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. -- Jens Axboe
