linux-kernel - Re: [PATCH v2 repost 7/7] virtio-balloon: tell host vm's free page info

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Date:	Fri, 29 Jul 2016 00:37:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Li, Liang Z" <liang.z.li@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"quintela@...hat.com" <quintela@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Amit Shah <amit.shah@...hat.com>
Subject: Re: [PATCH v2 repost 7/7] virtio-balloon: tell host vm's free page
 info

On Thu, Jul 28, 2016 at 07:50:52AM +0000, Li, Liang Z wrote:
> > >  }
> > >
> > > +static void update_free_pages_stats(struct virtio_balloon *vb,
> > 
> > why _stats?
> 
> Will change.
> 
> > > +	max_pfn = get_max_pfn();
> > > +	mutex_lock(&vb->balloon_lock);
> > > +	while (pfn < max_pfn) {
> > > +		memset(vb->page_bitmap, 0, vb->bmap_len);
> > > +		ret = get_free_pages(pfn, pfn + vb->pfn_limit,
> > > +			vb->page_bitmap, vb->bmap_len * BITS_PER_BYTE);
> > > +		hdr->cmd = cpu_to_virtio16(vb->vdev,
> > BALLOON_GET_FREE_PAGES);
> > > +		hdr->page_shift = cpu_to_virtio16(vb->vdev, PAGE_SHIFT);
> > > +		hdr->req_id = cpu_to_virtio64(vb->vdev, req_id);
> > > +		hdr->start_pfn = cpu_to_virtio64(vb->vdev, pfn);
> > > +		bmap_len = vb->pfn_limit / BITS_PER_BYTE;
> > > +		if (!ret) {
> > > +			hdr->flag = cpu_to_virtio16(vb->vdev,
> > > +
> > 	BALLOON_FLAG_DONE);
> > > +			if (pfn + vb->pfn_limit > max_pfn)
> > > +				bmap_len = (max_pfn - pfn) /
> > BITS_PER_BYTE;
> > > +		} else
> > > +			hdr->flag = cpu_to_virtio16(vb->vdev,
> > > +
> > 	BALLOON_FLAG_CONT);
> > > +		hdr->bmap_len = cpu_to_virtio64(vb->vdev, bmap_len);
> > > +		sg_init_one(&sg_out, hdr,
> > > +			 sizeof(struct balloon_bmap_hdr) + bmap_len);
> > 
> > Wait a second. This adds the same buffer multiple times in a loop.
> > We will overwrite the buffer without waiting for hypervisor to process it.
> > What did I miss?
> 
> I am no quite sure about this part, I though the virtqueue_kick(vq) will prevent
> the buffer from overwrite, I realized it's wrong.
> 
> > > +
> > > +		virtqueue_add_outbuf(vq, &sg_out, 1, vb, GFP_KERNEL);
> > 
> > this can fail. you want to maybe make sure vq has enough space before you
> > use it or check error and wait.
> > 
> > > +		virtqueue_kick(vq);
> > 
> > why kick here within loop? wait until done. in fact kick outside lock is better
> > for smp.
> 
> I will change this part in v3.
> 
> > 
> > > +		pfn += vb->pfn_limit;
> > > +	static const char * const names[] = { "inflate", "deflate", "stats",
> > > +						 "misc" };
> > >  	int err, nvqs;
> > >
> > >  	/*
> > >  	 * We expect two virtqueues: inflate and deflate, and
> > >  	 * optionally stat.
> > >  	 */
> > > -	nvqs = virtio_has_feature(vb->vdev,
> > VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
> > > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MISC_VQ))
> > > +		nvqs = 4;
> > 
> > Does misc vq depend on stats vq feature then? if yes please validate that.
> 
> Yes, what's you mean by 'validate' that?

Either handle misc vq without a stats vq, or
clear VIRTIO_BALLOON_F_MISC_VQ if stats vq is off.

> > 
> > 
> > > +	else
> > > +		nvqs = virtio_has_feature(vb->vdev,
> > > +					  VIRTIO_BALLOON_F_STATS_VQ) ? 3 :
> > 2;
> > 
> > Replace that ? with else too pls.
> 
> Will change.
> 
> Thanks!
> Liang