Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue

[Michael S. Tsirkin]

On Tue, Mar 31, 2020 at 12:35:24PM +0200, David Hildenbrand wrote:
> On 26.03.20 10:49, Michael S. Tsirkin wrote:
> > On Thu, Mar 26, 2020 at 08:54:04AM +0100, David Hildenbrand wrote:
> >>
> >>
> >>> Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <mst@xxxxxxxxxx>:
> >>>
> >>> ïOn Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbrand wrote:
> >>>>> On 12.03.20 09:47, Michael S. Tsirkin wrote:
> >>>>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote:
> >>>>>> 2. You are essentially stealing THPs in the guest. So the fastest
> >>>>>> mapping (THP in guest and host) is gone. The guest won't be able to make
> >>>>>> use of THP where it previously was able to. I can imagine this implies a
> >>>>>> performance degradation for some workloads. This needs a proper
> >>>>>> performance evaluation.
> >>>>>
> >>>>> I think the problem is more with the alloc_pages API.
> >>>>> That gives you exactly the given order, and if there's
> >>>>> a larger chunk available, it will split it up.
> >>>>>
> >>>>> But for balloon - I suspect lots of other users,
> >>>>> we do not want to stress the system but if a large
> >>>>> chunk is available anyway, then we could handle
> >>>>> that more optimally by getting it all in one go.
> >>>>>
> >>>>>
> >>>>> So if we want to address this, IMHO this calls for a new API.
> >>>>> Along the lines of
> >>>>>
> >>>>>    struct page *alloc_page_range(gfp_t gfp, unsigned int min_order,
> >>>>>                    unsigned int max_order, unsigned int *order)
> >>>>>
> >>>>> the idea would then be to return at a number of pages in the given
> >>>>> range.
> >>>>>
> >>>>> What do you think? Want to try implementing that?
> >>>>
> >>>> You can just start with the highest order and decrement the order until
> >>>> your allocation succeeds using alloc_pages(), which would be enough for
> >>>> a first version. At least I don't see the immediate need for a new
> >>>> kernel API.
> >>>
> >>> OK I remember now.  The problem is with reclaim. Unless reclaim is
> >>> completely disabled, any of these calls can sleep. After it wakes up,
> >>> we would like to get the larger order that has become available
> >>> meanwhile.
> >>>
> >>
> >> Yes, but thatâs a pure optimization IMHO.
> >> So I think we should do a trivial implementation first and then see what we gain from a new allocator API. Then we might also be able to justify it using real numbers.
> >>
> > 
> > Well how do you propose implement the necessary semantics?
> > I think we are both agreed that alloc_page_range is more or
> > less what's necessary anyway - so how would you approximate it
> > on top of existing APIs?
> 
> Looking at drivers/misc/vmw_balloon.c:vmballoon_inflate(), it first
> tries to allocate huge pages using
> 
> 	alloc_pages(__GFP_HIGHMEM|__GFP_NOWARN| __GFP_NOMEMALLOC, 
>                     VMW_BALLOON_2M_ORDER)
> 
> And then falls back to 4k allocations (balloon_page_alloc()) in case
> allocation fails.
> 
> I'm roughly thinking of something like the following, but with an
> optimized reporting interface/bigger pfn array so we can report >
> 1MB at a time. Also, it might make sense to remember the order that
> succeeded across some fill_balloon() calls.
> 
> Don't even expect it to compile ...
> 
> 
> 
> >From 4305f989672ccca4be9293e6d4167e929f3e299b Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@xxxxxxxxxx>
> Date: Tue, 31 Mar 2020 12:28:07 +0200
> Subject: [PATCH RFC] tmp
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_balloon.c    | 38 ++++++++++++++++++--------
>  include/linux/balloon_compaction.h |  7 ++++-
>  mm/balloon_compaction.c            | 43 +++++++++++++++++++++++-------
>  3 files changed, 67 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 8511d258dbb4..0660b1b988f0 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -187,7 +187,7 @@ int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
>  }
>  
>  static void set_page_pfns(struct virtio_balloon *vb,
> -			  __virtio32 pfns[], struct page *page)
> +			  __virtio32 pfns[], struct page *page, int order)
>  {
>  	unsigned int i;
>  
> @@ -197,7 +197,7 @@ static void set_page_pfns(struct virtio_balloon *vb,
>  	 * Set balloon pfns pointing at this page.
>  	 * Note that the first pfn points at start of the page.
>  	 */
> -	for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
> +	for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order); i++)
>  		pfns[i] = cpu_to_virtio32(vb->vdev,
>  					  page_to_balloon_pfn(page) + i);
>  }
> @@ -205,6 +205,7 @@ static void set_page_pfns(struct virtio_balloon *vb,
>  static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  {
>  	unsigned num_allocated_pages;
> +	int order = MAX_ORDER - 1;
>  	unsigned num_pfns;
>  	struct page *page;
>  	LIST_HEAD(pages);
> @@ -212,9 +213,20 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  	/* We can only do one array worth at a time. */
>  	num = min(num, ARRAY_SIZE(vb->pfns));
>  
> +	/*
> +	 * Note: we will currently never allocate more than 1MB due to the
> +	 * pfn array size, so we will not allocate MAX_ORDER - 1 ...
> +	 */
> +
>  	for (num_pfns = 0; num_pfns < num;
> -	     num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> -		struct page *page = balloon_page_alloc();
> +	     num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order)) {
> +		const unsigned long remaining = num - num_pfns;
> +
> +		order = MIN(order,
> +			    get_order(remaining << VIRTIO_BALLOON_PFN_SHIFT));
> +		if ((1 << order) * VIRTIO_BALLOON_PAGES_PER_PAGE > remaining)
> +			order--;
> +		page = balloon_pages_alloc(order);
>  
>  		if (!page) {
>  			dev_info_ratelimited(&vb->vdev->dev,
> @@ -225,6 +237,8 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  			break;
>  		}
>  
> +		/* Continue with the actual order that succeeded. */
> +		order = page_private(page);
>  		balloon_page_push(&pages, page);
>  	}
>  
> @@ -233,14 +247,16 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  	vb->num_pfns = 0;
>  
>  	while ((page = balloon_page_pop(&pages))) {
> +		order = page_order(page);
> +		/* enqueuing will split the page and clear the order */
>  		balloon_page_enqueue(&vb->vb_dev_info, page);
>  
> -		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> -		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
> +		set_page_pfns(vb, vb->pfns + vb->num_pfns, page, order);
> +		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order);
>  		if (!virtio_has_feature(vb->vdev,
>  					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> -			adjust_managed_page_count(page, -1);
> -		vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
> +			adjust_managed_page_count(page, -1 * (1 << order));
> +		vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE * (1 << order);
>  	}
>  
>  	num_allocated_pages = vb->num_pfns;
> @@ -284,7 +300,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
>  		page = balloon_page_dequeue(vb_dev_info);
>  		if (!page)
>  			break;
> -		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +		set_page_pfns(vb, vb->pfns + vb->num_pfns, page, 0);
>  		list_add(&page->lru, &pages);
>  		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
>  	}
> @@ -786,7 +802,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
>  	__count_vm_event(BALLOON_MIGRATE);
>  	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
>  	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -	set_page_pfns(vb, vb->pfns, newpage);
> +	set_page_pfns(vb, vb->pfns, newpage, 0);
>  	tell_host(vb, vb->inflate_vq);
>  
>  	/* balloon's page migration 2nd step -- deflate "page" */
> @@ -794,7 +810,7 @@ static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
>  	balloon_page_delete(page);
>  	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
>  	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -	set_page_pfns(vb, vb->pfns, page);
> +	set_page_pfns(vb, vb->pfns, page, 0);
>  	tell_host(vb, vb->deflate_vq);
>  
>  	mutex_unlock(&vb->balloon_lock);
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 338aa27e4773..ed93fe5704d1 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -60,7 +60,7 @@ struct balloon_dev_info {
>  	struct inode *inode;
>  };
>  
> -extern struct page *balloon_page_alloc(void);
> +extern struct page *balloon_pages_alloc(int order);
>  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);
> @@ -78,6 +78,11 @@ static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
>  	balloon->inode = NULL;
>  }
>  
> +static inline struct page *balloon_page_alloc(void)
> +{
> +	return balloon_pages_alloc(0);
> +}
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  extern const struct address_space_operations balloon_aops;
>  extern bool balloon_page_isolate(struct page *page,
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index 26de020aae7b..067810b32813 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -112,23 +112,35 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
>  EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
>  
>  /*
> - * balloon_page_alloc - allocates a new page for insertion into the balloon
> - *			page list.
> + * balloon_pages_alloc - allocates a new page (of at most the given order)
> + * 			 for insertion into the balloon page list.
>   *
>   * Driver must call this function to properly allocate a new balloon page.
>   * Driver must call balloon_page_enqueue before definitively removing the page
>   * from the guest system.
>   *
> + * Will fall back to smaller orders if allocation fails. The order of the
> + * allocated page is stored in page->private.
> + *
>   * Return: struct page for the allocated page or NULL on allocation failure.
>   */
> -struct page *balloon_page_alloc(void)
> +struct page *balloon_pages_alloc(int order)
>  {
> -	struct page *page = alloc_page(balloon_mapping_gfp_mask() |
> -				       __GFP_NOMEMALLOC | __GFP_NORETRY |
> -				       __GFP_NOWARN);
> -	return page;
> +	struct page *page;
> +
> +	while (order >= 0) {
> +		page = alloc_pages(balloon_mapping_gfp_mask() |
> +				   __GFP_NOMEMALLOC | __GFP_NORETRY |
> +				   __GFP_NOWARN, order);
> +		if (page) {
> +			set_page_private(page, order);
> +			return page;
> +		}
> +		order--;
> +	}
> +	return NULL;
>  }
> -EXPORT_SYMBOL_GPL(balloon_page_alloc);
> +EXPORT_SYMBOL_GPL(balloon_pages_alloc);
>  
>  /*
>   * balloon_page_enqueue - inserts a new page into the balloon page list.


I think this will try to invoke direct reclaim from the first iteration
to free up the max order.

> @@ -146,10 +158,23 @@ EXPORT_SYMBOL_GPL(balloon_page_alloc);
>  void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
>  			  struct page *page)
>  {
> +	const int order = page_private(page);
>  	unsigned long flags;
> +	int i;
> +
> +	/*
> +	 * We can only migrate single pages - and even if we could migrate
> +	 * bigger ones, we would want to split them on demand instead of
> +	 * trying to move around big chunks.
> +	 */
> +	if (order > 0)
> +		split_page(page, order);
> +	set_page_private(page, order);
>  
>  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> -	balloon_page_enqueue_one(b_dev_info, page);
> +	for (i = 0; i < (1 << order); i++)
> +		balloon_page_enqueue_one(b_dev_info, page + i);
> +
>  	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>  }
>  EXPORT_SYMBOL_GPL(balloon_page_enqueue);
> -- 
> 2.25.1
> 
> -- 
> Thanks,
> 
> David / dhildenb