virtio_balloon: fix deadlock on OOM

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.

  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 <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f0b3a0b9d42f8b8b3ea9c6943a4010c3d7e15f18..7960746f759788d545b9e85e384a56cbf99a7606 100644 (file)
--- 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 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 +163,19 @@ 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 fbbe6da40feda116cd90c8041bd551d56b8b933f..53051f3d8f2561207c4da0d4e12aa1d7e5a0470b 100644 (file)
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -50,6 +50,7 @@
 #include <linux/gfp.h>
 #include <linux/err.h>
 #include <linux/fs.h>
+#include <linux/list.h>
 
 /*
  * Balloon device information descriptor.
@@ -67,7 +68,9 @@ 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)
@@ -193,4 +196,34 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
 }
 
 #endif /* CONFIG_BALLOON_COMPACTION */
+
+/*
+ * 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;
+}
 #endif /* _LINUX_BALLOON_COMPACTION_H */

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 68d28924ba79d66cec76535f19772de6dc429522..ef858d547e2d7b69676aff410a52c32edc7986f0 100644 (file)
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -10,23 +10,38 @@
 #include <linux/export.h>
 #include <linux/balloon_compaction.h>
 
+/*
+ * 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 - allocates a new page and inserts it 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 allocated 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
@@ -39,7 +54,6 @@ struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info)
        __count_vm_event(BALLOON_INFLATE);
        spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
        unlock_page(page);
-       return page;
 }
 EXPORT_SYMBOL_GPL(balloon_page_enqueue);