> +/* Copy source offset from source block device to destination block
> + * device. Returns the length of bytes copied.
> + */
> +ssize_t blkdev_copy_offload_failfast(
> +             struct block_device *bdev_in, loff_t pos_in,
> +             struct block_device *bdev_out, loff_t pos_out,
> +             size_t len, gfp_t gfp_mask)

This is an odd and very misnamed interface.

Either we have a klkdev_copy() interface that automatically falls back
to a fallback (maybe with an opt-out), or we have separate
blkdev_copy_offload/blkdev_copy_emulated interface and let the caller
decide.  But none of that really is "failfast".

Also this needs to go into the helpers patch and not a patch that is
supposed to just wire copying up for block device node.

> index b07de77ef126..d27148a2543f 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1447,7 +1447,8 @@ static int generic_copy_file_checks(struct file 
> *file_in, loff_t pos_in,
>               return -EOVERFLOW;
>  
>       /* Shorten the copy to EOF */
> -     size_in = i_size_read(inode_in);
> +     size_in = i_size_read(file_in->f_mapping->host);

generic_copy_file_checks needs to be fixed to use ->mapping->host both
or inode_in and inode_out at the top of the file instead of this
band aid.  And that needs to be a separate patch with a Fixes tag.

> @@ -1708,7 +1709,9 @@ int generic_file_rw_checks(struct file *file_in, struct 
> file *file_out)
>       /* Don't copy dirs, pipes, sockets... */
>       if (S_ISDIR(inode_in->i_mode) || S_ISDIR(inode_out->i_mode))
>               return -EISDIR;
> -     if (!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode))
> +
> +     if ((!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode)) &&
> +             (!S_ISBLK(inode_in->i_mode) || !S_ISBLK(inode_out->i_mode)))

This is using weird indentation, and might also not be doing
exactly what we want.  I think the better thing to do here is to:

 1) check for the accetable types only on the in inode
 2) have a check that the mode matches for the in and out inodes

And please do this as a separate prep patch instead of hiding it here.

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