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

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Message-ID: <20171013153513.GA3755@hercules.tuxera.com>
Date:   Fri, 13 Oct 2017 18:35:13 +0300
From:   Rakesh Pandit <rakesh@...era.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     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 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.