On Wed, Nov 27, 2024 at 10:52 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On Wed, Nov 27, 2024 at 10:16?PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > > On 11/27/24 2:08 PM, Kent Overstreet wrote: > > > > On Wed, Nov 27, 2024 at 09:44:21PM +0100, Jann Horn wrote: > > > >> On Wed, Nov 27, 2024 at 9:25?PM Kent Overstreet > > > >> <kent.overstreet@xxxxxxxxx> wrote: > > > >>> 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? > > > >> > > > >> An io_uring instance is primarily exposed to userspace as a file > > > >> descriptor, so AFAIK it is possible for the io_uring instance to live > > > >> beyond when the last mmput() happens. io_uring initially only holds an > > > >> mmgrab() reference on the MM (a comment above that explains: "This is > > > >> just grabbed for accounting purposes"), which I think is not enough to > > > >> make it stable enough for kthread_use_mm(); I think in io_uring, only > > > >> the worker threads actually keep the MM fully alive (and AFAIK the > > > >> uring worker threads can exit before the uring instance itself is torn > > > >> down). > > > >> > > > >> To receive io_uring completions, there are multiple ways, but they > > > >> don't use a pointer from the io_uring instance to the MM to access > > > >> userspace memory. Instead, you can have a VMA that points to the > > > >> io_uring instance, created by calling mmap() on the io_uring fd; or > > > >> alternatively, with IORING_SETUP_NO_MMAP, you can have io_uring grab > > > >> references to userspace-provided pages. > > > >> > > > >> On top of that, I think it might currently be possible to use the > > > >> io_uring file descriptor from another task to submit work. (That would > > > >> probably be fairly nonsensical, but I think the kernel does not > > > >> currently prevent it.) > > > > > > > > Ok, that's a wrinkle. > > > > > > I'd argue the fact that you are using an mm from a different process > > > without grabbing a reference is the wrinkle. I just don't think it's a > > > problem right now, but it could be... aio is tied to the mm because of > > > how it does completions, potentially, and hence needs this exit_aio() > > > hack because of that. aio also doesn't care, because it doesn't care > > > about blocking - it'll happily block during issue. > > > > > > > Jens, is it really FMODE_NOWAIT that controls whether we can hit this? A > > > > very cursory glance leads me to suspect "no", it seems like this is a > > > > bug if io_uring is allowed on bcachefs at all. > > > > > > I just looked at bcachefs dio writes, which look to be the only case of > > > this. And yes, for writes, if FMODE_NOWAIT isn't set, then io-wq is > > > always involved for the IO. > > > > I guess it could be an issue if the iocb can outlive the io-wq thread? > > Like, a userspace task creates a uring instance and starts a write; > > the write will be punted to a uring worker because of missing > > FMODE_NOWAIT; then the uring worker enters io_write() and starts a > > write on a kiocb. Can this write initiated from the worker be async? > > And could the uring worker exit (for example, because the userspace > > task exited) while the kiocb is still in flight? > > No, any write (or read, whatever) from an io-wq worker is always > blocking / sync. That's the main reason for them existing, to be able to > do blocking issue. And that's what they always do. Aaah, ok, thanks for the explanation.
