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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, May 01, 2025 at 12:18:35PM -0400, Peter Xu wrote: > On Thu, Apr 24, 2025 at 03:57:09PM -0400, Peter Xu wrote: > > On Thu, Apr 24, 2025 at 01:20:46PM -0600, Jens Axboe wrote: > > > On 4/24/25 1:13 PM, Peter Xu wrote: > > > > > > (skipping to this bit as I think we're mostly in agreement on the above) > > > > > > >>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > > >>> index 296d294142c8..fa721525d93a 100644 > > > >>> --- a/arch/x86/mm/fault.c > > > >>> +++ b/arch/x86/mm/fault.c > > > >>> @@ -1300,9 +1300,14 @@ void do_user_addr_fault(struct pt_regs *regs, > > > >>> * We set FAULT_FLAG_USER based on the register state, not > > > >>> * based on X86_PF_USER. User space accesses that cause > > > >>> * system page faults are still user accesses. > > > >>> + * > > > >>> + * When we're in user mode, allow fast response on non-fatal > > > >>> + * signals. Do not set this in kernel mode faults because normally > > > >>> + * a kernel fault means the fault must be resolved anyway before > > > >>> + * going back to userspace. > > > >>> */ > > > >>> if (user_mode(regs)) > > > >>> - flags |= FAULT_FLAG_USER; > > > >>> + flags |= FAULT_FLAG_USER | FAULT_FLAG_INTERRUPTIBLE; > > > >>> > > > >>> #ifdef CONFIG_X86_64 > > > >>> /* > > > >>> diff --git a/include/linux/mm.h b/include/linux/mm.h > > > >>> index 9b701cfbef22..a80f3f609b37 100644 > > > >>> --- a/include/linux/mm.h > > > >>> +++ b/include/linux/mm.h > > > >>> @@ -487,8 +487,7 @@ extern unsigned int kobjsize(const void *objp); > > > >>> * arch-specific page fault handlers. > > > >>> */ > > > >>> #define FAULT_FLAG_DEFAULT (FAULT_FLAG_ALLOW_RETRY | \ > > > >>> - FAULT_FLAG_KILLABLE | \ > > > >>> - FAULT_FLAG_INTERRUPTIBLE) > > > >>> + FAULT_FLAG_KILLABLE) > > > >>> ===8<=== > > > >>> > > > >>> That also kind of matches with what we do with fault_signal_pending(). > > > >>> Would it make sense? > > > >> > > > >> I don't think doing a non-bounded non-interruptible sleep for a > > > >> condition that may never resolve (eg userfaultfd never fills the fault) > > > >> is a good idea. What happens if the condition never becomes true? You > > > > > > > > If page fault is never going to be resolved, normally we sigkill the > > > > program as it can't move any further with no way to resolve the page fault. > > > > > > > > But yeah that's based on the fact sigkill will work first.. > > > > > > Yep > > > > > > >> can't even kill the task at that point... Generally UNINTERRUPTIBLE > > > >> sleep should only be used if it's a bounded wait. > > > >> > > > >> For example, if I ran my previous write(2) reproducer here and the task > > > >> got killed or exited before the userfaultfd fills the fault, then you'd > > > >> have the task stuck in 'D' forever. Can't be killed, can't get > > > >> reclaimed. > > > >> > > > >> In other words, this won't work. > > > > > > > > .. Would you help explain why it didn't work even for SIGKILL? Above will > > > > still set FAULT_FLAG_KILLABLE, hence I thought SIGKILL would always work > > > > regardless. > > > > > > > > For such kernel user page access, IIUC it should respond to SIGKILL in > > > > handle_userfault(), then fault_signal_pending() would trap the SIGKILL this > > > > time -> going kernel fixups. Then the upper stack should get -EFAULT in the > > > > exception fixup path. > > > > > > > > I could have missed something.. > > > > > > It won't work because sending the signal will not wake the process in > > > question as it's sleeping uninterruptibly, forever. My looping approach > > > still works for fatal signals as we abort the loop every now and then, > > > hence we know it won't be stuck forever. But if you don't have a timeout > > > on that uninterruptible sleep, it's not waking from being sent a signal > > > alone. > > > > > > Example: > > > > > > axboe@m2max-kvm ~> sudo ./tufd > > > got buf 0xffff89800000 > > > child will write > > > Page fault > > > flags = 0; address = ffff89800000 > > > wait on child > > > fish: Job 1, 'sudo ./tufd' terminated by signal SIGKILL (Forced quit) > > > > > > meanwhile in ps: > > > > > > root 837 837 0.0 2 0.0 14628 1220 ? Dl 12:37 0:00 ./tufd > > > root 837 838 0.0 2 0.0 14628 1220 ? Sl 12:37 0:00 ./tufd > > > > I don't know TASK_WAKEKILL well, but I was hoping it would work in this > > case.. E.g., even if with the patch, we still have chance to not use any > > timeout at [1] below? > > > > if (likely(must_wait && !READ_ONCE(ctx->released))) { > > wake_up_poll(&ctx->fd_wqh, EPOLLIN); > > - schedule(); > > + /* See comment in userfaultfd_get_blocking_state() */ > > + if (!wait_mode.timeout) > > + schedule(); <----------------------------- [1] > > + else > > + schedule_timeout(HZ / 10); > > } > > > > So my understanding is sigkill also need to work always for [1] if > > FAULT_FLAG_KILLABLE is set (which should always be, iiuc). > > > > Did I miss something else? It would be helpful too if you could share the > > reproducer; I can give it a shot. > > Since the signal issue alone can definitely be reproduced with any > reproducer that triggers the fault in the kernel.. I wrote one today with > write() syscall, I'll attach that at the end. > > I did try this patch, meanwhile I also verified that actually what I > provided previously (at the end of the reply) can also avoid the cpu > spinning, and it is also killable at least here.. > > https://lore.kernel.org/all/aAqCXfPirHqWMlb4@x1.local/ > > Jens, could you help me to find why that simpler (and IMHO must cleaner) > change wouldn't work for your case? > > The important thing is, as I mentioned above sigkill need to also work for > [1], and I really want to know when it won't.. meanwhile it's logically > incorrect to set INTERRUPTIBLE for kernel faults, which this patch didn't > really address. My reproducer: $ cat uffd-kernel-sig.c #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define PAGE_SIZE 4096 #define BUFFER_PAGES 2 void sigusr1_handler(int signum) { printf("SIGUSR1 SIGNAL\n"); } static int setup_userfaultfd(void *addr, size_t len) { int uffd = syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK); if (uffd == -1) { perror("userfaultfd"); exit(1); } struct uffdio_api ua = { .api = UFFD_API }; if (ioctl(uffd, UFFDIO_API, &ua) == -1) { perror("UFFDIO_API"); exit(1); } struct uffdio_register ur = { .range = { .start = (unsigned long)addr, .len = len }, .mode = UFFDIO_REGISTER_MODE_MISSING }; if (ioctl(uffd, UFFDIO_REGISTER, &ur) == -1) { perror("UFFDIO_REGISTER"); exit(1); } return uffd; } void *signal_sender(void *arg) { pid_t pid = getpid(); usleep(100000); kill(pid, SIGUSR1); return NULL; } int main() { struct sigaction sa; sa.sa_handler = sigusr1_handler; sigemptyset(&sa.sa_mask); sa.sa_flags = 0; if (sigaction(SIGUSR1, &sa, NULL) == -1) { perror("sigaction"); exit(1); } size_t buffer_size = BUFFER_PAGES * PAGE_SIZE; void *src_buf = mmap(NULL, buffer_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (src_buf == MAP_FAILED) { perror("mmap src_buf"); exit(1); } if (madvise(src_buf, buffer_size, MADV_DONTNEED) == -1) { perror("madvise"); exit(1); } void *dst_buf = malloc(buffer_size); if (!dst_buf) { perror("malloc dst_buf"); exit(1); } int uffd = setup_userfaultfd(src_buf, buffer_size); pthread_t thread; if (pthread_create(&thread, NULL, signal_sender, NULL) != 0) { perror("pthread_create"); exit(1); } int tmp = open("/tmp/file", O_WRONLY | O_CREAT, 0644); if (tmp < 0) { exit(EXIT_FAILURE); } ssize_t written = write(tmp, src_buf, buffer_size); printf("write returned：%zd\n", written); close(tmp); return 0; } -- Peter Xu