Re: bdev size not updated correctly after underlying device isresized

From: Andrew Morton
Date: Tue Apr 15 2008 - 19:04:43 EST


On Wed, 09 Apr 2008 17:29:42 -0600
Andrew Patterson <andrew.patterson@xxxxxx> wrote:

> I ran into this problem while trying to resize a mounted file-system
> after growing/shrinking the size of the underlying block device (in this
> case, a fibre-channel LUN). The kernel recognizes the device size change
> when revalidate_disk() is called, but the bdev->bd_inode->i_size will
> not be updated for any new openers if there are already openers of the
> device. In my case I was using LVM thusly:
>
> 1. Create a volume group with a physical volume on something that
> can be resized (usually some sort of SCSI RAID device).
> 2. Create a logical volume on that VG. This holds the underlying
> PV block device open as long at the LV is activated
> 3. Run blockdev --getsize <block dev>
> 4. Resize the underlying block device.
> 5. Get the OS to notice the change. For fibre-channel LUN's you
> can use /sys/class/scsi_device/<device>/device/rescan.
> 6. Size is correctly changed in /dev/block/<device>/size
> 7. Run blockdev --getsize again (no change in size reported)
> 8. Inactivate the LV (there are now no longer any openers on the
> block device)
> 9. Run blockdev --getsize again. Size is now correct as there are
> no openers on the device when blockdev is run.
>
> This problem has been reported before at:
>
> http://lkml.org/lkml/2007/7/3/83
>
> The following patch is a suggestion on how to fix this problem. It is
> not a complete solution as it is probably a bad thing to change other
> openers device size without at least protecting the change with a lock.
> And user-apps and other sub-systems might not like the reported device
> size being changed underneath them. It looks like the following
> sub-systems access this value:
>
> ndb
> dm
> md
> affs
> hfs
> jfs
> reiserfs
> udf
>
>
> 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.
>
> Andrew Patterson
>
> 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.

Presumably increasing the device size will cause les problems than
decreasing it would. Do we even support device shrinking?

> 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.


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