On Fri 25-08-23 02:58:43, Al Viro wrote:

On Fri, Aug 11, 2023 at 01:04:31PM +0200, Jan Kara wrote:

quoted

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?

I can see the appeal of not having to introduce the new bdev_handle type
and just using struct file which unifies in-kernel and userspace block
device opens. But I can see downsides too - the last fput() happening from
task work makes me a bit nervous whether it will not break something
somewhere with exclusive bdev opens. Getting from struct file to bdev is
somewhat harder but I guess a helper like F_BDEV() would solve that just
fine.

So besides my last fput() worry about I think this could work and would be
probably a bit nicer than what I have. But before going and redoing the whole
series let me gather some more feedback so that we don't go back and forth.
Christoph, Christian, Jens, any opinion?

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR