Re: bcachefs: suspicious mm pointer in struct dio_write

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

On Wed, Nov 27, 2024 at 01:01:31PM -0700, Jens Axboe wrote:
> On 11/27/24 12:43 PM, Jann Horn wrote:
> > 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.
> 
> Yep I think that would definitely make it more sturdy, and also less
> headscratchy in terms of being able to verify it's actually safe.
> 
> > 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?
> 
> Not aware of anything else doing it like this, where it's punted to a
> kthread and then the mm used from there. The upfront page
> getting/mapping is the common approach, like you described. Which does
> seem like a much better choice, rather than needing to rely on the mm in
> a kworker.

More common, but not necessarily better: the "pin everything up front"
approach had the disadvantage that - well, you pinned everything up
front: if userspace requests a single multi-gigabyte IO, that is likely
not what you want.

The old dio code didn't pin everything up front for this reason (but
IIRC wasn't fully asynchronous, and also had a silly baked in 64 page
limit).