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2018-01-24 Thread Solen win 

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Re: [PATCH v24 2/2] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

2018-01-24 Thread Wei Wang 

On 01/25/2018 01:15 AM, Michael S. Tsirkin wrote:

On Wed, Jan 24, 2018 at 06:42:42PM +0800, Wei Wang wrote:
+
+static void report_free_page_func(struct work_struct *work)
+{
+   struct virtio_balloon *vb;
+   unsigned long flags;
+
+   vb = container_of(work, struct virtio_balloon, report_free_page_work);
+
+   /* Start by sending the obtained cmd id to the host with an outbuf */
+   send_cmd_id(vb, >start_cmd_id);
+
+   /*
+* Set start_cmd_id to VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID to
+* indicate a new request can be queued.
+*/
+   spin_lock_irqsave(>stop_update_lock, flags);
+   vb->start_cmd_id = cpu_to_virtio32(vb->vdev,
+   VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
+   spin_unlock_irqrestore(>stop_update_lock, flags);
+
+   walk_free_mem_block(vb, 0, _balloon_send_free_pages);
Can you teach walk_free_mem_block to return the && of all
return calls, so caller knows whether it completed?


There will be two cases that can cause walk_free_mem_block to return 
without completing:

1) host requests to stop in advance
2) vq->broken

How about letting walk_free_mem_block simply return the value returned 
by its callback (i.e. virtio_balloon_send_free_pages)?


For host requests to stop, it returns "1", and the above only bails out 
when walk_free_mem_block return a "< 0" value.


Best,
Wei
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Re: [PATCH net 1/2] vhost: use mutex_lock_nested() in vhost_dev_lock_vqs()

2018-01-24 Thread David Miller 
From: "Michael S. Tsirkin" 
Date: Wed, 24 Jan 2018 23:46:19 +0200

> On Wed, Jan 24, 2018 at 04:38:30PM -0500, David Miller wrote:
>> From: Jason Wang 
>> Date: Tue, 23 Jan 2018 17:27:25 +0800
>> 
>> > We used to call mutex_lock() in vhost_dev_lock_vqs() which tries to
>> > hold mutexes of all virtqueues. This may confuse lockdep to report a
>> > possible deadlock because of trying to hold locks belong to same
>> > class. Switch to use mutex_lock_nested() to avoid false positive.
>> > 
>> > Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
>> > Reported-by: syzbot+dbb7c1161485e61b0...@syzkaller.appspotmail.com
>> > Signed-off-by: Jason Wang 
>> 
>> Michael, I see you ACK'd this, meaning that you're OK with these two
>> fixes going via my net tree?
>> 
>> Thanks.
> 
> Yes - this seems to be what Jason wanted (judging by the net
> tag in the subject) and I'm fine with it.
> Thanks a lot.

Great, not a problem, done.
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Re: [PATCH net 1/2] vhost: use mutex_lock_nested() in vhost_dev_lock_vqs()

2018-01-24 Thread Michael S. Tsirkin 
On Wed, Jan 24, 2018 at 04:38:30PM -0500, David Miller wrote:
> From: Jason Wang 
> Date: Tue, 23 Jan 2018 17:27:25 +0800
> 
> > We used to call mutex_lock() in vhost_dev_lock_vqs() which tries to
> > hold mutexes of all virtqueues. This may confuse lockdep to report a
> > possible deadlock because of trying to hold locks belong to same
> > class. Switch to use mutex_lock_nested() to avoid false positive.
> > 
> > Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> > Reported-by: syzbot+dbb7c1161485e61b0...@syzkaller.appspotmail.com
> > Signed-off-by: Jason Wang 
> 
> Michael, I see you ACK'd this, meaning that you're OK with these two
> fixes going via my net tree?
> 
> Thanks.

Yes - this seems to be what Jason wanted (judging by the net
tag in the subject) and I'm fine with it.
Thanks a lot.

-- 
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Re: [PATCH net 1/2] vhost: use mutex_lock_nested() in vhost_dev_lock_vqs()

2018-01-24 Thread David Miller 
From: Jason Wang 
Date: Tue, 23 Jan 2018 17:27:25 +0800

> We used to call mutex_lock() in vhost_dev_lock_vqs() which tries to
> hold mutexes of all virtqueues. This may confuse lockdep to report a
> possible deadlock because of trying to hold locks belong to same
> class. Switch to use mutex_lock_nested() to avoid false positive.
> 
> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> Reported-by: syzbot+dbb7c1161485e61b0...@syzkaller.appspotmail.com
> Signed-off-by: Jason Wang 

Michael, I see you ACK'd this, meaning that you're OK with these two
fixes going via my net tree?

Thanks.
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Re: [PATCH v24 2/2] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

2018-01-24 Thread Michael S. Tsirkin 
On Wed, Jan 24, 2018 at 06:42:42PM +0800, Wei Wang wrote:
> Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_HINT feature indicates the
> support of reporting hints of guest free pages to host via virtio-balloon.
> 
> Host requests the guest to report free pages by sending a new cmd
> id to the guest via the free_page_report_cmd_id configuration register.
> 
> When the guest starts to report, the first element added to the free page
> vq is the cmd id given by host. When the guest finishes the reporting
> of all the free pages, VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID is added
> to the vq to tell host that the reporting is done. Host may also requests
> the guest to stop the reporting in advance by sending the stop cmd id to
> the guest via the configuration register.
> 
> Signed-off-by: Wei Wang 
> Signed-off-by: Liang Li 
> Cc: Michael S. Tsirkin 
> Cc: Michal Hocko 
> ---
>  drivers/virtio/virtio_balloon.c | 265 
> +++-
>  include/uapi/linux/virtio_balloon.h |   7 +
>  2 files changed, 236 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index a1fb52c..4440873 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -51,9 +51,21 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
>  static struct vfsmount *balloon_mnt;
>  #endif
>  
> +/* The number of virtqueues supported by virtio-balloon */
> +#define VIRTIO_BALLOON_VQ_NUM4
> +#define VIRTIO_BALLOON_VQ_ID_INFLATE 0
> +#define VIRTIO_BALLOON_VQ_ID_DEFLATE 1
> +#define VIRTIO_BALLOON_VQ_ID_STATS   2
> +#define VIRTIO_BALLOON_VQ_ID_FREE_PAGE   3
> +

Please do an enum instead of defines. VQ_ID can be just VQ
(it's not an ID, it's just the number).


>  struct virtio_balloon {
>   struct virtio_device *vdev;
> - struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
> + struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
> +
> + /* Balloon's own wq for cpu-intensive work items */
> + struct workqueue_struct *balloon_wq;
> + /* The free page reporting work item submitted to the balloon wq */
> + struct work_struct report_free_page_work;
>  
>   /* The balloon servicing is delegated to a freezable workqueue. */
>   struct work_struct update_balloon_stats_work;
> @@ -63,6 +75,13 @@ struct virtio_balloon {
>   spinlock_t stop_update_lock;
>   bool stop_update;
>  
> + /* Start to report free pages */
> + bool report_free_page;
> + /* Stores the cmd id given by host to start the free page reporting */
> + __virtio32 start_cmd_id;
> + /* Stores STOP_ID as a sign to tell host that the reporting is done */
> + __virtio32 stop_cmd_id;
> +
>   /* Waiting for host to ack the pages we released. */
>   wait_queue_head_t acked;
>  
> @@ -281,6 +300,53 @@ static unsigned int update_balloon_stats(struct 
> virtio_balloon *vb)
>   return idx;
>  }
>  
> +static int add_one_sg(struct virtqueue *vq, unsigned long pfn, uint32_t len)
> +{
> + struct scatterlist sg;
> + unsigned int unused;
> + int ret = 0;
> +
> + sg_init_table(, 1);
> + sg_set_page(, pfn_to_page(pfn), len, 0);
> +
> + /* Detach all the used buffers from the vq */
> + while (virtqueue_get_buf(vq, ))
> + ;
> +
> + /*
> +  * Since this is an optimization feature, losing a couple of free
> +  * pages to report isn't important. We simply return without adding
> +  * the page if the vq is full.
> +  * We are adding one entry each time, which essentially results in no
> +  * memory allocation, so the GFP_KERNEL flag below can be ignored.
> +  * There is always one entry reserved for the cmd id to use.
> +  */
> + if (vq->num_free > 1)
> + ret = virtqueue_add_inbuf(vq, , 1, vq, GFP_KERNEL);
> +
> + if (vq->num_free < virtqueue_get_vring_size(vq) / 2)
> + virtqueue_kick(vq);
> +
> + return ret;
> +}
> +
> +static void send_cmd_id(struct virtio_balloon *vb, __virtio32 *cmd_id)
> +{
> + struct scatterlist sg;
> + struct virtqueue *vq = vb->free_page_vq;
> +
> + if (unlikely(!virtio_has_feature(vb->vdev,
> +  VIRTIO_BALLOON_F_FREE_PAGE_HINT)))
> + return;
> +
> + sg_init_one(, cmd_id, sizeof(*cmd_id));
> +
> + if (virtqueue_add_outbuf(vq, , 1, vb, GFP_KERNEL))
> + __virtio_clear_bit(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT);

What is this doing? Basically handling the case where vq is broken?
It's kind of ugly to tweak feature bits, most code assumes they never
change.  Please just return an error to caller instead and handle it
there.

You can then avoid sprinking the check for the feature bit
all over the code.

> +
> + virtqueue_kick(vq);
> +}
> +
>  /*
>   * While most virtqueues communicate

Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

2018-01-24 Thread Wei Wang 

On 01/24/2018 12:29 PM, Michael S. Tsirkin wrote:

On Mon, Jan 22, 2018 at 07:25:45PM +0800, Wei Wang wrote:

On 01/19/2018 08:39 PM, Michael S. Tsirkin wrote:

On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote:

On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:

On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:


+   vb->start_cmd_id = cmd_id;
+   queue_work(vb->balloon_wq, >report_free_page_work);

It seems that if a command was already queued (with a different id),
this will result in new command id being sent to host twice, which will
likely confuse the host.

I think that case won't happen, because
- the host sends a cmd id to the guest via the config, while the guest acks
back the received cmd id via the virtqueue;
- the guest ack back a cmd id only when a new cmd id is received from the
host, that is the above check:

  if (cmd_id != vb->start_cmd_id) { --> the driver only queues the
reporting work only when a new cmd id is received
  /*
   * Host requests to start the reporting by sending a
   * new cmd id.
   */
  WRITE_ONCE(vb->report_free_page, true);
  vb->start_cmd_id = cmd_id;
  queue_work(vb->balloon_wq,
>report_free_page_work);
  }

So the same cmd id wouldn't queue the reporting work twice.


Like this:

vb->start_cmd_id = cmd_id;
queue_work(vb->balloon_wq, >report_free_page_work);

command id changes

vb->start_cmd_id = cmd_id;

work executes

queue_work(vb->balloon_wq, >report_free_page_work);

work executes again


If we think about the whole working flow, I think this case couldn't happen:

1) device send cmd_id=1 to driver;
2) driver receives cmd_id=1 in the config and acks cmd_id=1 to the device
via the vq;
3) device revives cmd_id=1;
4) device wants to stop the reporting by sending cmd_id=STOP;
5) driver receives cmd_id=STOP from the config, and acks cmd_id=STOP to the
device via the vq;
6) device sends cmd_id=2 to driver;
...

cmd_id=2 won't come after cmd_id=1, there will be a STOP cmd in between them
(STOP won't queue the work).

How about defining the correct device behavior in the spec:
The device Should NOT send a second cmd id to the driver until a STOP cmd
ack for the previous cmd id has been received from the guest.


Best,
Wei

I think we should just fix races in the driver rather than introduce
random restrictions in the device.

If device wants to start a new sequence, it should be able to
do just that without a complicated back and forth with several
roundtrips through the driver.



OK, I've fixed it in the new version, v24. Please have a check there. 
Thanks.

(Other changes based on the comments on v23 have also been included)

Best,
Wei



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[PATCH v24 2/2] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

2018-01-24 Thread Wei Wang 
Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_HINT feature indicates the
support of reporting hints of guest free pages to host via virtio-balloon.

Host requests the guest to report free pages by sending a new cmd
id to the guest via the free_page_report_cmd_id configuration register.

When the guest starts to report, the first element added to the free page
vq is the cmd id given by host. When the guest finishes the reporting
of all the free pages, VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID is added
to the vq to tell host that the reporting is done. Host may also requests
the guest to stop the reporting in advance by sending the stop cmd id to
the guest via the configuration register.

Signed-off-by: Wei Wang 
Signed-off-by: Liang Li 
Cc: Michael S. Tsirkin 
Cc: Michal Hocko 
---
 drivers/virtio/virtio_balloon.c | 265 +++-
 include/uapi/linux/virtio_balloon.h |   7 +
 2 files changed, 236 insertions(+), 36 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index a1fb52c..4440873 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -51,9 +51,21 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
 static struct vfsmount *balloon_mnt;
 #endif
 
+/* The number of virtqueues supported by virtio-balloon */
+#define VIRTIO_BALLOON_VQ_NUM  4
+#define VIRTIO_BALLOON_VQ_ID_INFLATE   0
+#define VIRTIO_BALLOON_VQ_ID_DEFLATE   1
+#define VIRTIO_BALLOON_VQ_ID_STATS 2
+#define VIRTIO_BALLOON_VQ_ID_FREE_PAGE 3
+
 struct virtio_balloon {
struct virtio_device *vdev;
-   struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
+   struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
+
+   /* Balloon's own wq for cpu-intensive work items */
+   struct workqueue_struct *balloon_wq;
+   /* The free page reporting work item submitted to the balloon wq */
+   struct work_struct report_free_page_work;
 
/* The balloon servicing is delegated to a freezable workqueue. */
struct work_struct update_balloon_stats_work;
@@ -63,6 +75,13 @@ struct virtio_balloon {
spinlock_t stop_update_lock;
bool stop_update;
 
+   /* Start to report free pages */
+   bool report_free_page;
+   /* Stores the cmd id given by host to start the free page reporting */
+   __virtio32 start_cmd_id;
+   /* Stores STOP_ID as a sign to tell host that the reporting is done */
+   __virtio32 stop_cmd_id;
+
/* Waiting for host to ack the pages we released. */
wait_queue_head_t acked;
 
@@ -281,6 +300,53 @@ static unsigned int update_balloon_stats(struct 
virtio_balloon *vb)
return idx;
 }
 
+static int add_one_sg(struct virtqueue *vq, unsigned long pfn, uint32_t len)
+{
+   struct scatterlist sg;
+   unsigned int unused;
+   int ret = 0;
+
+   sg_init_table(, 1);
+   sg_set_page(, pfn_to_page(pfn), len, 0);
+
+   /* Detach all the used buffers from the vq */
+   while (virtqueue_get_buf(vq, ))
+   ;
+
+   /*
+* Since this is an optimization feature, losing a couple of free
+* pages to report isn't important. We simply return without adding
+* the page if the vq is full.
+* We are adding one entry each time, which essentially results in no
+* memory allocation, so the GFP_KERNEL flag below can be ignored.
+* There is always one entry reserved for the cmd id to use.
+*/
+   if (vq->num_free > 1)
+   ret = virtqueue_add_inbuf(vq, , 1, vq, GFP_KERNEL);
+
+   if (vq->num_free < virtqueue_get_vring_size(vq) / 2)
+   virtqueue_kick(vq);
+
+   return ret;
+}
+
+static void send_cmd_id(struct virtio_balloon *vb, __virtio32 *cmd_id)
+{
+   struct scatterlist sg;
+   struct virtqueue *vq = vb->free_page_vq;
+
+   if (unlikely(!virtio_has_feature(vb->vdev,
+VIRTIO_BALLOON_F_FREE_PAGE_HINT)))
+   return;
+
+   sg_init_one(, cmd_id, sizeof(*cmd_id));
+
+   if (virtqueue_add_outbuf(vq, , 1, vb, GFP_KERNEL))
+   __virtio_clear_bit(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT);
+
+   virtqueue_kick(vq);
+}
+
 /*
  * While most virtqueues communicate guest-initiated requests to the 
hypervisor,
  * the stats queue operates in reverse.  The driver initializes the virtqueue
@@ -316,17 +382,6 @@ static void stats_handle_request(struct virtio_balloon *vb)
virtqueue_kick(vq);
 }
 
-static void virtballoon_changed(struct virtio_device *vdev)
-{
-   struct virtio_balloon *vb = vdev->priv;
-   unsigned long flags;
-
-   spin_lock_irqsave(>stop_update_lock, flags);
-   if (!vb->stop_update)
-   queue_work(system_freezable_wq, >update_balloon_size_work);
-   spin_unlock_irqrestore(>stop_update_lock, flags);
-}
-

[PATCH v24 0/2] Virtio-balloon: support free page reporting

2018-01-24 Thread Wei Wang 
This patch series is separated from the previous "Virtio-balloon
Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,  
implemented by this series enables the virtio-balloon driver to report
hints of guest free pages to the host. It can be used to accelerate live
migration of VMs. Here is an introduction of this usage:

Live migration needs to transfer the VM's memory from the source machine
to the destination round by round. For the 1st round, all the VM's memory
is transferred. From the 2nd round, only the pieces of memory that were
written by the guest (after the 1st round) are transferred. One method
that is popularly used by the hypervisor to track which part of memory is
written is to write-protect all the guest memory.

The second feature enables the optimization of the 1st round memory
transfer - the hypervisor can skip the transfer of guest free pages in the
1st round. It is not concerned that the memory pages are used after they
are given to the hypervisor as a hint of the free pages, because they will
be tracked by the hypervisor and transferred in the next round if they are
used and written.

ChangeLog:
v23->v24:
- change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
  VIRTIO_BALLOON_F_FREE_PAGE_HINT
- kick when vq->num_free < half full, instead of "= half full"
- replace BUG_ON with bailing out
- check vb->balloon_wq in probe(), if null, bail out
- add a new feature bit for page poisoning
- solve the corner case that one cmd id being sent to host twice
v22->v23:
- change to kick the device when the vq is half-way full;
- open-code batch_free_page_sg into add_one_sg;
- change cmd_id from "uint32_t" to "__virtio32";
- reserver one entry in the vq for teh driver to send cmd_id, instead
  of busywaiting for an available entry;
- add "stop_update" check before queue_work for prudence purpose for
  now, will have a separate patch to discuss this flag check later;
- init_vqs: change to put some variables on stack to have simpler
  implementation;
- add destroy_workqueue(vb->balloon_wq);

v21->v22:
- add_one_sg: some code and comment re-arrangement
- send_cmd_id: handle a cornercase

For previous ChangeLog, please reference
https://lwn.net/Articles/743660/

Wei Wang (2):
  mm: support reporting free page blocks
  virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

 drivers/virtio/virtio_balloon.c | 265 +++-
 include/linux/mm.h  |   6 +
 include/uapi/linux/virtio_balloon.h |   7 +
 mm/page_alloc.c |  91 +
 4 files changed, 333 insertions(+), 36 deletions(-)

-- 
2.7.4

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[PATCH v24 1/2] mm: support reporting free page blocks

2018-01-24 Thread Wei Wang 
This patch adds support to walk through the free page blocks in the
system and report them via a callback function. Some page blocks may
leave the free list after zone->lock is released, so it is the caller's
responsibility to either detect or prevent the use of such pages.

One use example of this patch is to accelerate live migration by skipping
the transfer of free pages reported from the guest. A popular method used
by the hypervisor to track which part of memory is written during live
migration is to write-protect all the guest memory. So, those pages that
are reported as free pages but are written after the report function
returns will be captured by the hypervisor, and they will be added to the
next round of memory transfer.

Signed-off-by: Wei Wang 
Signed-off-by: Liang Li 
Cc: Michal Hocko 
Cc: Michael S. Tsirkin 
Acked-by: Michal Hocko 
---
 include/linux/mm.h |  6 
 mm/page_alloc.c| 91 ++
 2 files changed, 97 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ea818ff..b3077dd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1938,6 +1938,12 @@ 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 void walk_free_mem_block(void *opaque,
+   int min_order,
+   bool (*report_pfn_range)(void *opaque,
+unsigned long pfn,
+unsigned long num));
+
 /*
  * 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 76c9688..705de22 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4899,6 +4899,97 @@ void show_free_areas(unsigned int filter, nodemask_t 
*nodemask)
show_swap_cache_info();
 }
 
+/*
+ * Walk through a free page list and report the found pfn range via the
+ * callback.
+ *
+ * Return false if the callback requests to stop reporting. Otherwise,
+ * return true.
+ */
+static bool walk_free_page_list(void *opaque,
+   struct zone *zone,
+   int order,
+   enum migratetype mt,
+   bool (*report_pfn_range)(void *,
+unsigned long,
+unsigned long))
+{
+   struct page *page;
+   struct list_head *list;
+   unsigned long pfn, flags;
+   bool ret;
+
+   spin_lock_irqsave(>lock, flags);
+   list = >free_area[order].free_list[mt];
+   list_for_each_entry(page, list, lru) {
+   pfn = page_to_pfn(page);
+   ret = report_pfn_range(opaque, pfn, 1 << order);
+   if (!ret)
+   break;
+   }
+   spin_unlock_irqrestore(>lock, flags);
+
+   return ret;
+}
+
+/**
+ * walk_free_mem_block - Walk through the free page blocks in the system
+ * @opaque: the context passed from the caller
+ * @min_order: the minimum order of free lists to check
+ * @report_pfn_range: the callback to report the pfn range of the free pages
+ *
+ * If the callback returns false, stop iterating the list of free page blocks.
+ * Otherwise, continue to report.
+ *
+ * Please note that there are no locking guarantees for the callback and
+ * that the reported pfn range might be freed or disappear after the
+ * callback returns so the caller has to be very careful how it is used.
+ *
+ * The callback itself must not sleep or perform any operations which would
+ * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC)
+ * or via any lock dependency. It is generally advisable to implement
+ * the callback as simple as possible and defer any heavy lifting to a
+ * different context.
+ *
+ * There is no guarantee that each free range will be reported only once
+ * during one walk_free_mem_block invocation.
+ *
+ * pfn_to_page on the given range is strongly discouraged and if there is
+ * an absolute need for that make sure to contact MM people to discuss
+ * potential problems.
+ *
+ * The function itself might sleep so it cannot be called from atomic
+ * contexts.
+ *
+ * In general low orders tend to be very volatile and so it makes more
+ * sense to query larger ones first for various optimizations which like
+ * ballooning etc... This will reduce the overhead as well.
+ */
+void walk_free_mem_block(void *opaque,
+int min_order,
+bool (*report_pfn_range)(void *opaque,
+ unsigned long pfn,