Message-ID: <de0371da-3bf8-7873-559d-665a0e7c0fc5@redhat.com>
Date:   Wed, 1 Apr 2020 13:21:28 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <namit@...are.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "pagupta@...hat.com" <pagupta@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "mojha@...eaurora.org" <mojha@...eaurora.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Hui Zhu <teawaterz@...ux.alibaba.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        Hui Zhu <teawater@...il.com>
Subject: Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to
 handle THP spilt issue

On 31.03.20 18:27, Nadav Amit wrote:
>> On Mar 31, 2020, at 6:32 AM, David Hildenbrand <david@...hat.com> wrote:
>>
>> On 31.03.20 15:24, Michael S. Tsirkin wrote:
>>> 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@...hat.com>:
>>>>>>>
>>>>>>> On 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 that‘s 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@...hat.com>
>>>> Date: Tue, 31 Mar 2020 12:28:07 +0200
>>>> Subject: [PATCH RFC] tmp
>>>>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>> 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.
>>
>> %__GFP_NORETRY: The VM implementation will try only very lightweight
>> memory direct reclaim to get some memory under memory pressure (thus it
>> can sleep). It will avoid disruptive actions like OOM killer.
>>
>> Certainly good enough for a first version I would say, no? Looking at
>> the vmware balloon, they don't even set __GFP_NORETRY.
> 
> Yes, it does seem that we are missing __GFP_NORETRY. I really do not know
> what I was thinking when I did not add it for huge-pages allocation. I will
> send a patch. Thanks for noticing :)
> 
> In regard to your patch, I would be happy to consolidate the allocation
> mechanisms, so VMware balloon driver would also use your code. In general
> your code looks good, take-away some style issues.

Yeah, let's see in which direction we'll be bringing
balloon_page_alloc(), I think there are still some questions to be
answered (mostly performance implications).

Cheers!

-- 
Thanks,

David / dhildenb

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