Re: [RFC v4 linux-next 19/19] fs & block: remove bdev->bd_inode

From: Al Viro <viro@zeniv.linux.org.uk>
To: Yu Kuai <yukuai1@huaweicloud.com>
Cc: Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>,
	brauner@kernel.org, axboe@kernel.dk,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	yi.zhang@huawei.com, yangerkun@huawei.com,
	"yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [RFC v4 linux-next 19/19] fs & block: remove bdev->bd_inode
Date: Fri, 22 Mar 2024 15:43:47 +0000	[thread overview]
Message-ID: <20240322154347.GO538574@ZenIV> (raw)
In-Reply-To: <170c544c-164e-368c-474a-74ae4055d55f@huaweicloud.com>

On Fri, Mar 22, 2024 at 02:52:16PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2024/03/22 14:39, Al Viro 写道:
> > On Fri, Mar 22, 2024 at 06:37:18AM +0000, Al Viro wrote:
> > > On Thu, Mar 21, 2024 at 08:15:06PM +0800, Yu Kuai wrote:
> > > 
> > > > > blkdev_iomap_begin() etc. may be an arbitrary filesystem block device
> > > > > inode. But why can't you use I_BDEV(inode->i_mapping->host) to get to the
> > > > > block device instead of your file_bdev(inode->i_private)? I don't see any
> > > > > advantage in stashing away that special bdev_file into inode->i_private but
> > > > > perhaps I'm missing something...
> > > > > 
> > > > 
> > > > Because we're goning to remove the 'block_device' from iomap and
> > > > buffer_head, and replace it with a 'bdev_file'.
> > > 
> > > What of that?  file_inode(file)->f_mapping->host will give you bdevfs inode
> > > just fine...
> > 
> > file->f_mapping->host, obviously - sorry.
> > .
> 
> Yes, we already get bdev_inode this way, and use it in
> blkdev_iomap_begin() and blkdev_get_block(), the problem is that if we
> want to let iomap and buffer_head to use bdev_file for raw block fops as
> well, we need a 'bdev_file' somehow.

Explain, please.  Why would anything care whether the file is bdevfs
one or coming from devtmpfs/xfs/ext2/whatnot?