Re: [PATCH v2 0/29] block: Make blkdev_get_by_*() return handle

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Aug 11, 2023 at 01:04:31PM +0200, Jan Kara wrote:
> Hello,
> 
> this is a v2 of the patch series which implements the idea of blkdev_get_by_*()
> calls returning bdev_handle which is then passed to blkdev_put() [1]. This
> makes the get and put calls for bdevs more obviously matching and allows us to
> propagate context from get to put without having to modify all the users
> (again!).  In particular I need to propagate used open flags to blkdev_put() to
> be able count writeable opens and add support for blocking writes to mounted
> block devices. I'll send that series separately.
> 
> The series is based on Christian's vfs tree as of yesterday as there is quite
> some overlap. Patches have passed some reasonable testing - I've tested block
> changes, md, dm, bcache, xfs, btrfs, ext4, swap. This obviously doesn't cover
> everything so I'd like to ask respective maintainers to review / test their
> changes. Thanks! I've pushed out the full branch to:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git bdev_handle
> 
> to ease review / testing.

Hmm...  Completely Insane Idea(tm): how about turning that thing inside out and
having your bdev_open_by... return an actual opened struct file?

After all, we do that for sockets and pipes just fine and that's a whole lot
hotter area.

Suppose we leave blkdev_open()/blkdev_release() as-is.  No need to mess with
what we have for normal opened files for block devices.  And have block_open_by_dev()
that would find bdev, etc., same yours does and shove it into anon file.

Paired with plain fput() - no need to bother with new primitives for closing.
With a helper returning I_BDEV(bdev_file_inode(file)) to get from those to bdev.

NOTE: I'm not suggesting replacing ->s_bdev with struct file * if we do that -
we want that value cached, obviously.  Just store both...

Not saying it's a good idea, but... might be interesting to look into.
Comments?