Re: bcachefs: suspicious mm pointer in struct dio_write

[Jann Horn <jannh@google.com>]

On Wed, Nov 27, 2024 at 7:09 PM Jens Axboe <axboe@kernel.dk> 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.

Ah, thanks for looking into it! I missed this implication of not
setting FMODE_NOWAIT.

Anyway, what you said sounds like it would be cleaner for bcachefs to
grab its own extra reference, maybe by initially grabbing an mm
reference with mmgrab() in bch2_direct_write(), and then use
mmget_not_zero() in bch2_dio_write_continue() to ensure the MM is
stable.

What do other file systems do for this? I think they normally grab
page references so that they don't need the MM anymore when
asynchronously fulfilling the request, right? Like in
iomap_dio_bio_iter(), which uses bio_iov_iter_get_pages() to grab
references to the pages corresponding to the userspace regions in
dio->submit.iter?