Linux Virtualization - Dec 2016 - [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

This patch set contains two parts of changes to the virtio-balloon.
 
One is the change for speeding up the inflating & deflating process,
the main idea of this optimization is to use bitmap to send the page
information to host instead of the PFNs, to reduce the overhead of
virtio data transmission, address translation and madvise(). This can
help to improve the performance by about 85%.
 
Another change is for speeding up live migration. By skipping process
guest's unused pages in the first round of data copy, to reduce needless
data processing, this can help to save quite a lot of CPU cycles and
network bandwidth. We put guest's unused page information in a bitmap
and send it to host with the virt queue of virtio-balloon. For an idle
guest with 8GB RAM, this can help to shorten the total live migration
time from 2Sec to about 500ms in 10Gbps network environment.
 
Changes from v4 to v5:
    * Drop the code to get the max_pfn, use another way instead.
    * Simplify the API to get the unused page information from mm. 

Changes from v3 to v4:
    * Use the new scheme suggested by Dave Hansen to encode the bitmap.
    * Add code which is missed in v3 to handle migrate page. 
    * Free the memory for bitmap intime once the operation is done.
    * Address some of the comments in v3.

Changes from v2 to v3:
    * Change the name of 'free page' to 'unused page'.
    * Use the scatter & gather bitmap instead of a 1MB page bitmap.
    * Fix overwriting the page bitmap after kicking.
    * Some of MST's comments for v2.
 
Changes from v1 to v2:
    * Abandon the patch for dropping page cache.
    * Put some structures to uapi head file.
    * Use a new way to determine the page bitmap size.
    * Use a unified way to send the free page information with the bitmap
    * Address the issues referred in MST's comments

Liang Li (5):
  virtio-balloon: rework deflate to add page to a list
  virtio-balloon: define new feature bit and head struct
  virtio-balloon: speed up inflate/deflate process
  virtio-balloon: define flags and head for host request vq
  virtio-balloon: tell host vm's unused page info

 drivers/virtio/virtio_balloon.c     | 539 ++++++++++++++++++++++++++++++++----
 include/linux/mm.h                  |   3 +-
 include/uapi/linux/virtio_balloon.h |  41 +++
 mm/page_alloc.c                     |  72 +++++
 4 files changed, 607 insertions(+), 48 deletions(-)

-- 
1.8.3.1

When doing the inflating/deflating operation, the current virtio-balloon
implementation uses an array to save 256 PFNS, then send these PFNS to
host through virtio and process each PFN one by one. This way is not
efficient when inflating/deflating a large mount of memory because too
many times of the following operations:

    1. Virtio data transmission
    2. Page allocate/free
    3. Address translation(GPA->HVA)
    4. madvise

The over head of these operations will consume a lot of CPU cycles and
will take a long time to complete, it may impact the QoS of the guest as
well as the host. The overhead will be reduced a lot if batch processing
is used. E.g. If there are several pages whose address are physical
contiguous in the guest, these bulk pages can be processed in one
operation.

The main idea for the optimization is to reduce the above operations as
much as possible. And it can be achieved by using a bitmap instead of an
PFN array. Comparing with PFN array, for a specific size buffer, bitmap
can present more pages, which is very important for batch processing.

Using bitmap instead of PFN is not very helpful when inflating/deflating
a small mount of pages, in this case, using PFNs is better. But using
bitmap will not impact the QoS of guest or host heavily because the
operation will be completed very soon for a small mount of pages, and we
will use some methods to make sure the efficiency not drop too much.

This patch saves the deflated pages to a list instead of the PFN array,
which will allow faster notifications using a bitmap down the road.
balloon_pfn_to_page() can be removed because it's useless.

Signed-off-by: Liang Li <liang.z.li at intel.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Cornelia Huck <cornelia.huck at de.ibm.com>
Cc: Amit Shah <amit.shah at redhat.com>
Cc: Dave Hansen <dave.hansen at intel.com>
---
 drivers/virtio/virtio_balloon.c | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 181793f..f59cb4f 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -103,12 +103,6 @@ static u32 page_to_balloon_pfn(struct page *page)
 	return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
 }
 
-static struct page *balloon_pfn_to_page(u32 pfn)
-{
-	BUG_ON(pfn % VIRTIO_BALLOON_PAGES_PER_PAGE);
-	return pfn_to_page(pfn / VIRTIO_BALLOON_PAGES_PER_PAGE);
-}
-
 static void balloon_ack(struct virtqueue *vq)
 {
 	struct virtio_balloon *vb = vq->vdev->priv;
@@ -181,18 +175,16 @@ static unsigned fill_balloon(struct virtio_balloon *vb,
size_t num)
 	return num_allocated_pages;
 }
 
-static void release_pages_balloon(struct virtio_balloon *vb)
+static void release_pages_balloon(struct virtio_balloon *vb,
+				 struct list_head *pages)
 {
-	unsigned int i;
-	struct page *page;
+	struct page *page, *next;
 
-	/* Find pfns pointing at start of each page, get pages and free them. */
-	for (i = 0; i < vb->num_pfns; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
-		page = balloon_pfn_to_page(virtio32_to_cpu(vb->vdev,
-							   vb->pfns[i]));
+	list_for_each_entry_safe(page, next, pages, lru) {
 		if (!virtio_has_feature(vb->vdev,
 					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 			adjust_managed_page_count(page, 1);
+		list_del(&page->lru);
 		put_page(page); /* balloon reference */
 	}
 }
@@ -202,6 +194,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 	unsigned num_freed_pages;
 	struct page *page;
 	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
+	LIST_HEAD(pages);
 
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
@@ -215,6 +208,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 		if (!page)
 			break;
 		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+		list_add(&page->lru, &pages);
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
 
@@ -226,7 +220,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 	 */
 	if (vb->num_pfns != 0)
 		tell_host(vb, vb->deflate_vq);
-	release_pages_balloon(vb);
+	release_pages_balloon(vb, &pages);
 	mutex_unlock(&vb->balloon_lock);
 	return num_freed_pages;
 }
-- 
1.8.3.1

Add a new feature which supports sending the page information with
a bitmap. The current implementation uses PFNs array, which is not
very efficient. Using bitmap can improve the performance of
inflating/deflating significantly

The page bitmap header will used to tell the host some information
about the page bitmap. e.g. the page size, page bitmap length and
start pfn.

Signed-off-by: Liang Li <liang.z.li at intel.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Cornelia Huck <cornelia.huck at de.ibm.com>
Cc: Amit Shah <amit.shah at redhat.com>
Cc: Dave Hansen <dave.hansen at intel.com>
---
 include/uapi/linux/virtio_balloon.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/uapi/linux/virtio_balloon.h
b/include/uapi/linux/virtio_balloon.h
index 343d7dd..1be4b1f 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -34,6 +34,7 @@
 #define VIRTIO_BALLOON_F_MUST_TELL_HOST	0 /* Tell before reclaiming pages */
 #define VIRTIO_BALLOON_F_STATS_VQ	1 /* Memory Stats virtqueue */
 #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
+#define VIRTIO_BALLOON_F_PAGE_BITMAP	3 /* Send page info with bitmap */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
@@ -82,4 +83,22 @@ struct virtio_balloon_stat {
 	__virtio64 val;
 } __attribute__((packed));
 
+/* Response header structure */
+struct virtio_balloon_resp_hdr {
+	__le64 cmd : 8; /* Distinguish different requests type */
+	__le64 flag: 8; /* Mark status for a specific request type */
+	__le64 id : 16; /* Distinguish requests of a specific type */
+	__le64 data_len: 32; /* Length of the following data, in bytes */
+};
+
+/* Page bitmap header structure */
+struct virtio_balloon_bmap_hdr {
+	struct {
+		__le64 start_pfn : 52; /* start pfn for the bitmap */
+		__le64 page_shift : 6; /* page shift width, in bytes */
+		__le64 bmap_len : 6;  /* bitmap length, in bytes */
+	} head;
+	__le64 bmap[0];
+};
+
 #endif /* _LINUX_VIRTIO_BALLOON_H */
-- 
1.8.3.1

The implementation of the current virtio-balloon is not very
efficient, the time spends on different stages of inflating
the balloon to 7GB of a 8GB idle guest:

a. allocating pages (6.5%)
b. sending PFNs to host (68.3%)
c. address translation (6.1%)
d. madvise (19%)

It takes about 4126ms for the inflating process to complete.
Debugging shows that the bottle neck are the stage b and stage d.

If using a bitmap to send the page info instead of the PFNs, we
can reduce the overhead in stage b quite a lot. Furthermore, we
can do the address translation and call madvise() with a bulk of
RAM pages, instead of the current page per page way, the overhead
of stage c and stage d can also be reduced a lot.

This patch is the kernel side implementation which is intended to
speed up the inflating & deflating process by adding a new feature
to the virtio-balloon device. With this new feature, inflating the
balloon to 7GB of a 8GB idle guest only takes 590ms, the
performance improvement is about 85%.

TODO: optimize stage a by allocating/freeing a chunk of pages
instead of a single page at a time.

Signed-off-by: Liang Li <liang.z.li at intel.com>
Suggested-by: Michael S. Tsirkin <mst at redhat.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Cornelia Huck <cornelia.huck at de.ibm.com>
Cc: Amit Shah <amit.shah at redhat.com>
Cc: Dave Hansen <dave.hansen at intel.com>
---
 drivers/virtio/virtio_balloon.c | 395 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 367 insertions(+), 28 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f59cb4f..c3ddec3 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -42,6 +42,10 @@
 #define OOM_VBALLOON_DEFAULT_PAGES 256
 #define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
 
+#define BALLOON_BMAP_SIZE	(8 * PAGE_SIZE)
+#define PFNS_PER_BMAP		(BALLOON_BMAP_SIZE * BITS_PER_BYTE)
+#define BALLOON_BMAP_COUNT	32
+
 static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
 module_param(oom_pages, int, S_IRUSR | S_IWUSR);
 MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
@@ -67,6 +71,18 @@ struct virtio_balloon {
 
 	/* Number of balloon pages we've told the Host we're not using. */
 	unsigned int num_pages;
+	/* Pointer to the response header. */
+	void *resp_hdr;
+	/* Pointer to the start address of response data. */
+	unsigned long *resp_data;
+	/* Pointer offset of the response data. */
+	unsigned long resp_pos;
+	/* Bitmap and bitmap count used to tell the host the pages */
+	unsigned long *page_bitmap[BALLOON_BMAP_COUNT];
+	/* Number of split page bitmaps */
+	unsigned int nr_page_bmap;
+	/* Used to record the processed pfn range */
+	unsigned long min_pfn, max_pfn, start_pfn, end_pfn;
 	/*
 	 * The pages we've told the Host we're not using are enqueued
 	 * at vb_dev_info->pages list.
@@ -110,20 +126,228 @@ static void balloon_ack(struct virtqueue *vq)
 	wake_up(&vb->acked);
 }
 
-static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
+static inline void init_bmap_pfn_range(struct virtio_balloon *vb)
 {
-	struct scatterlist sg;
+	vb->min_pfn = ULONG_MAX;
+	vb->max_pfn = 0;
+}
+
+static inline void update_bmap_pfn_range(struct virtio_balloon *vb,
+				 struct page *page)
+{
+	unsigned long balloon_pfn = page_to_balloon_pfn(page);
+
+	vb->min_pfn = min(balloon_pfn, vb->min_pfn);
+	vb->max_pfn = max(balloon_pfn, vb->max_pfn);
+}
+
+static void extend_page_bitmap(struct virtio_balloon *vb,
+				unsigned long nr_pfn)
+{
+	int i, bmap_count;
+	unsigned long bmap_len;
+
+	bmap_len = ALIGN(nr_pfn, BITS_PER_LONG) / BITS_PER_BYTE;
+	bmap_len = ALIGN(bmap_len, BALLOON_BMAP_SIZE);
+	bmap_count = min((int)(bmap_len / BALLOON_BMAP_SIZE),
+				 BALLOON_BMAP_COUNT);
+
+	for (i = 1; i < bmap_count; i++) {
+		vb->page_bitmap[i] = kmalloc(BALLOON_BMAP_SIZE, GFP_KERNEL);
+		if (vb->page_bitmap[i])
+			vb->nr_page_bmap++;
+		else
+			break;
+	}
+}
+
+static void free_extended_page_bitmap(struct virtio_balloon *vb)
+{
+	int i, bmap_count = vb->nr_page_bmap;
+
+
+	for (i = 1; i < bmap_count; i++) {
+		kfree(vb->page_bitmap[i]);
+		vb->page_bitmap[i] = NULL;
+		vb->nr_page_bmap--;
+	}
+}
+
+static void kfree_page_bitmap(struct virtio_balloon *vb)
+{
+	int i;
+
+	for (i = 0; i < vb->nr_page_bmap; i++)
+		kfree(vb->page_bitmap[i]);
+}
+
+static void clear_page_bitmap(struct virtio_balloon *vb)
+{
+	int i;
+
+	for (i = 0; i < vb->nr_page_bmap; i++)
+		memset(vb->page_bitmap[i], 0, BALLOON_BMAP_SIZE);
+}
+
+static unsigned long do_set_resp_bitmap(struct virtio_balloon *vb,
+	unsigned long *bitmap,	unsigned long base_pfn,
+	unsigned long pos, int nr_page)
+
+{
+	struct virtio_balloon_bmap_hdr *hdr;
+	unsigned long end, new_pos, new_end, nr_left, proccessed = 0;
+
+	new_pos = pos;
+	new_end = end = pos + nr_page;
+
+	if (pos % BITS_PER_LONG) {
+		unsigned long pos_s;
+
+		pos_s = rounddown(pos, BITS_PER_LONG);
+		hdr = (struct virtio_balloon_bmap_hdr *)(vb->resp_data
+							 + vb->resp_pos);
+		hdr->head.start_pfn = base_pfn + pos_s;
+		hdr->head.page_shift = PAGE_SHIFT;
+		hdr->head.bmap_len = sizeof(unsigned long);
+		hdr->bmap[0] = cpu_to_virtio64(vb->vdev,
+				 bitmap[pos_s / BITS_PER_LONG]);
+		vb->resp_pos += 2;
+		if (pos_s + BITS_PER_LONG >= end)
+			return roundup(end, BITS_PER_LONG) - pos;
+		new_pos = roundup(pos, BITS_PER_LONG);
+	}
+
+	if (end % BITS_PER_LONG) {
+		unsigned long pos_e;
+
+		pos_e = roundup(end, BITS_PER_LONG);
+		hdr = (struct virtio_balloon_bmap_hdr *)(vb->resp_data
+							 + vb->resp_pos);
+		hdr->head.start_pfn = base_pfn + pos_e - BITS_PER_LONG;
+		hdr->head.page_shift = PAGE_SHIFT;
+		hdr->head.bmap_len = sizeof(unsigned long);
+		hdr->bmap[0] = bitmap[pos_e / BITS_PER_LONG - 1];
+		vb->resp_pos += 2;
+		if (new_pos + BITS_PER_LONG >= pos_e)
+			return pos_e - pos;
+		new_end = rounddown(end, BITS_PER_LONG);
+	}
+
+	nr_left = nr_page = new_end - new_pos;
+
+	while (proccessed < nr_page) {
+		int bulk, order;
+
+		order = get_order(nr_left << PAGE_SHIFT);
+		if ((1 << order) > nr_left)
+			order--;
+		hdr = (struct virtio_balloon_bmap_hdr *)(vb->resp_data
+							 + vb->resp_pos);
+		hdr->head.start_pfn = base_pfn + new_pos + proccessed;
+		hdr->head.page_shift = order + PAGE_SHIFT;
+		hdr->head.bmap_len = 0;
+		bulk = 1 << order;
+		nr_left -= bulk;
+		proccessed += bulk;
+		vb->resp_pos++;
+	}
+
+	return roundup(end, BITS_PER_LONG) - pos;
+}
+
+static void send_resp_data(struct virtio_balloon *vb, struct virtqueue *vq,
+			bool busy_wait)
+{
+	struct scatterlist sg[2];
+	struct virtio_balloon_resp_hdr *hdr = vb->resp_hdr;
 	unsigned int len;
 
-	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+	len = hdr->data_len = vb->resp_pos * sizeof(unsigned long);
+	sg_init_table(sg, 2);
+	sg_set_buf(&sg[0], hdr, sizeof(struct virtio_balloon_resp_hdr));
+	sg_set_buf(&sg[1], vb->resp_data, len);
+
+	if (virtqueue_add_outbuf(vq, sg, 2, vb, GFP_KERNEL) == 0) {
+		virtqueue_kick(vq);
+		if (busy_wait)
+			while (!virtqueue_get_buf(vq, &len)
+				&& !virtqueue_is_broken(vq))
+				cpu_relax();
+		else
+			wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+		vb->resp_pos = 0;
+		free_extended_page_bitmap(vb);
+	}
+}
 
-	/* We should always be able to add one buffer to an empty queue. */
-	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
-	virtqueue_kick(vq);
+static void set_bulk_pages(struct virtio_balloon *vb, struct virtqueue *vq,
+		unsigned long start_pfn, unsigned long *bitmap,
+		unsigned long len, bool busy_wait)
+{
+	unsigned long pos = 0, end = len * BITS_PER_BYTE;
+
+	while (pos < end) {
+		unsigned long one = find_next_bit(bitmap, end, pos);
+
+		if ((vb->resp_pos + 64) * sizeof(unsigned long) >
+			 BALLOON_BMAP_SIZE)
+			send_resp_data(vb, vq, busy_wait);
+		if (one < end) {
+			unsigned long pages, zero;
+
+			zero = find_next_zero_bit(bitmap, end, one + 1);
+			if (zero >= end)
+				pages = end - one;
+			else
+				pages = zero - one;
+			if (pages) {
+				pages = do_set_resp_bitmap(vb, bitmap,
+					 start_pfn, one, pages);
+			}
+			pos = one + pages;
+		} else
+			pos = one;
+	}
+}
 
-	/* When host has read buffer, this completes via balloon_ack */
-	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
+{
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP)) {
+		int nr_pfn, nr_used_bmap, i;
+		unsigned long start_pfn, bmap_len;
+
+		start_pfn = vb->start_pfn;
+		nr_pfn = vb->end_pfn - start_pfn + 1;
+		nr_pfn = roundup(nr_pfn, BITS_PER_LONG);
+		nr_used_bmap = nr_pfn / PFNS_PER_BMAP;
+		if (nr_pfn % PFNS_PER_BMAP)
+			nr_used_bmap++;
+		bmap_len = nr_pfn / BITS_PER_BYTE;
+
+		for (i = 0; i < nr_used_bmap; i++) {
+			unsigned int bmap_size = BALLOON_BMAP_SIZE;
+
+			if (i + 1 == nr_used_bmap)
+				bmap_size = bmap_len - BALLOON_BMAP_SIZE * i;
+			set_bulk_pages(vb, vq, start_pfn + i * PFNS_PER_BMAP,
+				 vb->page_bitmap[i], bmap_size, false);
+		}
+		if (vb->resp_pos > 0)
+			send_resp_data(vb, vq, false);
+	} else {
+		struct scatterlist sg;
+		unsigned int len;
 
+		sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+
+		/* We should always be able to add one buffer to an
+		 * empty queue
+		 */
+		virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+		virtqueue_kick(vq);
+		/* When host has read buffer, this completes via balloon_ack */
+		wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+	}
 }
 
 static void set_page_pfns(struct virtio_balloon *vb,
@@ -138,13 +362,59 @@ static void set_page_pfns(struct virtio_balloon *vb,
 					  page_to_balloon_pfn(page) + i);
 }
 
-static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
+static void set_page_bitmap(struct virtio_balloon *vb,
+			 struct list_head *pages, struct virtqueue *vq)
+{
+	unsigned long pfn, pfn_limit;
+	struct page *page;
+	bool found;
+	int bmap_idx;
+
+	vb->min_pfn = rounddown(vb->min_pfn, BITS_PER_LONG);
+	vb->max_pfn = roundup(vb->max_pfn, BITS_PER_LONG);
+	pfn_limit = PFNS_PER_BMAP * vb->nr_page_bmap;
+
+	if (vb->nr_page_bmap == 1)
+		extend_page_bitmap(vb, vb->max_pfn - vb->min_pfn + 1);
+	for (pfn = vb->min_pfn; pfn < vb->max_pfn; pfn += pfn_limit) {
+		unsigned long end_pfn;
+
+		clear_page_bitmap(vb);
+		vb->start_pfn = pfn;
+		end_pfn = pfn;
+		found = false;
+		list_for_each_entry(page, pages, lru) {
+			unsigned long pos, balloon_pfn;
+
+			balloon_pfn = page_to_balloon_pfn(page);
+			if (balloon_pfn < pfn || balloon_pfn >= pfn + pfn_limit)
+				continue;
+			bmap_idx = (balloon_pfn - pfn) / PFNS_PER_BMAP;
+			pos = (balloon_pfn - pfn) % PFNS_PER_BMAP;
+			set_bit(pos, vb->page_bitmap[bmap_idx]);
+			if (balloon_pfn > end_pfn)
+				end_pfn = balloon_pfn;
+			found = true;
+		}
+		if (found) {
+			vb->end_pfn = end_pfn;
+			tell_host(vb, vq);
+		}
+	}
+}
+
+static unsigned int fill_balloon(struct virtio_balloon *vb, size_t num)
 {
 	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
-	unsigned num_allocated_pages;
+	unsigned int num_allocated_pages;
+	bool use_bmap = virtio_has_feature(vb->vdev,
+				 VIRTIO_BALLOON_F_PAGE_BITMAP);
 
-	/* We can only do one array worth at a time. */
-	num = min(num, ARRAY_SIZE(vb->pfns));
+	if (use_bmap)
+		init_bmap_pfn_range(vb);
+	else
+		/* We can only do one array worth at a time. */
+		num = min(num, ARRAY_SIZE(vb->pfns));
 
 	mutex_lock(&vb->balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
@@ -159,7 +429,10 @@ static unsigned fill_balloon(struct virtio_balloon *vb,
size_t num)
 			msleep(200);
 			break;
 		}
-		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+		if (use_bmap)
+			update_bmap_pfn_range(vb, page);
+		else
+			set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		if (!virtio_has_feature(vb->vdev,
 					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
@@ -168,8 +441,13 @@ static unsigned fill_balloon(struct virtio_balloon *vb,
size_t num)
 
 	num_allocated_pages = vb->num_pfns;
 	/* Did we get any? */
-	if (vb->num_pfns != 0)
-		tell_host(vb, vb->inflate_vq);
+	if (vb->num_pfns != 0) {
+		if (use_bmap)
+			set_page_bitmap(vb, &vb_dev_info->pages,
+					vb->inflate_vq);
+		else
+			tell_host(vb, vb->inflate_vq);
+	}
 	mutex_unlock(&vb->balloon_lock);
 
 	return num_allocated_pages;
@@ -189,15 +467,20 @@ static void release_pages_balloon(struct virtio_balloon
*vb,
 	}
 }
 
-static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned int leak_balloon(struct virtio_balloon *vb, size_t num)
 {
-	unsigned num_freed_pages;
+	unsigned int num_freed_pages;
 	struct page *page;
 	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
 	LIST_HEAD(pages);
+	bool use_bmap = virtio_has_feature(vb->vdev,
+			 VIRTIO_BALLOON_F_PAGE_BITMAP);
 
-	/* We can only do one array worth at a time. */
-	num = min(num, ARRAY_SIZE(vb->pfns));
+	if (use_bmap)
+		init_bmap_pfn_range(vb);
+	else
+		/* We can only do one array worth at a time. */
+		num = min(num, ARRAY_SIZE(vb->pfns));
 
 	mutex_lock(&vb->balloon_lock);
 	/* We can't release more pages than taken */
@@ -207,7 +490,10 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 		page = balloon_page_dequeue(vb_dev_info);
 		if (!page)
 			break;
-		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+		if (use_bmap)
+			update_bmap_pfn_range(vb, page);
+		else
+			set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
 		list_add(&page->lru, &pages);
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
@@ -218,8 +504,12 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
 	 * is true, we *have* to do it in this order
 	 */
-	if (vb->num_pfns != 0)
-		tell_host(vb, vb->deflate_vq);
+	if (vb->num_pfns != 0) {
+		if (use_bmap)
+			set_page_bitmap(vb, &pages, vb->deflate_vq);
+		else
+			tell_host(vb, vb->deflate_vq);
+	}
 	release_pages_balloon(vb, &pages);
 	mutex_unlock(&vb->balloon_lock);
 	return num_freed_pages;
@@ -431,6 +721,20 @@ static int init_vqs(struct virtio_balloon *vb)
 }
 
 #ifdef CONFIG_BALLOON_COMPACTION
+static void tell_host_one_page(struct virtio_balloon *vb,
+	struct virtqueue *vq, struct page *page)
+{
+	struct virtio_balloon_bmap_hdr *bmap_hdr;
+
+	bmap_hdr = (struct virtio_balloon_bmap_hdr *)(vb->resp_data
+							 + vb->resp_pos);
+	bmap_hdr->head.start_pfn = page_to_pfn(page);
+	bmap_hdr->head.page_shift = PAGE_SHIFT;
+	bmap_hdr->head.bmap_len = 0;
+	vb->resp_pos++;
+	send_resp_data(vb, vq, false);
+}
+
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
  *			     a compation thread.     (called under page lock)
@@ -455,6 +759,8 @@ static int virtballoon_migratepage(struct balloon_dev_info
*vb_dev_info,
 	struct virtio_balloon *vb = container_of(vb_dev_info,
 			struct virtio_balloon, vb_dev_info);
 	unsigned long flags;
+	bool use_bmap = virtio_has_feature(vb->vdev,
+				 VIRTIO_BALLOON_F_PAGE_BITMAP);
 
 	/*
 	 * In order to avoid lock contention while migrating pages concurrently
@@ -475,15 +781,23 @@ static int virtballoon_migratepage(struct balloon_dev_info
*vb_dev_info,
 	vb_dev_info->isolated_pages--;
 	__count_vm_event(BALLOON_MIGRATE);
 	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
-	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
-	set_page_pfns(vb, vb->pfns, newpage);
-	tell_host(vb, vb->inflate_vq);
+	if (use_bmap)
+		tell_host_one_page(vb, vb->inflate_vq, newpage);
+	else {
+		vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+		set_page_pfns(vb, vb->pfns, newpage);
+		tell_host(vb, vb->inflate_vq);
+	}
 
 	/* balloon's page migration 2nd step -- deflate "page" */
 	balloon_page_delete(page);
-	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
-	set_page_pfns(vb, vb->pfns, page);
-	tell_host(vb, vb->deflate_vq);
+	if (use_bmap)
+		tell_host_one_page(vb, vb->deflate_vq, page);
+	else {
+		vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+		set_page_pfns(vb, vb->pfns, page);
+		tell_host(vb, vb->deflate_vq);
+	}
 
 	mutex_unlock(&vb->balloon_lock);
 
@@ -533,6 +847,28 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	spin_lock_init(&vb->stop_update_lock);
 	vb->stop_update = false;
 	vb->num_pages = 0;
+	vb->resp_hdr = kzalloc(sizeof(struct virtio_balloon_resp_hdr),
+				 GFP_KERNEL);
+	/* Clear the feature bit if memory allocation fails */
+	if (!vb->resp_hdr)
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
+	else {
+		vb->page_bitmap[0] = kmalloc(BALLOON_BMAP_SIZE, GFP_KERNEL);
+		if (!vb->page_bitmap[0]) {
+			__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
+			kfree(vb->resp_hdr);
+		} else {
+			vb->nr_page_bmap = 1;
+			vb->resp_data = kmalloc(BALLOON_BMAP_SIZE, GFP_KERNEL);
+			if (!vb->resp_data) {
+				__virtio_clear_bit(vdev,
+						VIRTIO_BALLOON_F_PAGE_BITMAP);
+				kfree(vb->page_bitmap[0]);
+				kfree(vb->resp_hdr);
+			}
+		}
+	}
+	vb->resp_pos = 0;
 	mutex_init(&vb->balloon_lock);
 	init_waitqueue_head(&vb->acked);
 	vb->vdev = vdev;
@@ -611,6 +947,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	remove_common(vb);
 	if (vb->vb_dev_info.inode)
 		iput(vb->vb_dev_info.inode);
+	kfree_page_bitmap(vb);
+	kfree(vb->resp_hdr);
 	kfree(vb);
 }
 
@@ -649,6 +987,7 @@ static int virtballoon_restore(struct virtio_device *vdev)
 	VIRTIO_BALLOON_F_MUST_TELL_HOST,
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
+	VIRTIO_BALLOON_F_PAGE_BITMAP,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
-- 
1.8.3.1

Define the flags and head struct for a new host request virtual
queue. Guest can get requests from host and then responds to them on
this new virtual queue.
Host can make use of this virtual queue to request the guest do some
operations, e.g. drop page cache, synchronize file system, etc.
And the hypervisor can get some of guest's runtime information
through this virtual queue too, e.g. the guest's unused page
information, which can be used for live migration optimization.

Signed-off-by: Liang Li <liang.z.li at intel.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Cornelia Huck <cornelia.huck at de.ibm.com>
Cc: Amit Shah <amit.shah at redhat.com>
Cc: Dave Hansen <dave.hansen at intel.com>
---
 include/uapi/linux/virtio_balloon.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/uapi/linux/virtio_balloon.h
b/include/uapi/linux/virtio_balloon.h
index 1be4b1f..5ac3a40 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -35,6 +35,7 @@
 #define VIRTIO_BALLOON_F_STATS_VQ	1 /* Memory Stats virtqueue */
 #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
 #define VIRTIO_BALLOON_F_PAGE_BITMAP	3 /* Send page info with bitmap */
+#define VIRTIO_BALLOON_F_HOST_REQ_VQ	4 /* Host request virtqueue */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
@@ -101,4 +102,25 @@ struct virtio_balloon_bmap_hdr {
 	__le64 bmap[0];
 };
 
+enum virtio_balloon_req_id {
+	/* Get unused page information */
+	BALLOON_GET_UNUSED_PAGES,
+};
+
+enum virtio_balloon_flag {
+	/* Have more data for a request */
+	BALLOON_FLAG_CONT,
+	/* No more data for a request */
+	BALLOON_FLAG_DONE,
+};
+
+struct virtio_balloon_req_hdr {
+	/* Used to distinguish different requests */
+	__le16 cmd;
+	/* Reserved */
+	__le16 reserved[3];
+	/* Request parameter */
+	__le64 param;
+};
+
 #endif /* _LINUX_VIRTIO_BALLOON_H */
-- 
1.8.3.1

This patch contains two parts:

One is to add a new API to mm go get the unused page information.
The virtio balloon driver will use this new API added to get the
unused page info and send it to hypervisor(QEMU) to speed up live
migration. During sending the bitmap, some the pages may be modified
and are used by the guest, this inaccuracy can be corrected by the
dirty page logging mechanism.

One is to add support the request for vm's unused page information,
QEMU can make use of unused page information and the dirty page
logging mechanism to skip the transportation of some of these unused
pages, this is very helpful to reduce the network traffic and speed
up the live migration process.

Signed-off-by: Liang Li <liang.z.li at intel.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Cornelia Huck <cornelia.huck at de.ibm.com>
Cc: Amit Shah <amit.shah at redhat.com>
Cc: Dave Hansen <dave.hansen at intel.com>
---
 drivers/virtio/virtio_balloon.c | 126 +++++++++++++++++++++++++++++++++++++---
 include/linux/mm.h              |   3 +-
 mm/page_alloc.c                 |  72 +++++++++++++++++++++++
 3 files changed, 193 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index c3ddec3..2626cc0 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -56,7 +56,7 @@
 
 struct virtio_balloon {
 	struct virtio_device *vdev;
-	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
+	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *req_vq;
 
 	/* The balloon servicing is delegated to a freezable workqueue. */
 	struct work_struct update_balloon_stats_work;
@@ -75,6 +75,8 @@ struct virtio_balloon {
 	void *resp_hdr;
 	/* Pointer to the start address of response data. */
 	unsigned long *resp_data;
+	/* Size of response data buffer. */
+	unsigned long resp_buf_size;
 	/* Pointer offset of the response data. */
 	unsigned long resp_pos;
 	/* Bitmap and bitmap count used to tell the host the pages */
@@ -83,6 +85,8 @@ struct virtio_balloon {
 	unsigned int nr_page_bmap;
 	/* Used to record the processed pfn range */
 	unsigned long min_pfn, max_pfn, start_pfn, end_pfn;
+	/* Request header */
+	struct virtio_balloon_req_hdr req_hdr;
 	/*
 	 * The pages we've told the Host we're not using are enqueued
 	 * at vb_dev_info->pages list.
@@ -551,6 +555,58 @@ static void update_balloon_stats(struct virtio_balloon *vb)
 				pages_to_bytes(available));
 }
 
+static void send_unused_pages_info(struct virtio_balloon *vb,
+				unsigned long req_id)
+{
+	struct scatterlist sg_in;
+	unsigned long pos = 0;
+	struct virtqueue *vq = vb->req_vq;
+	struct virtio_balloon_resp_hdr *hdr = vb->resp_hdr;
+	int ret, order;
+
+	mutex_lock(&vb->balloon_lock);
+
+	for (order = MAX_ORDER - 1; order >= 0; order--) {
+		pos = 0;
+		ret = get_unused_pages(vb->resp_data,
+			 vb->resp_buf_size / sizeof(unsigned long),
+			 order, &pos);
+		if (ret == -ENOSPC) {
+			void *new_resp_data;
+
+			new_resp_data = kmalloc(2 * vb->resp_buf_size,
+						GFP_KERNEL);
+			if (new_resp_data) {
+				kfree(vb->resp_data);
+				vb->resp_data = new_resp_data;
+				vb->resp_buf_size *= 2;
+				order++;
+				continue;
+			} else
+				dev_warn(&vb->vdev->dev,
+					 "%s: omit some %d order pages\n",
+					 __func__, order);
+		}
+
+		if (pos > 0) {
+			vb->resp_pos = pos;
+			hdr->cmd = BALLOON_GET_UNUSED_PAGES;
+			hdr->id = req_id;
+			if (order > 0)
+				hdr->flag = BALLOON_FLAG_CONT;
+			else
+				hdr->flag = BALLOON_FLAG_DONE;
+
+			send_resp_data(vb, vq, true);
+		}
+	}
+
+	mutex_unlock(&vb->balloon_lock);
+	sg_init_one(&sg_in, &vb->req_hdr, sizeof(vb->req_hdr));
+	virtqueue_add_inbuf(vq, &sg_in, 1, &vb->req_hdr, GFP_KERNEL);
+	virtqueue_kick(vq);
+}
+
 /*
  * While most virtqueues communicate guest-initiated requests to the
hypervisor,
  * the stats queue operates in reverse.  The driver initializes the virtqueue
@@ -685,18 +741,56 @@ static void update_balloon_size_func(struct work_struct
*work)
 		queue_work(system_freezable_wq, work);
 }
 
+static void misc_handle_rq(struct virtio_balloon *vb)
+{
+	struct virtio_balloon_req_hdr *ptr_hdr;
+	unsigned int len;
+
+	ptr_hdr = virtqueue_get_buf(vb->req_vq, &len);
+	if (!ptr_hdr || len != sizeof(vb->req_hdr))
+		return;
+
+	switch (ptr_hdr->cmd) {
+	case BALLOON_GET_UNUSED_PAGES:
+		send_unused_pages_info(vb, ptr_hdr->param);
+		break;
+	default:
+		break;
+	}
+}
+
+static void misc_request(struct virtqueue *vq)
+{
+	struct virtio_balloon *vb = vq->vdev->priv;
+
+	misc_handle_rq(vb);
+}
+
 static int init_vqs(struct virtio_balloon *vb)
 {
-	struct virtqueue *vqs[3];
-	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
-	static const char * const names[] = { "inflate",
"deflate", "stats" };
+	struct virtqueue *vqs[4];
+	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack,
+					 stats_request, misc_request };
+	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_HOST_REQ_VQ))
+		nvqs = 4;
+	else if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
+		nvqs = 3;
+	else
+		nvqs = 2;
+
+	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
+		__virtio_clear_bit(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
+		__virtio_clear_bit(vb->vdev, VIRTIO_BALLOON_F_HOST_REQ_VQ);
+	}
+
 	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks,
names);
 	if (err)
 		return err;
@@ -717,6 +811,18 @@ static int init_vqs(struct virtio_balloon *vb)
 			BUG();
 		virtqueue_kick(vb->stats_vq);
 	}
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HOST_REQ_VQ)) {
+		struct scatterlist sg_in;
+
+		vb->req_vq = vqs[3];
+		sg_init_one(&sg_in, &vb->req_hdr, sizeof(vb->req_hdr));
+		if (virtqueue_add_inbuf(vb->req_vq, &sg_in, 1,
+		    &vb->req_hdr, GFP_KERNEL) < 0)
+			__virtio_clear_bit(vb->vdev,
+					VIRTIO_BALLOON_F_HOST_REQ_VQ);
+		else
+			virtqueue_kick(vb->req_vq);
+	}
 	return 0;
 }
 
@@ -850,11 +956,13 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	vb->resp_hdr = kzalloc(sizeof(struct virtio_balloon_resp_hdr),
 				 GFP_KERNEL);
 	/* Clear the feature bit if memory allocation fails */
-	if (!vb->resp_hdr)
+	if (!vb->resp_hdr) {
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
-	else {
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_HOST_REQ_VQ);
+	} else {
 		vb->page_bitmap[0] = kmalloc(BALLOON_BMAP_SIZE, GFP_KERNEL);
 		if (!vb->page_bitmap[0]) {
+			__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_HOST_REQ_VQ);
 			__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_BITMAP);
 			kfree(vb->resp_hdr);
 		} else {
@@ -863,12 +971,15 @@ static int virtballoon_probe(struct virtio_device *vdev)
 			if (!vb->resp_data) {
 				__virtio_clear_bit(vdev,
 						VIRTIO_BALLOON_F_PAGE_BITMAP);
+				__virtio_clear_bit(vdev,
+						VIRTIO_BALLOON_F_HOST_REQ_VQ);
 				kfree(vb->page_bitmap[0]);
 				kfree(vb->resp_hdr);
 			}
 		}
 	}
 	vb->resp_pos = 0;
+	vb->resp_buf_size = BALLOON_BMAP_SIZE;
 	mutex_init(&vb->balloon_lock);
 	init_waitqueue_head(&vb->acked);
 	vb->vdev = vdev;
@@ -988,6 +1099,7 @@ static int virtballoon_restore(struct virtio_device *vdev)
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
 	VIRTIO_BALLOON_F_PAGE_BITMAP,
+	VIRTIO_BALLOON_F_HOST_REQ_VQ,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index a92c8d7..e05ca86 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1772,7 +1772,8 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm,
pmd_t *pmd)
 extern void free_area_init_node(int nid, unsigned long * zones_size,
 		unsigned long zone_start_pfn, unsigned long *zholes_size);
 extern void free_initmem(void);
-
+extern int get_unused_pages(unsigned long *unused_pages, unsigned long size,
+				int order, unsigned long *pos);
 /*
  * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
  * into the buddy system. The freed pages will be poisoned with pattern
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6de9440..d5a5952 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4428,6 +4428,78 @@ void show_free_areas(unsigned int filter)
 	show_swap_cache_info();
 }
 
+struct page_info_item {
+	__le64 start_pfn : 52; /* start pfn for the bitmap */
+	__le64 page_shift : 6; /* page shift width, in bytes */
+	__le64 bmap_len : 6;  /* bitmap length, in bytes */
+};
+
+static int  mark_unused_pages(struct zone *zone,
+		unsigned long *unused_pages, unsigned long size,
+		int order, unsigned long *pos)
+{
+	unsigned long pfn, flags;
+	unsigned int t;
+	struct list_head *curr;
+	struct page_info_item *info;
+
+	if (zone_is_empty(zone))
+		return 0;
+
+	spin_lock_irqsave(&zone->lock, flags);
+
+	if (*pos + zone->free_area[order].nr_free > size)
+		return -ENOSPC;
+	for (t = 0; t < MIGRATE_TYPES; t++) {
+		list_for_each(curr, &zone->free_area[order].free_list[t]) {
+			pfn = page_to_pfn(list_entry(curr, struct page, lru));
+			info = (struct page_info_item *)(unused_pages + *pos);
+			info->start_pfn = pfn;
+			info->page_shift = order + PAGE_SHIFT;
+			*pos += 1;
+		}
+	}
+
+	spin_unlock_irqrestore(&zone->lock, flags);
+
+	return 0;
+}
+
+/*
+ * During live migration, page is always discardable unless it's
+ * content is needed by the system.
+ * get_unused_pages provides an API to get the unused pages, these
+ * unused pages can be discarded if there is no modification since
+ * the request. Some other mechanism, like the dirty page logging
+ * can be used to track the modification.
+ *
+ * This function scans the free page list to get the unused pages
+ * with the specified order, and set the corresponding element in
+ * the bitmap if an unused page is found.
+ *
+ * return -EINVAL if parameters are invalid
+ * return -ENOSPC when bitmap can't contain the pages
+ * return 0 when sccess
+ */
+int get_unused_pages(unsigned long *unused_pages, unsigned long size,
+		int order, unsigned long *pos)
+{
+	struct zone *zone;
+	int ret = 0;
+
+	if (unused_pages == NULL || pos == NULL || *pos < 0)
+		return -EINVAL;
+
+	for_each_populated_zone(zone) {
+		ret = mark_unused_pages(zone, unused_pages, size, order, pos);
+		if (ret < 0)
+			break;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(get_unused_pages);
+
 static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
 {
 	zoneref->zone = zone;
-- 
1.8.3.1

On 11/30/2016 12:43 AM, Liang Li wrote:
> +static void send_unused_pages_info(struct virtio_balloon *vb,
> +				unsigned long req_id)
> +{
> +	struct scatterlist sg_in;
> +	unsigned long pos = 0;
> +	struct virtqueue *vq = vb->req_vq;
> +	struct virtio_balloon_resp_hdr *hdr = vb->resp_hdr;
> +	int ret, order;
> +
> +	mutex_lock(&vb->balloon_lock);
> +
> +	for (order = MAX_ORDER - 1; order >= 0; order--) {
I scratched my head for a bit on this one.  Why are you walking over
orders, *then* zones.  I *think* you're doing it because you can
efficiently fill the bitmaps at a given order for all zones, then move
to a new bitmap.  But, it would be interesting to document this.
> +		pos = 0;
> +		ret = get_unused_pages(vb->resp_data,
> +			 vb->resp_buf_size / sizeof(unsigned long),
> +			 order, &pos);
FWIW, get_unsued_pages() is a pretty bad name.  "get" usually implies
bumping reference counts or consuming something.  You're just
"recording" or "marking" them.
> +		if (ret == -ENOSPC) {
> +			void *new_resp_data;
> +
> +			new_resp_data = kmalloc(2 * vb->resp_buf_size,
> +						GFP_KERNEL);
> +			if (new_resp_data) {
> +				kfree(vb->resp_data);
> +				vb->resp_data = new_resp_data;
> +				vb->resp_buf_size *= 2;
What happens to the data in ->resp_data at this point?  Doesn't this
just throw it away?

...
> +struct page_info_item {
> +	__le64 start_pfn : 52; /* start pfn for the bitmap */
> +	__le64 page_shift : 6; /* page shift width, in bytes */
> +	__le64 bmap_len : 6;  /* bitmap length, in bytes */
> +};
Is 'bmap_len' too short?  a 64-byte buffer is a bit tiny.  Right?
> +static int  mark_unused_pages(struct zone *zone,
> +		unsigned long *unused_pages, unsigned long size,
> +		int order, unsigned long *pos)
> +{
> +	unsigned long pfn, flags;
> +	unsigned int t;
> +	struct list_head *curr;
> +	struct page_info_item *info;
> +
> +	if (zone_is_empty(zone))
> +		return 0;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +
> +	if (*pos + zone->free_area[order].nr_free > size)
> +		return -ENOSPC;
Urg, so this won't partially fill?  So, what the nr_free pages limit
where we no longer fit in the kmalloc()'d buffer where this simply won't
work?
> +	for (t = 0; t < MIGRATE_TYPES; t++) {
> +		list_for_each(curr, &zone->free_area[order].free_list[t]) {
> +			pfn = page_to_pfn(list_entry(curr, struct page, lru));
> +			info = (struct page_info_item *)(unused_pages + *pos);
> +			info->start_pfn = pfn;
> +			info->page_shift = order + PAGE_SHIFT;
> +			*pos += 1;
> +		}
> +	}
Do we need to fill in ->bmap_len here?

Am 30.11.2016 um 09:43 schrieb Liang Li:
> This patch set contains two parts of changes to the virtio-balloon.
>
> One is the change for speeding up the inflating & deflating process,
> the main idea of this optimization is to use bitmap to send the page
> information to host instead of the PFNs, to reduce the overhead of
> virtio data transmission, address translation and madvise(). This can
> help to improve the performance by about 85%.
Do you have some statistics/some rough feeling how many consecutive bits 
are usually set in the bitmaps? Is it really just purely random or is 
there some granularity that is usually consecutive?

IOW in real examples, do we have really large consecutive areas or are 
all pages just completely distributed over our memory?

Thanks!

-- 

David

> Am 30.11.2016 um 09:43 schrieb Liang Li:
> > This patch set contains two parts of changes to the virtio-balloon.
> >
> > One is the change for speeding up the inflating & deflating
process,
> > the main idea of this optimization is to use bitmap to send the page
> > information to host instead of the PFNs, to reduce the overhead of
> > virtio data transmission, address translation and madvise(). This can
> > help to improve the performance by about 85%.
> 
> Do you have some statistics/some rough feeling how many consecutive bits
are
> usually set in the bitmaps? Is it really just purely random or is there
some
> granularity that is usually consecutive?
> 
I did something similar. Filled the balloon with 15GB for a 16GB idle guest, by
using bitmap, the madvise count was reduced to 605. when using the PFNs, the
madvise count
was 3932160. It means there are quite a lot consecutive bits in the bitmap.
I didn't test for a guest with heavy memory workload. 
> IOW in real examples, do we have really large consecutive areas or are all
> pages just completely distributed over our memory?
> 
The buddy system of Linux kernel memory management shows there should be quite a
lot of
 consecutive pages as long as there are a portion of free memory in the guest.
If all pages just completely distributed over our memory, it means the memory 
fragmentation is very serious, the kernel has the mechanism to avoid this
happened.
In the other hand, the inflating should not happen at this time because the
guest is almost
'out of memory'.

Liang
> Thanks!
> 
> --
> 
> David