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Subject[PATCH v4 2/3] virtio_balloon: introduce migration primitives to balloon pages
Date
Besides making balloon pages movable at allocation time and introducing
the necessary primitives to perform balloon page migration/compaction,
this patch also introduces the following locking scheme to provide the
proper synchronization and protection for struct virtio_balloon elements
against concurrent accesses due to parallel operations introduced by
memory compaction / page migration.
- balloon_lock (mutex) : synchronizes the access demand to elements of
struct virtio_balloon and its queue operations;
- pages_lock (spinlock): special protection to balloon pages list against
concurrent list handling operations;

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
drivers/virtio/virtio_balloon.c | 145 ++++++++++++++++++++++++++++++++++++---
include/linux/virtio_balloon.h | 4 ++
2 files changed, 141 insertions(+), 8 deletions(-)
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 0908e60..7669bc8 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -27,6 +27,7 @@
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/module.h>
+#include <linux/fs.h>

/*
* Balloon device works in 4K page units. So each page is pointed to by
@@ -35,6 +36,9 @@
*/
#define VIRTIO_BALLOON_PAGES_PER_PAGE (PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)

+/* Synchronize accesses/updates to the struct virtio_balloon elements */
+DEFINE_MUTEX(balloon_lock);
+
struct virtio_balloon
{
struct virtio_device *vdev;
@@ -51,6 +55,10 @@ struct virtio_balloon

/* Number of balloon pages we've told the Host we're not using. */
unsigned int num_pages;
+
+ /* Protect 'pages' list against concurrent handling */
+ spinlock_t pages_lock;
+
/*
* The pages we've told the Host we're not using.
* Each page on this list adds VIRTIO_BALLOON_PAGES_PER_PAGE
@@ -125,10 +133,12 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
/* We can only do one array worth at a time. */
num = min(num, ARRAY_SIZE(vb->pfns));

+ mutex_lock(&balloon_lock);
for (vb->num_pfns = 0; vb->num_pfns < num;
vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
- struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
- __GFP_NOMEMALLOC | __GFP_NOWARN);
+ struct page *page = alloc_page(GFP_HIGHUSER_MOVABLE |
+ __GFP_NORETRY | __GFP_NOWARN |
+ __GFP_NOMEMALLOC);
if (!page) {
if (printk_ratelimit())
dev_printk(KERN_INFO, &vb->vdev->dev,
@@ -141,7 +151,10 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
set_page_pfns(vb->pfns + vb->num_pfns, page);
vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
totalram_pages--;
+ spin_lock(&vb->pages_lock);
list_add(&page->lru, &vb->pages);
+ page->mapping = balloon_mapping;
+ spin_unlock(&vb->pages_lock);
}

/* Didn't get any? Oh well. */
@@ -149,6 +162,7 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
return;

tell_host(vb, vb->inflate_vq);
+ mutex_unlock(&balloon_lock);
}

static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
@@ -169,10 +183,22 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
/* We can only do one array worth at a time. */
num = min(num, ARRAY_SIZE(vb->pfns));

+ mutex_lock(&balloon_lock);
for (vb->num_pfns = 0; vb->num_pfns < num;
vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+ /*
+ * We can race against virtballoon_isolatepage() and end up
+ * stumbling across a _temporarily_ empty 'pages' list.
+ */
+ spin_lock(&vb->pages_lock);
+ if (unlikely(list_empty(&vb->pages))) {
+ spin_unlock(&vb->pages_lock);
+ break;
+ }
page = list_first_entry(&vb->pages, struct page, lru);
+ page->mapping = NULL;
list_del(&page->lru);
+ spin_unlock(&vb->pages_lock);
set_page_pfns(vb->pfns + vb->num_pfns, page);
vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
}
@@ -182,8 +208,11 @@ static void 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
*/
- tell_host(vb, vb->deflate_vq);
- release_pages_by_pfn(vb->pfns, vb->num_pfns);
+ if (vb->num_pfns > 0) {
+ tell_host(vb, vb->deflate_vq);
+ release_pages_by_pfn(vb->pfns, vb->num_pfns);
+ }
+ mutex_unlock(&balloon_lock);
}

static inline void update_stat(struct virtio_balloon *vb, int idx,
@@ -239,6 +268,7 @@ static void stats_handle_request(struct virtio_balloon *vb)
struct scatterlist sg;
unsigned int len;

+ mutex_lock(&balloon_lock);
vb->need_stats_update = 0;
update_balloon_stats(vb);

@@ -249,6 +279,7 @@ static void stats_handle_request(struct virtio_balloon *vb)
if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
BUG();
virtqueue_kick(vq);
+ mutex_unlock(&balloon_lock);
}

static void virtballoon_changed(struct virtio_device *vdev)
@@ -261,22 +292,27 @@ static void virtballoon_changed(struct virtio_device *vdev)
static inline s64 towards_target(struct virtio_balloon *vb)
{
__le32 v;
- s64 target;
+ s64 target, actual;

+ mutex_lock(&balloon_lock);
+ actual = vb->num_pages;
vb->vdev->config->get(vb->vdev,
offsetof(struct virtio_balloon_config, num_pages),
&v, sizeof(v));
target = le32_to_cpu(v);
- return target - vb->num_pages;
+ mutex_unlock(&balloon_lock);
+ return target - actual;
}

static void update_balloon_size(struct virtio_balloon *vb)
{
- __le32 actual = cpu_to_le32(vb->num_pages);
-
+ __le32 actual;
+ mutex_lock(&balloon_lock);
+ actual = cpu_to_le32(vb->num_pages);
vb->vdev->config->set(vb->vdev,
offsetof(struct virtio_balloon_config, actual),
&actual, sizeof(actual));
+ mutex_unlock(&balloon_lock);
}

static int balloon(void *_vballoon)
@@ -339,6 +375,82 @@ static int init_vqs(struct virtio_balloon *vb)
return 0;
}

+/*
+ * Populate balloon_mapping->a_ops->migratepage method to perform the balloon
+ * page migration task.
+ *
+ * After a ballooned page gets isolated by compaction procedures, this is the
+ * function that performs the page migration on behalf of move_to_new_page(),
+ * when the last calls (page)->mapping->a_ops->migratepage.
+ *
+ * Page migration for virtio balloon is done in a simple swap fashion which
+ * follows these two steps:
+ * 1) insert newpage into vb->pages list and update the host about it;
+ * 2) update the host about the removed old page from vb->pages list;
+ */
+int virtballoon_migratepage(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+ struct virtio_balloon *vb = (void *)mapping->backing_dev_info;
+
+ mutex_lock(&balloon_lock);
+
+ /* balloon's page migration 1st step */
+ vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+ spin_lock(&vb->pages_lock);
+ list_add(&newpage->lru, &vb->pages);
+ spin_unlock(&vb->pages_lock);
+ set_page_pfns(vb->pfns, newpage);
+ tell_host(vb, vb->inflate_vq);
+
+ /* balloon's page migration 2nd step */
+ vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+ set_page_pfns(vb->pfns, page);
+ tell_host(vb, vb->deflate_vq);
+
+ mutex_unlock(&balloon_lock);
+
+ return 0;
+}
+
+/*
+ * Populate balloon_mapping->a_ops->invalidatepage method to help compaction on
+ * isolating a page from the balloon page list.
+ */
+void virtballoon_isolatepage(struct page *page, unsigned long mode)
+{
+ /*
+ * struct address_space.backing_dev_info specially holds the pointer
+ * to our struct virtio_balloon *vb
+ */
+ struct virtio_balloon *vb = (void *)page->mapping->backing_dev_info;
+ spin_lock(&vb->pages_lock);
+ list_del(&page->lru);
+ spin_unlock(&vb->pages_lock);
+}
+
+/*
+ * Populate balloon_mapping->a_ops->freepage method to help compaction on
+ * re-inserting an isolated page into the balloon page list.
+ */
+void virtballoon_putbackpage(struct page *page)
+{
+ /*
+ * struct address_space.backing_dev_info specially holds the pointer
+ * to our struct virtio_balloon *vb
+ */
+ struct virtio_balloon *vb = (void *)page->mapping->backing_dev_info;
+ spin_lock(&vb->pages_lock);
+ list_add(&page->lru, &vb->pages);
+ spin_unlock(&vb->pages_lock);
+}
+
+static const struct address_space_operations virtio_balloon_aops = {
+ .migratepage = virtballoon_migratepage,
+ .invalidatepage = virtballoon_isolatepage,
+ .freepage = virtballoon_putbackpage,
+};
+
static int virtballoon_probe(struct virtio_device *vdev)
{
struct virtio_balloon *vb;
@@ -351,12 +463,27 @@ static int virtballoon_probe(struct virtio_device *vdev)
}

INIT_LIST_HEAD(&vb->pages);
+ spin_lock_init(&vb->pages_lock);
+
vb->num_pages = 0;
init_waitqueue_head(&vb->config_change);
init_waitqueue_head(&vb->acked);
vb->vdev = vdev;
vb->need_stats_update = 0;

+ /* Init the ballooned page->mapping special balloon_mapping */
+ balloon_mapping = kmalloc(sizeof(*balloon_mapping), GFP_KERNEL);
+ if (!balloon_mapping) {
+ err = -ENOMEM;
+ goto out_free_vb;
+ }
+
+ INIT_RADIX_TREE(&balloon_mapping->page_tree, GFP_ATOMIC | __GFP_NOWARN);
+ INIT_LIST_HEAD(&balloon_mapping->i_mmap_nonlinear);
+ spin_lock_init(&balloon_mapping->tree_lock);
+ balloon_mapping->a_ops = &virtio_balloon_aops;
+ balloon_mapping->backing_dev_info = (void *)vb;
+
err = init_vqs(vb);
if (err)
goto out_free_vb;
@@ -373,6 +500,7 @@ out_del_vqs:
vdev->config->del_vqs(vdev);
out_free_vb:
kfree(vb);
+ kfree(balloon_mapping);
out:
return err;
}
@@ -397,6 +525,7 @@ static void __devexit virtballoon_remove(struct virtio_device *vdev)
kthread_stop(vb->thread);
remove_common(vb);
kfree(vb);
+ kfree(balloon_mapping);
}

#ifdef CONFIG_PM
diff --git a/include/linux/virtio_balloon.h b/include/linux/virtio_balloon.h
index 652dc8b..930f1b7 100644
--- a/include/linux/virtio_balloon.h
+++ b/include/linux/virtio_balloon.h
@@ -56,4 +56,8 @@ struct virtio_balloon_stat {
u64 val;
} __attribute__((packed));

+#if !defined(CONFIG_COMPACTION)
+struct address_space *balloon_mapping;
+#endif
+
#endif /* _LINUX_VIRTIO_BALLOON_H */
--
1.7.10.4


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