On Wed, Jul 20, 2022 at 08:03:16PM -0400, Peter Xu wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Will squash the hugetlb support too altogether, which is a one-liner
anyway:
---8<---
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a57e1be41401..4025a305d573 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6176,9 +6176,12 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
fault_flags |= FAULT_FLAG_WRITE;
else if (unshare)
fault_flags |= FAULT_FLAG_UNSHARE;
- if (locked)
+ if (locked) {
fault_flags |= FAULT_FLAG_ALLOW_RETRY |
FAULT_FLAG_KILLABLE;
+ if (flags & FOLL_INTERRUPTIBLE)
+ fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
+ }
if (flags & FOLL_NOWAIT)
fault_flags |= FAULT_FLAG_ALLOW_RETRY |
FAULT_FLAG_RETRY_NOWAIT;
---8<---
I'll still keep R-b for John and DavidH.
Thanks,
--
Peter Xu
