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

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Date:	Tue, 5 Oct 2010 11:59:16 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, 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 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.

> +	if (list_empty_careful(&vcpu->async_pf.done))
> +		return;
> +
> +	spin_lock(&vcpu->async_pf.lock);
> +	work = list_first_entry(&vcpu->async_pf.done, typeof(*work), link);
> +	list_del(&work->link);
> +	spin_unlock(&vcpu->async_pf.lock);
> +
> +	kvm_arch_async_page_present(vcpu, work);
> +
> +free:
> +	list_del(&work->queue);
> +	vcpu->async_pf.queued--;
> +	put_page(work->page);
> +	kmem_cache_free(async_pf_cache, work);
> +}

Better handle it here (and other sites).
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