Re: [PATCH -next] block: remove field 'bd_inode' from block_device

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Nov 25, 2023 at 05:39:12PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> block_devcie is allocated from bdev_alloc() by bdev_alloc_inode(), and
> currently block_device contains a pointer that point to the address of
> inode, while such inode is allocated together:
> 
> bdev_alloc
>  inode = new_inode()
>   // inode is &bdev_inode->vfs_inode
>  bdev = I_BDEV(inode)
>   // bdev is &bdev_inode->bdev
>  bdev->inode = inode
> 
> Add a new helper to get address of inode from bdev by add operation
> instead of memory access, which is more efficiency. Also prepare to
> add a new field 'bd_flags' in the first cacheline(64 bytes).
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/bdev.c                       | 39 +++++++++++++++++-------------
>  block/blk-zoned.c                  |  4 +--
>  block/fops.c                       |  4 +--
>  block/genhd.c                      |  8 +++---
>  block/ioctl.c                      |  8 +++---
>  block/partitions/core.c            |  9 ++++---
>  drivers/block/xen-blkback/xenbus.c |  2 +-
>  drivers/md/bcache/super.c          |  2 +-
>  drivers/mtd/devices/block2mtd.c    | 12 ++++-----
>  drivers/s390/block/dasd_ioctl.c    |  2 +-
>  drivers/scsi/scsicam.c             |  2 +-
>  fs/bcachefs/util.h                 |  2 +-
>  fs/btrfs/disk-io.c                 |  6 ++---
>  fs/btrfs/volumes.c                 |  4 +--
>  fs/btrfs/zoned.c                   |  2 +-
>  fs/buffer.c                        |  8 +++---
>  fs/cramfs/inode.c                  |  2 +-
>  fs/erofs/data.c                    |  2 +-
>  fs/ext4/dir.c                      |  2 +-
>  fs/ext4/ext4_jbd2.c                |  2 +-
>  fs/ext4/super.c                    |  8 +++---
>  fs/gfs2/glock.c                    |  2 +-
>  fs/gfs2/ops_fstype.c               |  2 +-
>  fs/jbd2/journal.c                  |  3 ++-
>  fs/jbd2/recovery.c                 |  2 +-
>  fs/nilfs2/segment.c                |  2 +-
>  include/linux/blk_types.h          | 10 ++++++--
>  include/linux/blkdev.h             |  4 +--
>  include/linux/buffer_head.h        |  4 +--
>  29 files changed, 86 insertions(+), 73 deletions(-)

You should do this as a patch series, add the helper function that does
nothing, convert all the different portions of the kernel as different
patches, and _then_ change the implementation of the block layer to
handle the change in the structure.

Otherwise this is going to be hard to get accepted.

Also, one note:

> @@ -85,6 +84,13 @@ struct block_device {
>  #define bdev_kobj(_bdev) \
>  	(&((_bdev)->bd_device.kobj))
>  
> +static inline struct inode *bdev_inode(struct block_device *bdev)
> +{
> +	void *inode = bdev + 1;

That's crazy, if something changes, this will keep working yet the
kernel will break and no one will know why.

Please use container_of(), that's what it is there for, this exact type
of thing.  Or if not, are you just assuming that the memory location
right after bdev is the inode?  That's a tough assumption, how are you
going to assure it really stays there?

thanks,

greg k-h