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block: add partition resize function to blkpg ioctl

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Add a new operation code (BLKPG_RESIZE_PARTITION) to the BLKPG ioctl that
allows altering the size of an existing partition, even if it is currently
in use.

This patch converts hd_struct->nr_sects into sequence counter because
One might extend a partition while IO is happening to it and update of
nr_sects can be non-atomic on 32bit machines with 64bit sector_t. This
can lead to issues like reading inconsistent size of a partition. Sequence
counter have been used so that readers don't have to take bdev mutex lock
as we call sector_in_part() very frequently.

Now all the access to hd_struct->nr_sects should happen using sequence
counter read/update helper functions part_nr_sects_read/part_nr_sects_write.
There is one exception though, set_capacity()/get_capacity(). I think
theoritically race should exist there too but this patch does not
modify set_capacity()/get_capacity() due to sheer number of call sites
and I am afraid that change might break something. I have left that as a
TODO item. We can handle it later if need be. This patch does not introduce
any new races as such w.r.t set_capacity()/get_capacity().

v2: Add CONFIG_LBDAF test to UP preempt case as suggested by Phillip.

Signed-off-by: Vivek Goyal <[email protected]>
Signed-off-by: Phillip Susi <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Vivek Goyal 2012-08-01 12:24:18 +02:00 • committed by Jens Axboe
commit c83f6bf98d
5 changed files with 132 additions and 9 deletions
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4
block/partition-generic.c
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@ -84,7 +84,7 @@ ssize_t part_size_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
struct hd_struct *p = dev_to_part(dev);
return sprintf(buf, "%llu\n",(unsigned long long)p->nr_sects);
return sprintf(buf, "%llu\n",(unsigned long long)part_nr_sects_read(p));
}
static ssize_t part_ro_show(struct device *dev,
@ -294,6 +294,8 @@ struct hd_struct *add_partition(struct gendisk *disk, int partno,
err = -ENOMEM;
goto out_free;
}
seqcount_init(&p->nr_sects_seq);
pdev = part_to_dev(p);
p->start_sect = start;