Re: [PATCH] lightnvm: prevent bd removal if busy

From	Rakesh Pandit <rakesh@tuxera.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] lightnvm: prevent bd removal if busy
Date	2017-09-10 21:20 +0200
Message-ID	<uofVU-8ls-5@gated-at.bofh.it> (permalink)
References	<un5Ff-To-1@gated-at.bofh.it> <unp1f-5MW-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Fri, Sep 08, 2017 at 12:42:47PM +0200, Matias Bjørling wrote:
> On 09/07/2017 03:58 PM, Rakesh Pandit wrote:
> > Removal of virtual block device by "nvm lnvm remove..." undergoing IO
> > and created by "nvme lnvm create... -t pblk" results in following and
> > is annoying.
> > 
> > 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
> > .....
> > 
> > This patch solves this by checking bd_openers for each partition
> > before removal can continue.  Note that this isn't full proof as
> > device can become busy as soon as it's bd_mutex is unlocked but it
> > needn't be full proof either.  It does work for general case where
> > device is mounted and removal can be prevented.
> > 
> > Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
[..]
> > +	while ((part = disk_part_iter_next(&piter))) {
> 
> A race condition can occur where disk_part_next tries to pblk (in
> block/genhd.c), and it in the meantime has been set to NULL. Leading to a
> kernel crash. Is there a better way to do it?
> 
> [root@localhost ~]# nvme lnvm remove -n pblk0
> [ 5262.338647] BUG: unable to handle kernel NULL pointer dereference at
> 0000000000000010
> [ 5262.340769] IP: disk_part_iter_next+0xd3/0xf0

Thanks, indeed partition can go away from our feet if we don't lock
the whole thing from changing and not just individual partition locks.

I have given it another go which should avoid taking mutex locks on
bdev.  Posted V2.

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[PATCH] lightnvm: prevent bd removal if busy Rakesh Pandit <rakesh@tuxera.com> - 2017-09-07 16:10 +0200
  Re: [PATCH] lightnvm: prevent bd removal if busy Matias Bjørling <mb@lightnvm.io> - 2017-09-08 12:50 +0200
    Re: [PATCH] lightnvm: prevent bd removal if busy Rakesh Pandit <rakesh@tuxera.com> - 2017-09-10 21:20 +0200

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