[PATCH v29 1/4] mm: support reporting free page blocks

[PATCH v29 1/4] mm: support reporting free page blocks

[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 <wei.w.wang@intel.com>
Signed-off-by: Liang Li <liang.z.li@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michal Hocko <mhocko@kernel.org>
---
 include/linux/mm.h |  6 ++++
 mm/page_alloc.c    | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ad06d42..c72b5a9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1944,6 +1944,12 @@ 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 int walk_free_mem_block(void *opaque,
+			       int min_order,
+			       int (*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 635d7dd..d58de87 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4912,6 +4912,102 @@ 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 0 if it completes the reporting. Otherwise, return the non-zero
+ * value returned from the callback.
+ */
+static int walk_free_page_list(void *opaque,
+			       struct zone *zone,
+			       int order,
+			       enum migratetype mt,
+			       int (*report_pfn_range)(void *,
+						       unsigned long,
+						       unsigned long))
+{
+	struct page *page;
+	struct list_head *list;
+	unsigned long pfn, flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&zone->lock, flags);
+	list = &zone->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(&zone->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 a non-zero value, 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.
+ *
+ * Return 0 if it completes the reporting. Otherwise, return the non-zero
+ * value returned from the callback.
+ */
+int walk_free_mem_block(void *opaque,
+			int min_order,
+			int (*report_pfn_range)(void *opaque,
+			unsigned long pfn,
+			unsigned long num))
+{
+	struct zone *zone;
+	int order;
+	enum migratetype mt;
+	int ret;
+
+	for_each_populated_zone(zone) {
+		for (order = MAX_ORDER - 1; order >= min_order; order--) {
+			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
+				ret = walk_free_page_list(opaque, zone,
+							  order, mt,
+							  report_pfn_range);
+				if (ret)
+					return ret;
+			}
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(walk_free_mem_block);
+
 static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
 {
 	zoneref->zone = zone;
-- 
2.7.4

[PATCH v29 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[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 page hints 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 polls the free
page vq after sending the starting cmd id, so the guest doesn't need to
kick after filling an element to the vq.

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 <wei.w.wang@intel.com>
Signed-off-by: Liang Li <liang.z.li@intel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
---
 drivers/virtio/virtio_balloon.c     | 257 +++++++++++++++++++++++++++++++-----
 include/uapi/linux/virtio_balloon.h |   4 +
 2 files changed, 225 insertions(+), 36 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index dfe5684..18d24a4 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -51,9 +51,22 @@ MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
 static struct vfsmount *balloon_mnt;
 #endif
 
+enum virtio_balloon_vq {
+	VIRTIO_BALLOON_VQ_INFLATE,
+	VIRTIO_BALLOON_VQ_DEFLATE,
+	VIRTIO_BALLOON_VQ_STATS,
+	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_MAX
+};
+
 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 +76,13 @@ struct virtio_balloon {
 	spinlock_t stop_update_lock;
 	bool stop_update;
 
+	/* The new cmd id received from host */
+	uint32_t cmd_id_received;
+	/* The cmd id that is in use */
+	__virtio32 cmd_id_use;
+	/* Buffer to store the stop sign */
+	__virtio32 stop_cmd_id;
+
 	/* Waiting for host to ack the pages we released. */
 	wait_queue_head_t acked;
 
@@ -320,17 +340,6 @@ 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(&vb->stop_update_lock, flags);
-	if (!vb->stop_update)
-		queue_work(system_freezable_wq, &vb->update_balloon_size_work);
-	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
-}
-
 static inline s64 towards_target(struct virtio_balloon *vb)
 {
 	s64 target;
@@ -347,6 +356,34 @@ static inline s64 towards_target(struct virtio_balloon *vb)
 	return target - vb->num_pages;
 }
 
+static void virtballoon_changed(struct virtio_device *vdev)
+{
+	struct virtio_balloon *vb = vdev->priv;
+	unsigned long flags;
+	s64 diff = towards_target(vb);
+
+	if (diff) {
+		spin_lock_irqsave(&vb->stop_update_lock, flags);
+		if (!vb->stop_update)
+			queue_work(system_freezable_wq,
+				   &vb->update_balloon_size_work);
+		spin_unlock_irqrestore(&vb->stop_update_lock, flags);
+	}
+
+	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+		virtio_cread(vdev, struct virtio_balloon_config,
+			     free_page_report_cmd_id, &vb->cmd_id_received);
+		if (vb->cmd_id_received !=
+		    VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID) {
+			spin_lock_irqsave(&vb->stop_update_lock, flags);
+			if (!vb->stop_update)
+				queue_work(vb->balloon_wq,
+					   &vb->report_free_page_work);
+			spin_unlock_irqrestore(&vb->stop_update_lock, flags);
+		}
+	}
+}
+
 static void update_balloon_size(struct virtio_balloon *vb)
 {
 	u32 actual = vb->num_pages;
@@ -421,42 +458,163 @@ static void update_balloon_size_func(struct work_struct *work)
 
 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" };
-	int err, nvqs;
+	struct virtqueue *vqs[VIRTIO_BALLOON_VQ_MAX];
+	vq_callback_t *callbacks[VIRTIO_BALLOON_VQ_MAX];
+	const char *names[VIRTIO_BALLOON_VQ_MAX];
+	struct scatterlist sg;
+	int ret;
 
 	/*
-	 * We expect two virtqueues: inflate and deflate, and
-	 * optionally stat.
+	 * Inflateq and deflateq are used unconditionally. The names[]
+	 * will be NULL if the related feature is not enabled, which will
+	 * cause no allocation for the corresponding virtqueue in find_vqs.
 	 */
-	nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
-	err = virtio_find_vqs(vb->vdev, nvqs, vqs, callbacks, names, NULL);
-	if (err)
-		return err;
+	callbacks[VIRTIO_BALLOON_VQ_INFLATE] = balloon_ack;
+	names[VIRTIO_BALLOON_VQ_INFLATE] = "inflate";
+	callbacks[VIRTIO_BALLOON_VQ_DEFLATE] = balloon_ack;
+	names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
+	names[VIRTIO_BALLOON_VQ_STATS] = NULL;
+	names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
 
-	vb->inflate_vq = vqs[0];
-	vb->deflate_vq = vqs[1];
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
-		struct scatterlist sg;
-		unsigned int num_stats;
-		vb->stats_vq = vqs[2];
+		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
+		callbacks[VIRTIO_BALLOON_VQ_STATS] = stats_request;
+	}
+
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+		names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "free_page_vq";
+		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
+	}
 
+	ret = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
+					 vqs, callbacks, names, NULL, NULL);
+	if (ret)
+		return ret;
+
+	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
+	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
+		vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];
 		/*
 		 * Prime this virtqueue with one buffer so the hypervisor can
 		 * use it to signal us later (it can't be broken yet!).
 		 */
-		num_stats = update_balloon_stats(vb);
-
-		sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
-		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
-		    < 0)
-			BUG();
+		sg_init_one(&sg, vb->stats, sizeof(vb->stats));
+		ret = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
+					   GFP_KERNEL);
+		if (ret) {
+			dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
+				 __func__);
+			return ret;
+		}
 		virtqueue_kick(vb->stats_vq);
 	}
+
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
+		vb->free_page_vq = vqs[VIRTIO_BALLOON_VQ_FREE_PAGE];
+
 	return 0;
 }
 
+static int add_one_sg(struct virtqueue *vq, unsigned long pfn, uint32_t len)
+{
+	struct scatterlist sg;
+	unsigned int unused;
+
+	sg_init_table(&sg, 1);
+	sg_set_page(&sg, pfn_to_page(pfn), len, 0);
+
+	/* Detach all the used buffers from the vq */
+	while (virtqueue_get_buf(vq, &unused))
+		;
+
+	/*
+	 * Since this is an optimization feature, losing a couple of free
+	 * pages to report isn't important. We simply return without adding
+	 * the page hint 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.
+	 * Host works by polling the free page vq for hints after sending the
+	 * starting cmd id, so the driver doesn't need to kick after filling
+	 * the vq.
+	 * Lastly, there is always one entry reserved for the cmd id to use.
+	 */
+	if (vq->num_free > 1)
+		return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
+
+	return 0;
+}
+
+static int virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
+					   unsigned long nr_pages)
+{
+	struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
+	uint32_t len = nr_pages << PAGE_SHIFT;
+
+	/*
+	 * If a stop id or a new cmd id was just received from host, stop
+	 * the reporting, and return 1 to indicate an active stop.
+	 */
+	if (virtio32_to_cpu(vb->vdev, vb->cmd_id_use) != vb->cmd_id_received)
+		return 1;
+
+	return add_one_sg(vb->free_page_vq, pfn, len);
+}
+
+static int send_start_cmd_id(struct virtio_balloon *vb, uint32_t cmd_id)
+{
+	struct scatterlist sg;
+	struct virtqueue *vq = vb->free_page_vq;
+
+	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
+	sg_init_one(&sg, &vb->cmd_id_use, sizeof(vb->cmd_id_use));
+	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+}
+
+static int send_stop_cmd_id(struct virtio_balloon *vb)
+{
+	struct scatterlist sg;
+	struct virtqueue *vq = vb->free_page_vq;
+
+	sg_init_one(&sg, &vb->stop_cmd_id, sizeof(vb->cmd_id_use));
+	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+}
+
+static void report_free_page_func(struct work_struct *work)
+{
+	struct virtio_balloon *vb;
+	struct virtqueue *vq;
+	unsigned int unused;
+	int ret;
+
+	vb = container_of(work, struct virtio_balloon, report_free_page_work);
+	vq = vb->free_page_vq;
+
+	/* Start by sending the received cmd id to host with an outbuf */
+	ret = send_start_cmd_id(vb, vb->cmd_id_received);
+	if (unlikely(ret))
+		goto err;
+
+	ret = walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
+	if (unlikely(ret == -EIO))
+		goto err;
+
+	/* End by sending a stop id to host with an outbuf */
+	ret = send_stop_cmd_id(vb);
+	if (likely(!ret)) {
+		/*
+		 * Ending: make sure all the used buffers have been detached
+		 * from the vq.
+		 */
+		while (vq->num_free != virtqueue_get_vring_size(vq))
+			virtqueue_get_buf(vq, &unused);
+		return;
+	}
+err:
+	dev_err(&vb->vdev->dev, "%s: free page vq failure, ret=%d\n",
+		__func__, ret);
+}
+
 #ifdef CONFIG_BALLOON_COMPACTION
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
@@ -570,18 +728,36 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	if (err)
 		goto out_free_vb;
 
+	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+		/*
+		 * There is always one entry reserved for cmd id, so the ring
+		 * size needs to be at least two to report free page hints.
+		 */
+		if (virtqueue_get_vring_size(vb->free_page_vq) < 2)
+			goto out_free_vb;
+		vb->balloon_wq = alloc_workqueue("balloon-wq",
+					WQ_FREEZABLE | WQ_CPU_INTENSIVE, 0);
+		if (!vb->balloon_wq) {
+			err = -ENOMEM;
+			goto out_del_vqs;
+		}
+		vb->stop_cmd_id = cpu_to_virtio32(vb->vdev,
+				VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
+		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
+	}
+
 	vb->nb.notifier_call = virtballoon_oom_notify;
 	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
 	err = register_oom_notifier(&vb->nb);
 	if (err < 0)
-		goto out_del_vqs;
+		goto out_del_balloon_wq;
 
 #ifdef CONFIG_BALLOON_COMPACTION
 	balloon_mnt = kern_mount(&balloon_fs);
 	if (IS_ERR(balloon_mnt)) {
 		err = PTR_ERR(balloon_mnt);
 		unregister_oom_notifier(&vb->nb);
-		goto out_del_vqs;
+		goto out_del_balloon_wq;
 	}
 
 	vb->vb_dev_info.migratepage = virtballoon_migratepage;
@@ -591,7 +767,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		kern_unmount(balloon_mnt);
 		unregister_oom_notifier(&vb->nb);
 		vb->vb_dev_info.inode = NULL;
-		goto out_del_vqs;
+		goto out_del_balloon_wq;
 	}
 	vb->vb_dev_info.inode->i_mapping->a_ops = &balloon_aops;
 #endif
@@ -602,6 +778,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		virtballoon_changed(vdev);
 	return 0;
 
+out_del_balloon_wq:
+	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
+		destroy_workqueue(vb->balloon_wq);
 out_del_vqs:
 	vdev->config->del_vqs(vdev);
 out_free_vb:
@@ -635,6 +814,11 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	cancel_work_sync(&vb->update_balloon_size_work);
 	cancel_work_sync(&vb->update_balloon_stats_work);
 
+	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+		cancel_work_sync(&vb->report_free_page_work);
+		destroy_workqueue(vb->balloon_wq);
+	}
+
 	remove_common(vb);
 #ifdef CONFIG_BALLOON_COMPACTION
 	if (vb->vb_dev_info.inode)
@@ -686,6 +870,7 @@ static unsigned int features[] = {
 	VIRTIO_BALLOON_F_MUST_TELL_HOST,
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
+	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index 4e8b830..b2d86c2 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -34,15 +34,19 @@
 #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_FREE_PAGE_HINT	3 /* VQ to report free pages */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
 
+#define VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID		0
 struct virtio_balloon_config {
 	/* Number of pages host wants Guest to give up. */
 	__u32 num_pages;
 	/* Number of pages we've actually got in balloon. */
 	__u32 actual;
+	/* Free page report command id, readonly by guest */
+	__u32 free_page_report_cmd_id;
 };
 
 #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
-- 
2.7.4

[PATCH v29 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

[Wei Wang]

In some usages, e.g. virtio-balloon, a kernel module needs to know if
page poisoning is in use. This patch exposes the page_poisoning_enabled
function to kernel modules.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
---
 mm/page_poison.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/page_poison.c b/mm/page_poison.c
index e83fd44..762b472 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -17,6 +17,11 @@ static int early_page_poison_param(char *buf)
 }
 early_param("page_poison", early_page_poison_param);
 
+/**
+ * page_poisoning_enabled - check if page poisoning is enabled
+ *
+ * Return true if page poisoning is enabled, or false if not.
+ */
 bool page_poisoning_enabled(void)
 {
 	/*
@@ -29,6 +34,7 @@ bool page_poisoning_enabled(void)
 		(!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
 		debug_pagealloc_enabled()));
 }
+EXPORT_SYMBOL_GPL(page_poisoning_enabled);
 
 static void poison_page(struct page *page)
 {
-- 
2.7.4

[PATCH v29 4/4] virtio-balloon: VIRTIO_BALLOON_F_PAGE_POISON

[Wei Wang]

The VIRTIO_BALLOON_F_PAGE_POISON feature bit is used to indicate if the
guest is using page poisoning. Guest writes to the poison_val config
field to tell host about the page poisoning value in use.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 drivers/virtio/virtio_balloon.c     | 10 ++++++++++
 include/uapi/linux/virtio_balloon.h |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 18d24a4..6de9339 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -699,6 +699,7 @@ static struct file_system_type balloon_fs = {
 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
+	__u32 poison_val;
 	int err;
 
 	if (!vdev->config->get) {
@@ -744,6 +745,11 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		vb->stop_cmd_id = cpu_to_virtio32(vb->vdev,
 				VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
 		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
+		if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) {
+			memset(&poison_val, PAGE_POISON, sizeof(poison_val));
+			virtio_cwrite(vb->vdev, struct virtio_balloon_config,
+				      poison_val, &poison_val);
+		}
 	}
 
 	vb->nb.notifier_call = virtballoon_oom_notify;
@@ -862,6 +868,9 @@ static int virtballoon_restore(struct virtio_device *vdev)
 
 static int virtballoon_validate(struct virtio_device *vdev)
 {
+	if (!page_poisoning_enabled())
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
+
 	__virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
 	return 0;
 }
@@ -871,6 +880,7 @@ static unsigned int features[] = {
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
 	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
+	VIRTIO_BALLOON_F_PAGE_POISON,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index b2d86c2..8b93581 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_FREE_PAGE_HINT	3 /* VQ to report free pages */
+#define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
@@ -47,6 +48,8 @@ struct virtio_balloon_config {
 	__u32 actual;
 	/* Free page report command id, readonly by guest */
 	__u32 free_page_report_cmd_id;
+	/* Stores PAGE_POISON if page poisoning is in use */
+	__u32 poison_val;
 };
 
 #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
-- 
2.7.4

RE: [PATCH v29 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

[Wang, Wei W]

On Monday, March 26, 2018 10:40 AM, Wang, Wei W wrote:
> Subject: [PATCH v29 3/4] mm/page_poison: expose page_poisoning_enabled
> to kernel modules
> 
> In some usages, e.g. virtio-balloon, a kernel module needs to know if page
> poisoning is in use. This patch exposes the page_poisoning_enabled function
> to kernel modules.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> ---
>  mm/page_poison.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/page_poison.c b/mm/page_poison.c index e83fd44..762b472
> 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -17,6 +17,11 @@ static int early_page_poison_param(char *buf)  }
> early_param("page_poison", early_page_poison_param);
> 
> +/**
> + * page_poisoning_enabled - check if page poisoning is enabled
> + *
> + * Return true if page poisoning is enabled, or false if not.
> + */
>  bool page_poisoning_enabled(void)
>  {
>  	/*
> @@ -29,6 +34,7 @@ bool page_poisoning_enabled(void)
> 
> 	(!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
>  		debug_pagealloc_enabled()));
>  }
> +EXPORT_SYMBOL_GPL(page_poisoning_enabled);
> 
>  static void poison_page(struct page *page)  {
> --
> 2.7.4


Could we get a review of this patch? We've reviewed other parts, and this one seems to be the last part of this feature. Thanks.

Best,
Wei

Re: [PATCH v29 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

[Andrew Morton]

On Mon, 26 Mar 2018 10:39:53 +0800 Wei Wang <wei.w.wang@intel.com> wrote:

> In some usages, e.g. virtio-balloon, a kernel module needs to know if
> page poisoning is in use. This patch exposes the page_poisoning_enabled
> function to kernel modules.

Acked-by: Andrew Morton <akpm@linux-foundation.org>

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Andrew Morton]

On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang@intel.com> wrote:

> 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.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4912,6 +4912,102 @@ 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 0 if it completes the reporting. Otherwise, return the non-zero
> + * value returned from the callback.
> + */
> +static int walk_free_page_list(void *opaque,
> +			       struct zone *zone,
> +			       int order,
> +			       enum migratetype mt,
> +			       int (*report_pfn_range)(void *,
> +						       unsigned long,
> +						       unsigned long))
> +{
> +	struct page *page;
> +	struct list_head *list;
> +	unsigned long pfn, flags;
> +	int ret = 0;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +	list = &zone->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(&zone->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 a non-zero value, 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.

I don't see how walk_free_mem_block() can sleep.

> + * 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.
> + *
> + * Return 0 if it completes the reporting. Otherwise, return the non-zero
> + * value returned from the callback.
> + */
> +int walk_free_mem_block(void *opaque,
> +			int min_order,
> +			int (*report_pfn_range)(void *opaque,
> +			unsigned long pfn,
> +			unsigned long num))
> +{
> +	struct zone *zone;
> +	int order;
> +	enum migratetype mt;
> +	int ret;
> +
> +	for_each_populated_zone(zone) {
> +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> +				ret = walk_free_page_list(opaque, zone,
> +							  order, mt,
> +							  report_pfn_range);
> +				if (ret)
> +					return ret;
> +			}
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(walk_free_mem_block);

This looks like it could take a long time.  Will we end up needing to
add cond_resched() in there somewhere?

>  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
>  {
>  	zoneref->zone = zone;
> -- 
> 2.7.4

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Wei Wang]

On 03/27/2018 05:22 AM, Andrew Morton wrote:
> On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang@intel.com> wrote:
>
>> 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.
>>
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4912,6 +4912,102 @@ 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 0 if it completes the reporting. Otherwise, return the non-zero
>> + * value returned from the callback.
>> + */
>> +static int walk_free_page_list(void *opaque,
>> +			       struct zone *zone,
>> +			       int order,
>> +			       enum migratetype mt,
>> +			       int (*report_pfn_range)(void *,
>> +						       unsigned long,
>> +						       unsigned long))
>> +{
>> +	struct page *page;
>> +	struct list_head *list;
>> +	unsigned long pfn, flags;
>> +	int ret = 0;
>> +
>> +	spin_lock_irqsave(&zone->lock, flags);
>> +	list = &zone->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(&zone->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 a non-zero value, 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.
> I don't see how walk_free_mem_block() can sleep.

OK, it would be better to remove this sentence for the current version. 
But I think we could probably keep it if we decide to add cond_resched() 
below.

>
>> + * 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.
>> + *
>> + * Return 0 if it completes the reporting. Otherwise, return the non-zero
>> + * value returned from the callback.
>> + */
>> +int walk_free_mem_block(void *opaque,
>> +			int min_order,
>> +			int (*report_pfn_range)(void *opaque,
>> +			unsigned long pfn,
>> +			unsigned long num))
>> +{
>> +	struct zone *zone;
>> +	int order;
>> +	enum migratetype mt;
>> +	int ret;
>> +
>> +	for_each_populated_zone(zone) {
>> +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
>> +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
>> +				ret = walk_free_page_list(opaque, zone,
>> +							  order, mt,
>> +							  report_pfn_range);
>> +				if (ret)
>> +					return ret;
>> +			}
==>
>> +		}
>> +	}
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> This looks like it could take a long time.  Will we end up needing to
> add cond_resched() in there somewhere?

OK. How about adding cond_resched at the above place "==>" (i.e. every 
order)?


Best,
Wei

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Michal Hocko]

On Tue 27-03-18 14:23:51, Wei Wang wrote:
> On 03/27/2018 05:22 AM, Andrew Morton wrote:
> > On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang@intel.com> wrote:
> > 
> > > 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.
> > > 
> > > ...
> > > 
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4912,6 +4912,102 @@ 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 0 if it completes the reporting. Otherwise, return the non-zero
> > > + * value returned from the callback.
> > > + */
> > > +static int walk_free_page_list(void *opaque,
> > > +			       struct zone *zone,
> > > +			       int order,
> > > +			       enum migratetype mt,
> > > +			       int (*report_pfn_range)(void *,
> > > +						       unsigned long,
> > > +						       unsigned long))
> > > +{
> > > +	struct page *page;
> > > +	struct list_head *list;
> > > +	unsigned long pfn, flags;
> > > +	int ret = 0;
> > > +
> > > +	spin_lock_irqsave(&zone->lock, flags);
> > > +	list = &zone->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(&zone->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 a non-zero value, 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.
> > I don't see how walk_free_mem_block() can sleep.
> 
> OK, it would be better to remove this sentence for the current version. But
> I think we could probably keep it if we decide to add cond_resched() below.

The point of this sentence was to make any user aware that the function
might sleep from the very begining rather than chase existing callers
when we need to add cond_resched or sleep for any other reason. So I
would rather keep it.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Michael S. Tsirkin]

On Tue, Mar 27, 2018 at 08:33:22AM +0200, Michal Hocko wrote:
> > > > + * The function itself might sleep so it cannot be called from atomic
> > > > + * contexts.
> > > I don't see how walk_free_mem_block() can sleep.
> > 
> > OK, it would be better to remove this sentence for the current version. But
> > I think we could probably keep it if we decide to add cond_resched() below.
> 
> The point of this sentence was to make any user aware that the function
> might sleep from the very begining rather than chase existing callers
> when we need to add cond_resched or sleep for any other reason. So I
> would rather keep it.

Let's say what it is then - "will be changed to sleep in the future".

> -- 
> Michal Hocko
> SUSE Labs

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Michal Hocko]

On Tue 27-03-18 19:07:22, Michael S. Tsirkin wrote:
> On Tue, Mar 27, 2018 at 08:33:22AM +0200, Michal Hocko wrote:
> > > > > + * The function itself might sleep so it cannot be called from atomic
> > > > > + * contexts.
> > > > I don't see how walk_free_mem_block() can sleep.
> > > 
> > > OK, it would be better to remove this sentence for the current version. But
> > > I think we could probably keep it if we decide to add cond_resched() below.
> > 
> > The point of this sentence was to make any user aware that the function
> > might sleep from the very begining rather than chase existing callers
> > when we need to add cond_resched or sleep for any other reason. So I
> > would rather keep it.
> 
> Let's say what it is then - "will be changed to sleep in the future".

Do we really want to describe the precise implementation in the
documentation? I thought the main purpose of the documentation is to
describe the _contract_. If I am curious about the implementation I can
look at the code. As I've said earlier in this patchset lifetime. This
interface is rather dangerous because we are exposing guts of our
internal data structures. So we better set expectations of what can and
cannot be done right from the beginning. I definitely do not want
somebody to simply look at the code and see that the interface is
sleepable and abuse that fact.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Michael S. Tsirkin]

On Mon, Mar 26, 2018 at 02:22:54PM -0700, Andrew Morton wrote:
> On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang@intel.com> wrote:
> 
> > 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.
> > 
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4912,6 +4912,102 @@ 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 0 if it completes the reporting. Otherwise, return the non-zero
> > + * value returned from the callback.
> > + */
> > +static int walk_free_page_list(void *opaque,
> > +			       struct zone *zone,
> > +			       int order,
> > +			       enum migratetype mt,
> > +			       int (*report_pfn_range)(void *,
> > +						       unsigned long,
> > +						       unsigned long))
> > +{
> > +	struct page *page;
> > +	struct list_head *list;
> > +	unsigned long pfn, flags;
> > +	int ret = 0;
> > +
> > +	spin_lock_irqsave(&zone->lock, flags);
> > +	list = &zone->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(&zone->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 a non-zero value, 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.
> 
> I don't see how walk_free_mem_block() can sleep.
> 
> > + * 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.
> > + *
> > + * Return 0 if it completes the reporting. Otherwise, return the non-zero
> > + * value returned from the callback.
> > + */
> > +int walk_free_mem_block(void *opaque,
> > +			int min_order,
> > +			int (*report_pfn_range)(void *opaque,
> > +			unsigned long pfn,
> > +			unsigned long num))
> > +{
> > +	struct zone *zone;
> > +	int order;
> > +	enum migratetype mt;
> > +	int ret;
> > +
> > +	for_each_populated_zone(zone) {
> > +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> > +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> > +				ret = walk_free_page_list(opaque, zone,
> > +							  order, mt,
> > +							  report_pfn_range);
> > +				if (ret)
> > +					return ret;
> > +			}
> > +		}
> > +	}
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> 
> This looks like it could take a long time.  Will we end up needing to
> add cond_resched() in there somewhere?

Andrew, were your questions answered? If yes could I bother you for an ack on this?


> >  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
> >  {
> >  	zoneref->zone = zone;
> > -- 
> > 2.7.4

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Andrew Morton]

On Tue, 10 Apr 2018 21:19:31 +0300 "Michael S. Tsirkin" <mst@redhat.com> wrote:

> 
> Andrew, were your questions answered? If yes could I bother you for an ack on this?
> 

Still not very happy that readers are told that "this function may
sleep" when it clearly doesn't do so.  If we wish to be able to change
it to sleep in the future then that should be mentioned.  And even put a
might_sleep() in there, to catch people who didn't read the comments...

Otherwise it looks OK.

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Michael S. Tsirkin]

On Tue, Apr 10, 2018 at 01:54:29PM -0700, Andrew Morton wrote:
> On Tue, 10 Apr 2018 21:19:31 +0300 "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 
> > Andrew, were your questions answered? If yes could I bother you for an ack on this?
> > 
> 
> Still not very happy that readers are told that "this function may
> sleep" when it clearly doesn't do so.  If we wish to be able to change
> it to sleep in the future then that should be mentioned.  And even put a
> might_sleep() in there, to catch people who didn't read the comments...
> 
> Otherwise it looks OK.

Oh, might_sleep with a comment explaining it's for the future sounds
good to me. I queued this - Wei, could you post a patch on top pls?

-- 
MST

Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Wei Wang]

On 04/11/2018 07:25 AM, Michael S. Tsirkin wrote:
> On Tue, Apr 10, 2018 at 01:54:29PM -0700, Andrew Morton wrote:
>> On Tue, 10 Apr 2018 21:19:31 +0300 "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>
>>> Andrew, were your questions answered? If yes could I bother you for an ack on this?
>>>
>> Still not very happy that readers are told that "this function may
>> sleep" when it clearly doesn't do so.  If we wish to be able to change
>> it to sleep in the future then that should be mentioned.  And even put a
>> might_sleep() in there, to catch people who didn't read the comments...
>>
>> Otherwise it looks OK.
> Oh, might_sleep with a comment explaining it's for the future sounds
> good to me. I queued this - Wei, could you post a patch on top pls?
>

I'm just thinking if it would be necessary to add another might_sleep, 
because we've had a cond_resched there which has wrapped a __might_sleep.

Best,
Wei