linux-kernel - Re: [PATCH v2 1/3] mm/gup: Add FOLL

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Date:   Tue, 16 Aug 2022 20:33:28 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v2 1/3] mm/gup: Add FOLL_INTERRUPTIBLE

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@...dia.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>

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