On Mon, Apr 02, 2018 at 07:10:58AM -0700, Matthew Wilcox wrote: > > Souptick and I have been auditing the various page fault handler routines > and we've noticed that graphics drivers assume that a signal should be > able to interrupt a page fault. In contrast, the page cache takes great > care to allow only fatal signals to interrupt a page fault. > > I believe (but have not verified) that a non-fatal signal being delivered > to a task which is in the middle of a page fault may well end up in an > infinite loop, attempting to handle the page fault and failing forever. > > Here's one of the simpler ones: > > ret = mutex_lock_interruptible(&etnaviv_obj->lock); > if (ret) > return VM_FAULT_NOPAGE; > > (many other drivers do essentially the same thing including i915) > > On seeing NOPAGE, the fault handler believes the PTE is in the page > table, so does nothing before it returns to arch code at which point > I get lost in the magic assembler macros. I believe it will end up > returning to userspace if the signal is non-fatal, at which point it'll > go right back into the page fault handler, and mutex_lock_interruptible() > will immediately fail. So we've converted a sleeping lock into the most > expensive spinlock. > > I don't think the graphics drivers really want to be interrupted by > any signal. I think they want to be interruptible by fatal signals > and should use the mutex_lock_killable / fatal_signal_pending family of > functions. That's going to be a bit of churn, funnelling TASK_KILLABLE > / TASK_INTERRUPTIBLE all the way down into the dma-fence code. Before > anyone gets started on that, I want to be sure that my analysis is > correct, and the drivers are doing the wrong thing by using interruptible > waits in a page fault handler.
So we've done some experiments for the case where the fault originated from kernel context (copy_to|from_user and friends). The fixup code seems to retry the copy once after the fault (in copy_user_handle_tail), if that fails again we get a short read/write. This might result in an -EFAULT, short read()/write() or anything else really, depending upon the syscall api. Except in some code paths in gpu drivers where we convert anything into -ERESTARTSYS/EINTR if there's a signal pending it won't ever result in the syscall getting restarted (well except maybe short read/writes if userspace bothers with that). So I guess gpu fault handlers indeed break the kernel's expectations, but then I think we're getting away with that because the inner workings of gpu memory objects is all heavily abstracted away by opengl/vulkan and friends. I guess what we could do is try to only do killable sleeps if it's a kernel fault, but that means wiring a flag through all the callchains. Not pretty. Except when there's a magic set of functions that would convert all interruptible sleeps to killable ones only for us. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch