Virtualization - Dec 2017 - Memory corruption in powerpc guests with virtio_balloon (was Re: [PATCH v3] 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 at I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Wei Wang <wei.w.wang at intel.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---

Changes since v2:
	drop an unused declaration, reported by Tetsuo Hocko
Changes from v1:
	fix a build warning

 drivers/virtio/virtio_balloon.c    | 24 +++++++++++++++++++-----
 include/linux/balloon_compaction.h | 35 ++++++++++++++++++++++++++++++++++-
 mm/balloon_compaction.c            | 28 +++++++++++++++++++++-------
 3 files changed, 74 insertions(+), 13 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f0b3a0b..7960746 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 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 fbbe6da..c4c8df9 100644
--- 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)
@@ -89,6 +92,36 @@ 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 68d2892..ef858d5 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
@@ -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);
 
-- 
MST

"Michael S. Tsirkin" <mst at redhat.com> writes:
> 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 at I-love.SAKURA.ne.jp>
> Cc: Michal Hocko <mhocko at suse.com>
> Cc: Wei Wang <wei.w.wang at intel.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
>
>  drivers/virtio/virtio_balloon.c    | 24 +++++++++++++++++++-----
>  include/linux/balloon_compaction.h | 35
++++++++++++++++++++++++++++++++++-
>  mm/balloon_compaction.c            | 28 +++++++++++++++++++++-------
>  3 files changed, 74 insertions(+), 13 deletions(-)

Somehow this commit seems to be killing powerpc guests.

The symptom is that the first page (64K) of the guests memory gets over
written with zeroes, which is where our interrupt handlers are, so the
system rapidly locks up due to illegal instructions in the illegal
instruction handler.

There seems to be some element of a race, because it doesn't always
crash. Sometimes I can boot to a shell, but not often. When it does
happen it's fairly late in boot, but before I get to a shell.

I had a few bisects go off into the weeds due to the intermittent
nature. But once I realised that I changed my script to boot 5 times
before declaring a kernel good, and that bisected straight here.

I can also revert this commit on v4.15-rc1 and everything's fine again -
I got through ~250 boots with that kernel.

So I'm pretty sure this commit is triggering/exposing/causing the bug.

The other data point is that the page that's overwritten is mapped read
only in the guest kernel. So either the guest kernel is writing to it in
real mode (MMU off), or the hypervisor/DMA is doing it.

I haven't isolated if it's host kernel/qemu version dependent, at the
moment I'm just using distro packaged versions of both.

Anyway I'll try and dig further into it on Monday, but I thought I'd let
you know in case this is a known bug with a fix in the pipeline, or
rings any bells or whatever.

cheers

> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index f0b3a0b..7960746 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 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 fbbe6da..c4c8df9 100644
> --- 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)
> @@ -89,6 +92,36 @@ 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 68d2892..ef858d5 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
> @@ -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);
>  
> -- 
> MST

On Fri, Dec 01, 2017 at 11:31:08PM +1100, Michael Ellerman
wrote:
> "Michael S. Tsirkin" <mst at redhat.com> writes:
> 
> > 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 at
I-love.SAKURA.ne.jp>
> > Cc: Michal Hocko <mhocko at suse.com>
> > Cc: Wei Wang <wei.w.wang at intel.com>
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> >
> >  drivers/virtio/virtio_balloon.c    | 24 +++++++++++++++++++-----
> >  include/linux/balloon_compaction.h | 35
++++++++++++++++++++++++++++++++++-
> >  mm/balloon_compaction.c            | 28 +++++++++++++++++++++-------
> >  3 files changed, 74 insertions(+), 13 deletions(-)
> 
> 
> Somehow this commit seems to be killing powerpc guests.
> 
> The symptom is that the first page (64K) of the guests memory gets over
> written with zeroes, which is where our interrupt handlers are, so the
> system rapidly locks up due to illegal instructions in the illegal
> instruction handler.
> 
> There seems to be some element of a race, because it doesn't always
> crash. Sometimes I can boot to a shell, but not often. When it does
> happen it's fairly late in boot, but before I get to a shell.
> 
> I had a few bisects go off into the weeds due to the intermittent
> nature. But once I realised that I changed my script to boot 5 times
> before declaring a kernel good, and that bisected straight here.
> 
> I can also revert this commit on v4.15-rc1 and everything's fine again
-
> I got through ~250 boots with that kernel.
> 
> So I'm pretty sure this commit is triggering/exposing/causing the bug.
> 
> The other data point is that the page that's overwritten is mapped read
> only in the guest kernel. So either the guest kernel is writing to it in
> real mode (MMU off), or the hypervisor/DMA is doing it.
> 
> I haven't isolated if it's host kernel/qemu version dependent, at
the
> moment I'm just using distro packaged versions of both.
> 
> Anyway I'll try and dig further into it on Monday, but I thought
I'd let
> you know in case this is a known bug with a fix in the pipeline, or
> rings any bells or whatever.
> 
> cheers
Thanks for the report!
A fix was just posted:
virtio_balloon: fix increment of vb->num_pfns in fill_balloon()

Would appreciate testing.
> 
> > diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> > index f0b3a0b..7960746 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 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 fbbe6da..c4c8df9 100644
> > --- 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)
> > @@ -89,6 +92,36 @@ 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 68d2892..ef858d5 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
> > @@ -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);
> >  
> > -- 
> > MST