[PATCH 4/5] io_uring: add support for dma pre-mapping

[Al Viro]

On Wed, Jul 27, 2022 at 08:48:29AM -0600, Keith Busch wrote:

> > This, BTW, is completely insane - what happens if you follow that
> > with close(map.fd)?  A bunch of dangling struct file references?
> 
> This should have been tied to files registered with the io_uring instance
> holding a reference, and cleaned up when the files are unregistered. I may be
> missing some cases here, so I'll fix that up.

???

Your code does the following sequence:
	file = fget(some number)
	store the obtained pointer in a lot of places
	fput(file)

What is "may be missing" and what kind of "registration" could possibly
help here?  As soon as fget() had returned the reference, another thread
might have removed it from the descriptor table, leaving you the sole holder
of reference to object.  In that case it will be destroyed by fput(), making
its memory free for reuse.

Looks like you have some very odd idea of what the struct file lifetime rules
are...

> > I really don't understand what you are trying to do here
> 
> We want to register userspace addresses with the block_device just once. We can
> skip costly per-IO setup this way.

Explain, please.  How will those be used afterwards and how will IO be matched
with the file you've passed here?