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

[Nadav Amit]

> On Mar 31, 2020, at 6:32 AM, David Hildenbrand <address@hidden> 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 <address@hidden>:
>>>>>> 
>>>>>> 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 <address@hidden>
>>> Date: Tue, 31 Mar 2020 12:28:07 +0200
>>> Subject: [PATCH RFC] tmp
>>> 
>>> Signed-off-by: David Hildenbrand <address@hidden>
>>> ---
>>> 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.