Wei Wang wrote:
> The balloon_lock was used to synchronize the access demand to elements
> of struct virtio_balloon and its queue operations (please see commit
> e22504296d). This prevents the concurrent run of the leak_balloon and
> fill_balloon functions, thereby resulting in a deadlock issue on OOM:
>
> fill_balloon: take balloon_lock and wait for OOM to get some memory;
> oom_notify: release some inflated memory via leak_balloon();
> leak_balloon: wait for balloon_lock to be released by fill_balloon.
>
> This patch breaks the lock into two fine-grained inflate_lock and
> deflate_lock, and eliminates the unnecessary use of the shared data
> (i.e. vb->pnfs, vb->num_pfns). This enables leak_balloon and
> fill_balloon to run concurrently and solves the deadlock issue.
>
> @@ -162,20 +160,20 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> msleep(200);
> break;
> }
> - set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> - vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> + set_page_pfns(vb, pfns + num_pfns, page);
> if (!virtio_has_feature(vb->vdev,
> VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> adjust_managed_page_count(page, -1);
> }
>
> - num_allocated_pages = vb->num_pfns;
> + mutex_lock(&vb->inflate_lock);
> /* Did we get any? */
> - if (vb->num_pfns != 0)
> - tell_host(vb, vb->inflate_vq);
> - mutex_unlock(&vb->balloon_lock);
> + if (num_pfns != 0)
> + tell_host(vb, vb->inflate_vq, pfns, num_pfns);
> + mutex_unlock(&vb->inflate_lock);
> + atomic64_add(num_pfns, &vb->num_pages);
Isn't this addition too late? If leak_balloon() is called due to
out_of_memory(), it will fail to find up to dated vb->num_pages value.
>
> - return num_allocated_pages;
> + return num_pfns;
> }
>
> static void release_pages_balloon(struct virtio_balloon *vb,
> @@ -194,38 +192,39 @@ static void release_pages_balloon(struct virtio_balloon *vb,
>
> static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
> {
> - unsigned num_freed_pages;
> struct page *page;
> struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
> LIST_HEAD(pages);
> + unsigned int num_pfns;
> + __virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
This array consumes 1024 bytes of kernel stack, doesn't it?
leak_balloon() might be called from out_of_memory() where kernel stack
is already largely consumed before entering __alloc_pages_nodemask().
For reducing possibility of stack overflow, since out_of_memory() is
serialized by oom_lock, I suggest using static (maybe kmalloc()ed as
vb->oom_pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX]) buffer when called from
out_of_memory().
>
> /* We can only do one array worth at a time. */
> - num = min(num, ARRAY_SIZE(vb->pfns));
> + num = min_t(size_t, num, VIRTIO_BALLOON_ARRAY_PFNS_MAX);
>
> - mutex_lock(&vb->balloon_lock);
> /* We can't release more pages than taken */
> - num = min(num, (size_t)vb->num_pages);
> - for (vb->num_pfns = 0; vb->num_pfns < num;
> - vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> + num = min_t(size_t, num, atomic64_read(&vb->num_pages));
> + for (num_pfns = 0; num_pfns < num;
> + num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> page = balloon_page_dequeue(vb_dev_info);
If balloon_page_dequeue() can be concurrently called by both host's request
and guest's OOM event, is (!dequeued_page) test in balloon_page_dequeue() safe?
Is such concurrency needed?
> if (!page)
> break;
> - set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> + set_page_pfns(vb, pfns + num_pfns, page);
> list_add(&page->lru, &pages);
> - vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
> }
>
> - num_freed_pages = vb->num_pfns;
> /*
> * Note that if
> * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> * is true, we *have* to do it in this order
> */
> - if (vb->num_pfns != 0)
> - tell_host(vb, vb->deflate_vq);
> + mutex_lock(&vb->deflate_lock);
> + if (num_pfns != 0)
> + tell_host(vb, vb->deflate_vq, pfns, num_pfns);
> + mutex_unlock(&vb->deflate_lock);
> release_pages_balloon(vb, &pages);
> - mutex_unlock(&vb->balloon_lock);
> - return num_freed_pages;
> + atomic64_sub(num_pfns, &vb->num_pages);
Isn't this subtraction too late?
> +
> + return num_pfns;
> }
>
> static inline void update_stat(struct virtio_balloon *vb, int idx,
> @@ -465,6 +464,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
> struct virtio_balloon *vb = container_of(vb_dev_info,
> struct virtio_balloon, vb_dev_info);
> unsigned long flags;
> + __virtio32 pfns[VIRTIO_BALLOON_PAGES_PER_PAGE];
If this is called from memory allocation path, maybe kmalloc()ed buffer is safer.
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