnvm: prevent bd removal if busy


> On 13 Oct 2017, at 17.35, Rakesh Pandit <rakesh@...era.com> wrote:
> 
>> On Fri, Oct 13, 2017 at 07:58:09AM -0700, Christoph Hellwig wrote:
>>> On Fri, Oct 13, 2017 at 02:45:51PM +0200, Matias Bjørling wrote:
>>> From: Rakesh Pandit <rakesh@...era.com>
>>> 
>>> When a virtual block device is formatted and mounted after creating
>>> with "nvme lnvm create... -t pblk", a removal from "nvm lnvm remove"
>>> would result in this:
>>> 
>>> 446416.309757] bdi-block not registered
>>> [446416.309773] ------------[ cut here ]------------
>>> [446416.309780] WARNING: CPU: 3 PID: 4319 at fs/fs-writeback.c:2159
>>>  __mark_inode_dirty+0x268/0x340
>>> 
>>> Ideally removal should return -EBUSY as block device is mounted after
>>> formatting.  This patch tries to address this checking if whole device
>>> or any partition of it already mounted or not before removal.
>> 
>> How is this different from any other block device that can be
>> removed even if a file system is mounted?
> 
> One can create many virtual block devices on top of physical using:
> nvme lnvm create ... -t pblk
> 
> And remove them using:
> nvme lnvm remove
> 
> Because the block devices are virtual in nature created by a program I was
> expecting removal to tell me they are busy instead of bdi-block not registered
> following by a WARNING (above).  My use case was writing automatic test case
> but I assumed this is useful in general.
> 
>> 
>>> 
>>> Whole device is checked using "bd_super" member of block device.  This
>>> member is always set once block device has been mounted using a
>>> filesystem.  Another member "bd_part_count" takes care of checking any
>>> if any partitions are under use.  "bd_part_count" is only updated
>>> under locks when partitions are opened or closed (first open and last
>>> release).  This at least does take care sending -EBUSY if removal is
>>> being attempted while whole block device or any partition is mounted.
>>> 
>> 
>> That's a massive layering violation, and a driver has no business
>> looking at these fields.
> 
> Okay, I didn't consider this earlier.  I would suggest a revert for this.

The use case is still valid, since a block device typically does not disappear under a file system - at least not because of a script suddenly removing it by mistake. 

Any suggestion on how we can do this better?

Javier.