[PATCH] virtio_balloon: fix deadlock on OOM

Michal Hocko
2017-10-13T13:44:38Z

On Fri 13-10-17 16:21:22, Michael S. Tsirkin wrote:
> 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.

OK, that sounds like a better fix. As long as there are no other
allocations or indirect waiting for an allocation this should work
correctly. Thanks!

> 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 at I-love.SAKURA.ne.jp>
> Cc: Michal Hocko <mhocko at suse.com>
> Cc: Wei Wang <wei.w.wang at 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

--
Michal Hocko
SUSE Labs