On Wed, Nov 27, 2024 at 7:09 PM Jens Axboe <axboe@xxxxxxxxx> 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?
