linux-kernel - Re: [PATCH RFC 1/4] mm/gup: Add FOLL

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Date:   Tue, 21 Jun 2022 10:23:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

On 17.06.22 03:41, 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.

This makes sense to me. I assume relevant callers will detect "GUP
failed" but also "well, there is a signal to handle" and cleanly back
off, correct?

-- 
Thanks,

David / dhildenb