linux-kernel - Re: [PATCH v18 10/10] virtio-balloon: don't report free pages when page poisoning is enabled

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Date:   Tue, 12 Dec 2017 20:21:31 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, penguin-kernel@...ove.SAKURA.ne.jp,
        cornelia.huck@...ibm.com, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        willy@...radead.org, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu@...yun.com, nilal@...hat.com,
        riel@...hat.com
Subject: Re: [PATCH v18 10/10] virtio-balloon: don't report free pages when
 page poisoning is enabled

On 12/11/2017 09:24 PM, Michael S. Tsirkin wrote:
> On Mon, Dec 11, 2017 at 02:38:45PM +0800, Wei Wang wrote:
>> On 12/01/2017 11:49 PM, Michael S. Tsirkin wrote:
>>> On Wed, Nov 29, 2017 at 09:55:26PM +0800, Wei Wang wrote:
>>>> The guest free pages should not be discarded by the live migration thread
>>>> when page poisoning is enabled with PAGE_POISONING_NO_SANITY=n, because
>>>> skipping the transfer of such poisoned free pages will trigger false
>>>> positive when new pages are allocated and checked on the destination.
>>>> This patch skips the reporting of free pages in the above case.
>>>>
>>>> Reported-by: Michael S. Tsirkin <mst@...hat.com>
>>>> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
>>>> Cc: Michal Hocko <mhocko@...e.com>
>>>> ---
>>>>    drivers/virtio/virtio_balloon.c | 4 +++-
>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>>>> index 035bd3a..6ac4cff 100644
>>>> --- a/drivers/virtio/virtio_balloon.c
>>>> +++ b/drivers/virtio/virtio_balloon.c
>>>> @@ -652,7 +652,9 @@ static void report_free_page(struct work_struct *work)
>>>>    	/* Start by sending the obtained cmd id to the host with an outbuf */
>>>>    	send_one_desc(vb, vb->free_page_vq, virt_to_phys(&vb->start_cmd_id),
>>>>    		      sizeof(uint32_t), false, true, false);
>>>> -	walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
>>>> +	if (!(page_poisoning_enabled() &&
>>>> +	    !IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY)))
>>>> +		walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
>>>>    	/*
>>>>    	 * End by sending the stop id to the host with an outbuf. Use the
>>>>    	 * non-batching mode here to trigger a kick after adding the stop id.
>>> PAGE_POISONING_ZERO is actually OK.
>>>
>>> But I really would prefer it that we still send pages to host,
>>> otherwise debugging becomes much harder.
>>>
>>> And it does not have to be completely useless, even though
>>> you can not discard them as they would be zero-filled then.
>>>
>>> How about a config field telling host what should be there in the free
>>> pages? This way even though host can not discard them, host can send
>>> them out without reading them, still a win.
>>>
>>>
>> Since this poison value comes with the free page reporting feature, how
>> about sending the poison value via the free_page_vq, along with the cmd id
>> in the outbuf? That is, use the following interface:
>>
>> struct virtio_balloon_free_page_vq_hdr {
>>      bool page_poisoning;
>>      __virtio32 poison_value;
>>      __virtio32 cmd_id;
>> }
> Can we put the value in config space instead?
>
>> We need "bool page_poisoning" because "poison_value=0" doesn't tell whether
>> page poising is in use by the guest.
> Can we use a feature bit for this?
>
>> PAGE_POISONING_ZERO sets
>> "page_poisoning=true, poisoning_value=0", and the host will send the
>> 0-filled pages to the destination (if not sending 0-filled pages, the
>> destination host would offer non-zero pages to the guest)
> Why would it? Linux zeroes all pages on alloc.
>

Thanks, that is the case. I think we don't need a feature bit then. 
Please have a check the v19 patches.

Best,
Wei