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

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Date:   Fri, 13 Oct 2017 07:58:09 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Matias Bjørling <m@...rling.me>
Cc:     axboe@...com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rakesh Pandit <rakesh@...era.com>
Subject: Re: [GIT PULL 02/58] lightnvm: prevent bd removal if busy

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?

> 
> 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.