On Wed, Nov 27, 2024 at 05:57:03PM +0100, 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 don't know about io_uring but aio guarantees that outstanding kiocbs are completed before exiting the mm_struct - I would expect some sort of similar guarantee to hold, because otherwise where are we going to deliver the completion?
