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.
