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

[Ming Lei <ming.lei@redhat.com>]

On Thu, Dec 03, 2020 at 09:29:51AM +0100, Christoph Hellwig wrote:
> 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.

I guess it could be done without fs code change, because now block device is
always allocated by bdev_alloc() since 22ae8ce8b892("block: simplify bdev/disk
lookup in blkdev_get"). And one manual inode allocation with a bit duplication
from new_inode_pseudo() should be fine:

	allocate big inode for disk, and small for partition
	inode_init_always(sb, inode);
    if (inode) {
            spin_lock(&inode->i_lock);
            inode->i_state = 0;
            spin_unlock(&inode->i_lock);
            INIT_LIST_HEAD(&inode->i_sb_list);
			inode_sb_list_add(inode);
    }

> Beause the hd_struct is gone we're
> still not using more structures in the I/O path than we did before.

Indeed, and block_device instance is often cached in IO path.


thanks,
Ming

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