Re: bcachefs: suspicious mm pointer in struct dio_write

[Kent Overstreet <kent.overstreet@linux.dev>]

On Wed, Nov 27, 2024 at 11:09:14AM -0700, Jens Axboe wrote:
> On 11/27/24 9:57 AM, Jann Horn wrote:
> > Hi!
> > 
> > In fs/bcachefs/fs-io-direct.c, "struct dio_write" contains a pointer
> > to an mm_struct. This pointer is grabbed in bch2_direct_write()
> > (without any kind of refcount increment), and used in
> > bch2_dio_write_continue() for kthread_use_mm()/kthread_unuse_mm()
> > which are used to enable userspace memory access from kthread context.
> > I believe kthread_use_mm()/kthread_unuse_mm() require that the caller
> > guarantees that the MM hasn't gone through exit_mmap() yet (normally
> > by holding an mmget() reference).
> > 
> > If we reach this codepath via io_uring, do we have a guarantee that
> > the mm_struct that called bch2_direct_write() is still alive and
> > hasn't yet gone through exit_mmap() when it is accessed from
> > bch2_dio_write_continue()?
> > 
> > I don't know the async direct I/O codepath particularly well, so I
> > cc'ed the uring maintainers, who probably know this better than me.
> 
> I _think_ this is fine as-is, even if it does look dubious and bcachefs
> arguably should grab an mm ref for this just for safety to avoid future
> problems. The reason is that bcachefs doesn't set FMODE_NOWAIT, which
> means that on the io_uring side it cannot do non-blocking issue of
> requests. This is slower as it always punts to an io-wq thread, which
> shares the same mm. Hence if the request is alive, there's always a
> thread with the same mm alive as well.
> 
> Now if FMODE_NOWAIT was set, then the original task could exit. I'd need
> to dig a bit deeper to verify that would always be safe and there's not
> a of time today with a few days off in the US looming, so I'll defer
> that to next week. It certainly would be fine with an mm ref grabbed.

Wouldn't delivery of completions be tied to an address space (not a
process) like it is for aio?