linux-kernel - Re: [PATCH 1/2] Add partition resize function to BLKPG ioctl

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Message-ID: <20111208155844.GA9281@redhat.com>
Date:	Thu, 8 Dec 2011 10:58:44 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Karel Zak <kzak@...hat.com>, linux-kernel@...r.kernel.org,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH 1/2] Add partition resize function to BLKPG ioctl

On Thu, Dec 08, 2011 at 10:25:26AM -0500, Phillip Susi wrote:
> On 12/8/2011 10:16 AM, Karel Zak wrote:
> >  Is it safe to alter the partition size in arbitrary way if the
> >  partition is used by any process?
> >
> >  Vivek's BLKPG_EXTEND_PARTITIONV seems more safety, because it extends
> >  a partition size only, so all offsets in all running stuff are still
> >  valid.
> >
> >  Maybe you need to check bdevp->bd_openers and returns -EBUSY if you
> >  want to alter the begin of the partition.
> 
> I disallowed altering the start of the partition ( that would just
> be crazy ), but altering the end has been supported on lvm for years
> now. Ext4 can not perform an online shrink, but btrfs can, and I was
> able to successfully have btrfs shrink the fs and then use
> BLKPG_RES_PARTITION to shrink the partition.

So, if there is an IO in flight while partition shrinking is happening,
then IO can end up happening outside the partition?

part->nr_sects can be 64 bits on 32bit machines and update will be
non-atomic. I had used sequence counter to make sure read is able
to get to intermediate value. May be it is a good idea to address
this concenrn.

Thanks
Vivek
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