[dm-devel] [PATCH 3/9] block: store a block_device pointer in struct bio

[Christoph Hellwig]

On Thu, Dec 03, 2020 at 03:10:55PM +0800, Ming Lei wrote:
> On Thu, Dec 03, 2020 at 02:40:04PM +0800, Ming Lei wrote:
> > On Tue, Dec 01, 2020 at 05:54:18PM +0100, Christoph Hellwig wrote:
> > > Replace the gendisk pointer in struct bio with a pointer to the newly
> > > improved struct block device.  From that the gendisk can be trivially
> > > accessed with an extra indirection, but it also allows to directly
> > > look up all information related to partition remapping.
> > 
> > The extra indirection is often done in fast path, so just wondering why
> > you don't consider to embed gendisk into block_device? Then the extra
> > indirection can be avoided.
> 
> oops, that is only possible for disk, and indirection is still needed
> for partitions.

I looked into that, but given that the block device is allocated as part
of the inode we'd need to tell ->alloc_inode if we want to allocate the
small inode without the gendisk, or the large one with it which doesn't
work with the current interface.  Beause the hd_struct is gone we're
still not using more structures in the I/O path than we did before.