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    SubjectRe: [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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