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SubjectRe: [PATCH] virtio_balloon: fix deadlock on OOM
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@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

--
Michal Hocko
SUSE Labs

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