linux-kernel - Re: [RFC v2 PATCH 13/13] KVM: Enable memfd based page invalidation/fallocate

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Date:   Mon, 22 Nov 2021 17:16:47 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, Wanpeng Li <wanpengli@...cent.com>,
        jun.nakajima@...el.com, david@...hat.com,
        "J . Bruce Fields" <bfields@...ldses.org>, dave.hansen@...el.com,
        "H . Peter Anvin" <hpa@...or.com>, ak@...ux.intel.com,
        Jonathan Corbet <corbet@....net>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        luto@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>, susie.li@...el.com,
        Jeff Layton <jlayton@...nel.org>, john.ji@...el.com,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC v2 PATCH 13/13] KVM: Enable memfd based page
 invalidation/fallocate

On Fri, Nov 19, 2021 at 09:47:39PM +0800, Chao Peng wrote:
> Since the memory backing store does not get notified when VM is
> destroyed so need check if VM is still live in these callbacks.
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> ---
>  virt/kvm/memfd.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/virt/kvm/memfd.c b/virt/kvm/memfd.c
> index bd930dcb455f..bcfdc685ce22 100644
> --- a/virt/kvm/memfd.c
> +++ b/virt/kvm/memfd.c
> @@ -12,16 +12,38 @@
>  #include <linux/memfd.h>
>  const static struct guest_mem_ops *memfd_ops;
>  
> +static bool vm_is_dead(struct kvm *vm)
> +{
> +	struct kvm *kvm;
> +
> +	list_for_each_entry(kvm, &vm_list, vm_list) {
> +		if (kvm == vm)
> +			return false;
> +	}

I don't think this is enough. The struct kvm can be freed and re-allocated
from the slab and this function will give false-negetive.

Maybe the kvm has to be tagged with a sequential id that incremented every
allocation. This id can be checked here.

> +
> +	return true;
> +}

-- 
 Kirill A. Shutemov