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[dm-devel] Re: bdev size not updated correctly after underlying device is resized



On Tue, 2008-04-15 at 16:03 -0700, Andrew Morton wrote:
> On Wed, 09 Apr 2008 17:29:42 -0600
> .
> > Subject: [PATCH] Reset bdev size regardless of other openers.
> > 
> > A block device may be resized while online.  If the revalidate_disk
> > routine is called after the resize, the gendisk->capacity value is
> > updated for the device.  However, the bdev->bd_inode->i_size is not
> > updated when the block device is opened if there are any other openers
> > on the device.  This means that apps like LVM are unlikely to see the
> > size change as they tend to keep their block devices open.  There is a
> > discussion of this problem at:
> > 
> >   http://lkml.org/lkml/2007/7/3/83
> > 
> > This patch changes block_dev.c:do_open() to call bd_set_size()
> > regardless if there are other openers on the device.  It should not be
> > applied in its existing state as changing i_size should be protected by
> > a lock. Also, there needs to be some analysis on the effects of changing
> > the device size underneath an app.
> 
> hm, tricky.
> 
> I don't know what problems a change like this might cause - probably few,
> given the rarity and slowness of block device resizing.

I have been looking through code where this might be a problem.  The
sort of things I was worried about is where something might try and do a
calculation based on the i_size and write/read data from there after it
has been resized, possibly corrupting data.  The COW code in dm seems to
come the closest, but then if you are resizing the device that has
snapshots on it, you might be getting what you deserve.

> 
> Presumably increasing the device size will cause les problems than
> decreasing it would. 

Agreed.

>  Do we even support device shrinking?

Yes, this common with LVM at least.  Whether it is a good idea to do
this with a mounted file-system on it is another matter.

> 
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 7d822fa..d13a4e5 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -992,6 +992,9 @@ static int do_open(struct block_device *bdev, struct file *file, int for_part)
> >  				ret = bdev->bd_disk->fops->open(bdev->bd_inode, file);
> >  				if (ret)
> >  					goto out;
> > +				/* device may have been resized with revalidate_disk */
> > +				if (!part)
> > +					bd_set_size(bdev, (loff_t)get_capacity(disk)<<9);
> >  			}
> >  			if (bdev->bd_invalidated)
> >  				rescan_partitions(bdev->bd_disk, bdev);
> 
> I'd have thought that an appropriate way to fix all this would be to
> perform the i_size update between freeze_bdev() and thaw_bdev(), when the
> fs is quiesced.  But it's not really in my comfort zone.

Except that this is not only done with file-systems.  In my case I am
just trying to extend an LVM logical volume after a resize but cannot
because it is open (activated). In practice, however, it is probably
only useful to do this with an online file-system.  Otherwise you could
just close all openers and the resize will work fine.

> 

-- 
Andrew Patterson


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