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Subject[PATCH] 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. 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 <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Wang <wei.w.wang@intel.com>
---

This is a replacement for
[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()
but unlike that patch it actually deflates on oom even in presence of
lock contention.

drivers/virtio/virtio_balloon.c | 30 ++++++++++++++++++++++--------
include/linux/balloon_compaction.h | 38 +++++++++++++++++++++++++++++++++++++-
mm/balloon_compaction.c | 27 +++++++++++++++++++++------
3 files changed, 80 insertions(+), 15 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f0b3a0b..725e366 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -143,16 +143,14 @@ 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 +160,22 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
msleep(200);
break;
}
+
+ balloon_page_push(&pages, page);
+ }
+
+ /* We can only do one array worth at a time. */
+ num = min(num, ARRAY_SIZE(vb->pfns));
+
+ 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..88cfac4 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.
@@ -66,9 +67,14 @@ 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);

+extern void balloon_devinfo_splice(struct balloon_dev_info *to_add,
+ struct balloon_dev_info *b_dev_info);
+
static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
{
balloon->isolated_pages = 0;
@@ -88,6 +94,36 @@ extern int balloon_page_migrate(struct address_space *mapping,
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
* the page->private assignment accordingly.
* @balloon : pointer to balloon device
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index b06d9fe..cd605bf 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -11,22 +11,37 @@
#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
--
MST
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