mm/gup: Add FOLL_INTERRUPTIBLE

We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
issue with it is that not all GUP paths are able to handle signal delivers
besides SIGKILL.

That's not ideal for the GUP users who are actually able to handle these
cases, like KVM.

KVM uses GUP extensively on faulting guest pages, during which we've got
existing infrastructures to retry a page fault at a later time.  Allowing
the GUP to be interrupted by generic signals can make KVM related threads
to be more responsive.  For examples:

  (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
      e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
      generated to kick the vcpus out of kernel context immediately,

  (2) SIGINT: which can be used with interactive hypervisor users to stop a
      virtual machine with Ctrl-C without any delays/hangs,

  (3) SIGTRAP: which grants GDB capability even during page faults that are
      stuck for a long time.

Normally hypervisor will be able to receive these signals properly, but not
if we're stuck in a GUP for a long time for whatever reason.  It happens
easily with a stucked postcopy migration when e.g. a network temp failure
happens, then some vcpu threads can hang death waiting for the pages.  With
the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
enable the ability to trap these signals.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20221011195809.557016-2-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 29 insertions, 4 deletions

diff --git a/mm/gup.c b/mm/gup.c
index fe195d47de74..90e372352e82 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -989,8 +989,17 @@ static int faultin_page(struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
-	if (locked)
+	if (locked) {
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+		/*
+		 * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set
+		 * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE.
+		 * That's because some callers may not be prepared to
+		 * handle early exits caused by non-fatal signals.
+		 */
+		if (*flags & FOLL_INTERRUPTIBLE)
+			fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
+	}
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
@@ -1392,6 +1401,22 @@ retry:
 EXPORT_SYMBOL_GPL(fixup_user_fault);
 
 /*
+ * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
+ * specified, it'll also respond to generic signals.  The caller of GUP
+ * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
+ */
+static bool gup_signal_pending(unsigned int flags)
+{
+	if (fatal_signal_pending(current))
+		return true;
+
+	if (!(flags & FOLL_INTERRUPTIBLE))
+		return false;
+
+	return signal_pending(current);
+}
+
+/*
  * Please note that this function, unlike __get_user_pages will not
  * return 0 for nr_pages > 0 without FOLL_NOWAIT
  */
@@ -1472,11 +1497,11 @@ retry:
 		 * Repeat on the address that fired VM_FAULT_RETRY
 		 * with both FAULT_FLAG_ALLOW_RETRY and
 		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
-		 * by fatal signals, so we need to check it before we
+		 * by fatal signals of even common signals, depending on
+		 * the caller's request. So we need to check it before we
 		 * start trying again otherwise it can loop forever.
 		 */
-
-		if (fatal_signal_pending(current)) {
+		if (gup_signal_pending(flags)) {
 			if (!pages_done)
 				pages_done = -EINTR;
 			break;