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

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Message-ID: <20220625003554.GJ23621@ziepe.ca>
Date:   Fri, 24 Jun 2022 21:35:54 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

On Wed, Jun 22, 2022 at 05:36:53PM -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.

Can you talk abit about what is required to use this new interface
correctly?

Lots of GUP callers are in simple system call contexts (like ioctl),
can/should they set this flag and if so what else do they need to do?

Thanks,
Jason