[Qemu-devel] [PATCH v17 3/6] mm/balloon_compaction.c: split balloon page allocation and enqueue

[Wei Wang]

From: "Michael S. Tsirkin" <mst.redhat.com>

fill_balloon doing memory allocations under balloon_lock
can cause a deadlock when leak_balloon is called from
virtballoon_oom_notify and tries to take same lock.

To fix, split page allocation and enqueue and do allocations outside
the lock.

Here's a detailed analysis of the deadlock by Tetsuo Handa:

In leak_balloon(), mutex_lock(&vb->balloon_lock) is called in order to
serialize against fill_balloon(). But in fill_balloon(),
alloc_page(GFP_HIGHUSER[_MOVABLE] | __GFP_NOMEMALLOC | __GFP_NORETRY) is
called with vb->balloon_lock mutex held. Since GFP_HIGHUSER[_MOVABLE]
implies __GFP_DIRECT_RECLAIM | __GFP_IO | __GFP_FS, despite __GFP_NORETRY
is specified, this allocation attempt might indirectly depend on somebody
else's __GFP_DIRECT_RECLAIM memory allocation. And such indirect
__GFP_DIRECT_RECLAIM memory allocation might call leak_balloon() via
virtballoon_oom_notify() via blocking_notifier_call_chain() callback via
out_of_memory() when it reached __alloc_pages_may_oom() and held oom_lock
mutex. Since vb->balloon_lock mutex is already held by fill_balloon(), it
will cause OOM lockup. Thus, do not wait for vb->balloon_lock mutex if
leak_balloon() is called from out_of_memory().

Thread1                                Thread2
fill_balloon()
 takes a balloon_lock
  balloon_page_enqueue()
   alloc_page(GFP_HIGHUSER_MOVABLE)
    direct reclaim (__GFP_FS context)  takes a fs lock
     waits for that fs lock             alloc_page(GFP_NOFS)
                                         __alloc_pages_may_oom()
                                          takes the oom_lock
                                           out_of_memory()
                                            blocking_notifier_call_chain()
                                             leak_balloon()
                                               tries to take that
                                               balloon_lock and deadlocks

Reported-by: Tetsuo Handa <address@hidden>
Signed-off-by: Michael S. Tsirkin <address@hidden>
Cc: Michal Hocko <address@hidden>
Cc: Wei Wang <address@hidden>
Reviewed-by: Wei Wang <address@hidden>

---
 drivers/virtio/virtio_balloon.c    | 23 ++++++++++++++++++-----
 include/linux/balloon_compaction.h | 34 +++++++++++++++++++++++++++++++++-
 mm/balloon_compaction.c            | 28 +++++++++++++++++++++-------
 3 files changed, 72 insertions(+), 13 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f0b3a0b..45fe6a8 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -143,16 +143,17 @@ static void set_page_pfns(struct virtio_balloon *vb,
 
 static unsigned 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_pfns;
+       struct page *page;
+       LIST_HEAD(pages);
 
        /* 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;
-            vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
-               struct page *page = balloon_page_enqueue(vb_dev_info);
+       for (num_pfns = 0; num_pfns < num;
+            num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+               struct page *page = balloon_page_alloc();
 
                if (!page) {
                        dev_info_ratelimited(&vb->vdev->dev,
@@ -162,6 +163,18 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
size_t num)
                        msleep(200);
                        break;
                }
+
+               balloon_page_push(&pages, page);
+       }
+
+       mutex_lock(&vb->balloon_lock);
+
+       vb->num_pfns = 0;
+       while ((page = balloon_page_pop(&pages))) {
+               balloon_page_enqueue(&vb->vb_dev_info, page);
+
+               vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
+
                set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
                vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
                if (!virtio_has_feature(vb->vdev,
diff --git a/include/linux/balloon_compaction.h 
b/include/linux/balloon_compaction.h
index 79542b2..bdc055a 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -49,6 +49,7 @@
 #include <linux/gfp.h>
 #include <linux/err.h>
 #include <linux/fs.h>
+#include <linux/list.h>
 
 /*
  * Balloon device information descriptor.
@@ -66,7 +67,9 @@ struct balloon_dev_info {
        struct inode *inode;
 };
 
-extern struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info);
+extern struct page *balloon_page_alloc(void);
+extern void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
+                                struct page *page);
 extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
 
 static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
@@ -86,6 +89,35 @@ extern void balloon_page_putback(struct page *page);
 extern int balloon_page_migrate(struct address_space *mapping,
                                struct page *newpage,
                                struct page *page, enum migrate_mode mode);
+/*
+ * balloon_page_push - insert a page into a page list.
+ * @head : pointer to list
+ * @page : page to be added
+ *
+ * Caller must ensure the page is private and protect the list.
+ */
+static inline void balloon_page_push(struct list_head *pages, struct page 
*page)
+{
+       list_add(&page->lru, pages);
+}
+
+/*
+ * balloon_page_pop - remove a page from a page list.
+ * @head : pointer to list
+ * @page : page to be added
+ *
+ * Caller must ensure the page is private and protect the list.
+ */
+static inline struct page *balloon_page_pop(struct list_head *pages)
+{
+       struct page *page = list_first_entry_or_null(pages, struct page, lru);
+
+       if (!page)
+               return NULL;
+
+       list_del(&page->lru);
+       return page;
+}
 
 /*
  * balloon_page_insert - insert a page into the balloon's page list and make
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 68d2892..16212c7 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -11,22 +11,36 @@
 #include <linux/balloon_compaction.h>
 
 /*
- * balloon_page_enqueue - allocates a new page and inserts it into the balloon
+ * balloon_page_alloc - allocates a new page for insertion into the balloon
  *                       page list.
+ *
+ * Driver must call it to properly allocate a new enlisted balloon page.
+ * Driver must call balloon_page_enqueue before definitively removing it from
+ * the guest system.  This function returns the page address for the recently
+ * allocated page or NULL in the case we fail to allocate a new page this turn.
+ */
+struct page *balloon_page_alloc(void)
+{
+       struct page *page = alloc_page(balloon_mapping_gfp_mask() |
+                                      __GFP_NOMEMALLOC | __GFP_NORETRY);
+       return page;
+}
+EXPORT_SYMBOL_GPL(balloon_page_alloc);
+
+/*
+ * balloon_page_enqueue - inserts a new page into the balloon page list.
  * @b_dev_info: balloon device descriptor where we will insert a new page to
+ * @page: new page to enqueue - allocated using balloon_page_alloc.
  *
- * Driver must call it to properly allocate a new enlisted balloon page
+ * Driver must call it to properly enqueue a new enlisted balloon page
  * before definitively removing it from the guest system.
  * This function returns the page address for the recently enqueued page or
  * NULL in the case we fail to allocate a new page this turn.
  */
-struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info)
+void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
+                         struct page *page)
 {
        unsigned long flags;
-       struct page *page = alloc_page(balloon_mapping_gfp_mask() |
-                                      __GFP_NOMEMALLOC | __GFP_NORETRY);
-       if (!page)
-               return NULL;
 
        /*
         * Block others from accessing the 'page' when we get around to
-- 
2.7.4