Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v3 6/6] virtio-balloon: Add support for providing unused
 page reports to host
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>, kvm@vger.kernel.org,
        david@redhat.com, dave.hansen@intel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
        pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com,
        willy@infradead.org, lcapitulino@redhat.com, wei.w.wang@intel.com,
        aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com
References: <20190801222158.22190.96964.stgit@localhost.localdomain>
 <20190801223829.22190.36831.stgit@localhost.localdomain>
 <1cff09a4-d302-639c-ab08-9d82e5fc1383@redhat.com>
 <ed48ecdb833808bf6b08bc54fa98503cbad493f3.camel@linux.intel.com>
 <20190806073047-mutt-send-email-mst@kernel.org>
 <dcd778623685079f66bfccb5dc0195e6f5bc992d.camel@linux.intel.com>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=nitesh@redhat.com; prefer-encrypt=mutual; keydata=
 mQINBFl4pQoBEADT/nXR2JOfsCjDgYmE2qonSGjkM1g8S6p9UWD+bf7YEAYYYzZsLtbilFTe
 z4nL4AV6VJmC7dBIlTi3Mj2eymD/2dkKP6UXlliWkq67feVg1KG+4UIp89lFW7v5Y8Muw3Fm
 uQbFvxyhN8n3tmhRe+ScWsndSBDxYOZgkbCSIfNPdZrHcnOLfA7xMJZeRCjqUpwhIjxQdFA7
 n0s0KZ2cHIsemtBM8b2WXSQG9CjqAJHVkDhrBWKThDRF7k80oiJdEQlTEiVhaEDURXq+2XmG
 jpCnvRQDb28EJSsQlNEAzwzHMeplddfB0vCg9fRk/kOBMDBtGsTvNT9OYUZD+7jaf0gvBvBB
 lbKmmMMX7uJB+ejY7bnw6ePNrVPErWyfHzR5WYrIFUtgoR3LigKnw5apzc7UIV9G8uiIcZEn
 C+QJCK43jgnkPcSmwVPztcrkbC84g1K5v2Dxh9amXKLBA1/i+CAY8JWMTepsFohIFMXNLj+B
 RJoOcR4HGYXZ6CAJa3Glu3mCmYqHTOKwezJTAvmsCLd3W7WxOGF8BbBjVaPjcZfavOvkin0u
 DaFvhAmrzN6lL0msY17JCZo046z8oAqkyvEflFbC0S1R/POzehKrzQ1RFRD3/YzzlhmIowkM
 BpTqNBeHEzQAlIhQuyu1ugmQtfsYYq6FPmWMRfFPes/4JUU/PQARAQABtCVOaXRlc2ggTmFy
 YXlhbiBMYWwgPG5pbGFsQHJlZGhhdC5jb20+iQI9BBMBCAAnBQJZeKUKAhsjBQkJZgGABQsJ
 CAcCBhUICQoLAgQWAgMBAh4BAheAAAoJEKOGQNwGMqM56lEP/A2KMs/pu0URcVk/kqVwcBhU
 SnvB8DP3lDWDnmVrAkFEOnPX7GTbactQ41wF/xwjwmEmTzLrMRZpkqz2y9mV0hWHjqoXbOCS
 6RwK3ri5e2ThIPoGxFLt6TrMHgCRwm8YuOSJ97o+uohCTN8pmQ86KMUrDNwMqRkeTRW9wWIQ
 EdDqW44VwelnyPwcmWHBNNb1Kd8j3xKlHtnS45vc6WuoKxYRBTQOwI/5uFpDZtZ1a5kq9Ak/
 MOPDDZpd84rqd+IvgMw5z4a5QlkvOTpScD21G3gjmtTEtyfahltyDK/5i8IaQC3YiXJCrqxE
 r7/4JMZeOYiKpE9iZMtS90t4wBgbVTqAGH1nE/ifZVAUcCtycD0f3egX9CHe45Ad4fsF3edQ
 ESa5tZAogiA4Hc/yQpnnf43a3aQ67XPOJXxS0Qptzu4vfF9h7kTKYWSrVesOU3QKYbjEAf95
 NewF9FhAlYqYrwIwnuAZ8TdXVDYt7Z3z506//sf6zoRwYIDA8RDqFGRuPMXUsoUnf/KKPrtR
 ceLcSUP/JCNiYbf1/QtW8S6Ca/4qJFXQHp0knqJPGmwuFHsarSdpvZQ9qpxD3FnuPyo64S2N
 Dfq8TAeifNp2pAmPY2PAHQ3nOmKgMG8Gn5QiORvMUGzSz8Lo31LW58NdBKbh6bci5+t/HE0H
 pnyVf5xhNC/FuQINBFl4pQoBEACr+MgxWHUP76oNNYjRiNDhaIVtnPRqxiZ9v4H5FPxJy9UD
 Bqr54rifr1E+K+yYNPt/Po43vVL2cAyfyI/LVLlhiY4yH6T1n+Di/hSkkviCaf13gczuvgz4
 KVYLwojU8+naJUsiCJw01MjO3pg9GQ+47HgsnRjCdNmmHiUQqksMIfd8k3reO9SUNlEmDDNB
 XuSzkHjE5y/R/6p8uXaVpiKPfHoULjNRWaFc3d2JGmxJpBdpYnajoz61m7XJlgwl/B5Ql/6B
 dHGaX3VHxOZsfRfugwYF9CkrPbyO5PK7yJ5vaiWre7aQ9bmCtXAomvF1q3/qRwZp77k6i9R3
 tWfXjZDOQokw0u6d6DYJ0Vkfcwheg2i/Mf/epQl7Pf846G3PgSnyVK6cRwerBl5a68w7xqVU
 4KgAh0DePjtDcbcXsKRT9D63cfyfrNE+ea4i0SVik6+N4nAj1HbzWHTk2KIxTsJXypibOKFX
 2VykltxutR1sUfZBYMkfU4PogE7NjVEU7KtuCOSAkYzIWrZNEQrxYkxHLJsWruhSYNRsqVBy
 KvY6JAsq/i5yhVd5JKKU8wIOgSwC9P6mXYRgwPyfg15GZpnw+Fpey4bCDkT5fMOaCcS+vSU1
 UaFmC4Ogzpe2BW2DOaPU5Ik99zUFNn6cRmOOXArrryjFlLT5oSOe4IposgWzdwARAQABiQIl
 BBgBCAAPBQJZeKUKAhsMBQkJZgGAAAoJEKOGQNwGMqM5ELoP/jj9d9gF1Al4+9bngUlYohYu
 0sxyZo9IZ7Yb7cHuJzOMqfgoP4tydP4QCuyd9Q2OHHL5AL4VFNb8SvqAxxYSPuDJTI3JZwI7
 d8JTPKwpulMSUaJE8ZH9n8A/+sdC3CAD4QafVBcCcbFe1jifHmQRdDrvHV9Es14QVAOTZhnJ
 vweENyHEIxkpLsyUUDuVypIo6y/Cws+EBCWt27BJi9GH/EOTB0wb+2ghCs/i3h8a+bi+bS7L
 FCCm/AxIqxRurh2UySn0P/2+2eZvneJ1/uTgfxnjeSlwQJ1BWzMAdAHQO1/lnbyZgEZEtUZJ
 x9d9ASekTtJjBMKJXAw7GbB2dAA/QmbA+Q+Xuamzm/1imigz6L6sOt2n/X/SSc33w8RJUyor
 SvAIoG/zU2Y76pKTgbpQqMDmkmNYFMLcAukpvC4ki3Sf086TdMgkjqtnpTkEElMSFJC8npXv
 3QnGGOIfFug/qs8z03DLPBz9VYS26jiiN7QIJVpeeEdN/LKnaz5LO+h5kNAyj44qdF2T2AiF
 HxnZnxO5JNP5uISQH3FjxxGxJkdJ8jKzZV7aT37sC+Rp0o3KNc+GXTR+GSVq87Xfuhx0LRST
 NK9ZhT0+qkiN7npFLtNtbzwqaqceq3XhafmCiw8xrtzCnlB/C4SiBr/93Ip4kihXJ0EuHSLn
 VujM7c/b4pps
Organization: Red Hat Inc,
Message-ID: <85b045b4-9d43-6929-49b9-786bd25eaed2@redhat.com>
Date:   Tue, 6 Aug 2019 11:40:13 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <dcd778623685079f66bfccb5dc0195e6f5bc992d.camel@linux.intel.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 8/6/19 11:16 AM, Alexander Duyck wrote:
> On Tue, 2019-08-06 at 07:31 -0400, Michael S. Tsirkin wrote:
>> On Mon, Aug 05, 2019 at 09:27:16AM -0700, Alexander Duyck wrote:
>>> On Mon, 2019-08-05 at 12:00 -0400, Nitesh Narayan Lal wrote:
>>>> On 8/1/19 6:38 PM, Alexander Duyck wrote:
>>>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>>>
>>>>> Add support for the page reporting feature provided by virtio-balloon.
>>>>> Reporting differs from the regular balloon functionality in that is is
>>>>> much less durable than a standard memory balloon. Instead of creating a
>>>>> list of pages that cannot be accessed the pages are only inaccessible
>>>>> while they are being indicated to the virtio interface. Once the
>>>>> interface has acknowledged them they are placed back into their respective
>>>>> free lists and are once again accessible by the guest system.
>>>>>
>>>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>>> ---
>>>>>  drivers/virtio/Kconfig              |    1 +
>>>>>  drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
>>>>>  include/uapi/linux/virtio_balloon.h |    1 +
>>>>>  3 files changed, 58 insertions(+)
>>>>>
>>>>> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
>>>>> index 078615cf2afc..4b2dd8259ff5 100644
>>>>> --- a/drivers/virtio/Kconfig
>>>>> +++ b/drivers/virtio/Kconfig
>>>>> @@ -58,6 +58,7 @@ config VIRTIO_BALLOON
>>>>>  	tristate "Virtio balloon driver"
>>>>>  	depends on VIRTIO
>>>>>  	select MEMORY_BALLOON
>>>>> +	select PAGE_REPORTING
>>>>>  	---help---
>>>>>  	 This driver supports increasing and decreasing the amount
>>>>>  	 of memory within a KVM guest.
>>>>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>>>>> index 2c19457ab573..971fe924e34f 100644
>>>>> --- a/drivers/virtio/virtio_balloon.c
>>>>> +++ b/drivers/virtio/virtio_balloon.c
>>>>> @@ -19,6 +19,7 @@
>>>>>  #include <linux/mount.h>
>>>>>  #include <linux/magic.h>
>>>>>  #include <linux/pseudo_fs.h>
>>>>> +#include <linux/page_reporting.h>
>>>>>  
>>>>>  /*
>>>>>   * Balloon device works in 4K page units.  So each page is pointed to by
>>>>> @@ -37,6 +38,9 @@
>>>>>  #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
>>>>>  	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
>>>>>  
>>>>> +/*  limit on the number of pages that can be on the reporting vq */
>>>>> +#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
>>>>> +
>>>>>  #ifdef CONFIG_BALLOON_COMPACTION
>>>>>  static struct vfsmount *balloon_mnt;
>>>>>  #endif
>>>>> @@ -46,6 +50,7 @@ enum virtio_balloon_vq {
>>>>>  	VIRTIO_BALLOON_VQ_DEFLATE,
>>>>>  	VIRTIO_BALLOON_VQ_STATS,
>>>>>  	VIRTIO_BALLOON_VQ_FREE_PAGE,
>>>>> +	VIRTIO_BALLOON_VQ_REPORTING,
>>>>>  	VIRTIO_BALLOON_VQ_MAX
>>>>>  };
>>>>>  
>>>>> @@ -113,6 +118,10 @@ struct virtio_balloon {
>>>>>  
>>>>>  	/* To register a shrinker to shrink memory upon memory pressure */
>>>>>  	struct shrinker shrinker;
>>>>> +
>>>>> +	/* Unused page reporting device */
>>>>> +	struct virtqueue *reporting_vq;
>>>>> +	struct page_reporting_dev_info ph_dev_info;
>>>>>  };
>>>>>  
>>>>>  static struct virtio_device_id id_table[] = {
>>>>> @@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
>>>>>  
>>>>>  }
>>>>>  
>>>>> +void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
>>>>> +				    unsigned int nents)
>>>>> +{
>>>>> +	struct virtio_balloon *vb =
>>>>> +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
>>>>> +	struct virtqueue *vq = vb->reporting_vq;
>>>>> +	unsigned int unused;
>>>>> +
>>>>> +	/* We should always be able to add these buffers to an empty queue. */
>>>>> +	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
>>>>> +			    GFP_NOWAIT | __GFP_NOWARN);
>>>> I think you should handle allocation failure here. It is a possibility, isn't?
>>>> Maybe return an error or even disable page hinting/reporting?
>>>>
>>> I don't think it is an issue I have to worry about. Specifically I am
>>> limiting the size of the scatterlist based on the size of the vq. As such
>>> I will never exceed the size and should be able to use it to store the
>>> scatterlist directly.
>> I agree. But it can't hurt to BUG_ON for good measure.
>>
> I wouldn't use a BUG_ON as that seems overkill. No need to panic the
> kernel just because we couldn't report some idle pages.
>
> I can probably do something like:
> 	if (WARN_ON(err))
> 		return;

+1

>
> That way the unused page reporting can run to completion still and the
> fact that we aren't really hinting on the pages would effectively be no
> different then if we had a direct assigned device or shared memory in the
> hypervisor.
>