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

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Message-ID: <20211123010639.GA32088@chaop.bj.intel.com>
Date:   Tue, 23 Nov 2021 09:06:39 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Wanpeng Li <wanpengli@...cent.com>, jun.nakajima@...el.com,
        kvm@...r.kernel.org, david@...hat.com, qemu-devel@...gnu.org,
        "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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jim Mattson <jmattson@...gle.com>, linux-mm@...ck.org,
        Sean Christopherson <seanjc@...gle.com>, susie.li@...el.com,
        Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org,
        john.ji@...el.com, Yu Zhang <yu.c.zhang@...ux.intel.com>,
        linux-fsdevel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC v2 PATCH 13/13] KVM: Enable memfd based page
 invalidation/fallocate

On Mon, Nov 22, 2021 at 05:16:47PM +0300, Kirill A. Shutemov wrote:
> 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.

Right.

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

Sounds like a sequential id will be needed, no existing fields in struct
kvm can work for this.

> 
> > +
> > +	return true;
> > +}
> 
> -- 
>  Kirill A. Shutemov