linux-kernel - Re: [PATCH v6 02/12] Halt vcpu if page it tries to access is swapped out.

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Message-ID: <4CAD98CE.7020502@redhat.com>
Date:	Thu, 07 Oct 2010 11:54:22 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, hpa@...or.com,
	riel@...hat.com, cl@...ux-foundation.org
Subject: Re: [PATCH v6 02/12] Halt vcpu if page it tries to access is swapped
 out.

  On 10/06/2010 12:52 PM, Gleb Natapov wrote:
> On Wed, Oct 06, 2010 at 12:50:01PM +0200, Avi Kivity wrote:
> >   On 10/05/2010 04:59 PM, Marcelo Tosatti wrote:
> >  >On Mon, Oct 04, 2010 at 05:56:24PM +0200, Gleb Natapov wrote:
> >  >>   If a guest accesses swapped out memory do not swap it in from vcpu thread
> >  >>   context. Schedule work to do swapping and put vcpu into halted state
> >  >>   instead.
> >  >>
> >  >>   Interrupts will still be delivered to the guest and if interrupt will
> >  >>   cause reschedule guest will continue to run another task.
> >  >>
> >  >>   Signed-off-by: Gleb Natapov<gleb@...hat.com>
> >  >>   ---
> >  >>    arch/x86/include/asm/kvm_host.h |   17 +++
> >  >>    arch/x86/kvm/Kconfig            |    1 +
> >  >>    arch/x86/kvm/Makefile           |    1 +
> >  >>    arch/x86/kvm/mmu.c              |   51 +++++++++-
> >  >>    arch/x86/kvm/paging_tmpl.h      |    4 +-
> >  >>    arch/x86/kvm/x86.c              |  109 +++++++++++++++++++-
> >  >>    include/linux/kvm_host.h        |   31 ++++++
> >  >>    include/trace/events/kvm.h      |   88 ++++++++++++++++
> >  >>    virt/kvm/Kconfig                |    3 +
> >  >>    virt/kvm/async_pf.c             |  220 +++++++++++++++++++++++++++++++++++++++
> >  >>    virt/kvm/async_pf.h             |   36 +++++++
> >  >>    virt/kvm/kvm_main.c             |   57 ++++++++--
> >  >>    12 files changed, 603 insertions(+), 15 deletions(-)
> >  >>    create mode 100644 virt/kvm/async_pf.c
> >  >>    create mode 100644 virt/kvm/async_pf.h
> >  >>
> >  >
> >  >>   +	async_pf_cache = NULL;
> >  >>   +}
> >  >>   +
> >  >>   +void kvm_async_pf_vcpu_init(struct kvm_vcpu *vcpu)
> >  >>   +{
> >  >>   +	INIT_LIST_HEAD(&vcpu->async_pf.done);
> >  >>   +	INIT_LIST_HEAD(&vcpu->async_pf.queue);
> >  >>   +	spin_lock_init(&vcpu->async_pf.lock);
> >  >>   +}
> >  >>   +
> >  >>   +static void async_pf_execute(struct work_struct *work)
> >  >>   +{
> >  >>   +	struct page *page;
> >  >>   +	struct kvm_async_pf *apf =
> >  >>   +		container_of(work, struct kvm_async_pf, work);
> >  >>   +	struct mm_struct *mm = apf->mm;
> >  >>   +	struct kvm_vcpu *vcpu = apf->vcpu;
> >  >>   +	unsigned long addr = apf->addr;
> >  >>   +	gva_t gva = apf->gva;
> >  >>   +
> >  >>   +	might_sleep();
> >  >>   +
> >  >>   +	use_mm(mm);
> >  >>   +	down_read(&mm->mmap_sem);
> >  >>   +	get_user_pages(current, mm, addr, 1, 1, 0,&page, NULL);
> >  >>   +	up_read(&mm->mmap_sem);
> >  >>   +	unuse_mm(mm);
> >  >>   +
> >  >>   +	spin_lock(&vcpu->async_pf.lock);
> >  >>   +	list_add_tail(&apf->link,&vcpu->async_pf.done);
> >  >>   +	apf->page = page;
> >  >>   +	spin_unlock(&vcpu->async_pf.lock);
> >  >
> >  >This can fail, and apf->page become NULL.
> >
> >  Does it even become NULL?  On error, get_user_pages() won't update
> >  the pages argument, so page becomes garbage here.
> >
> apf is allocated with kmem_cache_zalloc() and ->page is set to NULL in
> kvm_setup_async_pf() to be extra sure.
>

But you assign apf->page = page;, overriding it with garbage here.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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