[PATCH 25/45] block: reference struct block_device from struct hd_struct

[Tejun Heo]

Hello,

Please see lkml.kernel.org/r/X708BTJ5njtbC2z1 at mtj.duckdns.org for a few nits
on the previous version.

On Tue, Nov 24, 2020 at 02:27:31PM +0100, Christoph Hellwig wrote:
> To simplify block device lookup and a few other upcoming areas, make sure
> that we always have a struct block_device available for each disk and
> each partition.  The only downside of this is that each device and
> partition uses a little more memory.  The upside will be that a lot of
> code can be simplified.
> 
> With that all we need to look up the block device is to lookup the inode
> and do a few sanity checks on the gendisk, instead of the separate lookup
> for the gendisk.  These checks are in a new RCU critical section and
> the disk is now freed using kfree_rcu().

I might be confused but am wondering whether RCU is needed. It's currently
used to ensure that gendisk is accessible in the blkdev_get path but
wouldn't it be possible to simply pin gendisk from block_devices? The
gendisk and hd_structs hold the base refs of the block_devices and in turn
the block_devices pin the gendisk. When the gendisk gets deleted, it puts
the base refs of the block devices but stays around while the block devices
are being accessed.

Also, would it make sense to separate out lookup_sem removal? I *think* it's
there to ensure that the same bdev doesn't get associated with old and new
gendisks at the same time but can't wrap my head around how it works
exactly. I can see that this may not be needed once the lifetimes of gendisk
and block_devices are tied together but that may warrant a bit more
explanation.

Thanks.

-- 
tejun