Re: [PATCH 3/6] mm/balloon_compaction: list interfaces

[Nadav Amit <namit@vmware.com>]

> On Feb 6, 2019, at 4:32 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
> On Wed, Feb 06, 2019 at 03:57:03PM -0800, Nadav Amit wrote:
>> Introduce interfaces for ballooning enqueueing and dequeueing of a list
>> of pages. These interfaces reduce the overhead of storing and restoring
>> IRQs by batching the operations. In addition they do not panic if the
>> list of pages is empty.
>> 

[Snip]

First, thanks for the quick feedback.

>> +
>> +/**
>> + * balloon_page_list_enqueue() - inserts a list of pages into the balloon page
>> + *				 list.
>> + * @b_dev_info: balloon device descriptor where we will insert a new page to
>> + * @pages: pages to enqueue - allocated using balloon_page_alloc.
>> + *
>> + * Driver must call it to properly enqueue a balloon pages before definitively
>> + * removing it from the guest system.
>> + */
>> +void balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
>> +			       struct list_head *pages)
>> +{
>> +	struct page *page, *tmp;
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>> +	list_for_each_entry_safe(page, tmp, pages, lru)
>> +		balloon_page_enqueue_one(b_dev_info, page);
>> +	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> 
> As this is scanning pages one by one anyway, it will be useful
> to have this return the # of pages enqueued.

Sure.

> 
>> +}
>> +EXPORT_SYMBOL_GPL(balloon_page_list_enqueue);
>> +
>> +/**
>> + * balloon_page_list_dequeue() - removes pages from balloon's page list and
>> + *				 returns a list of the pages.
>> + * @b_dev_info: balloon device decriptor where we will grab a page from.
>> + * @pages: pointer to the list of pages that would be returned to the caller.
>> + * @n_req_pages: number of requested pages.
>> + *
>> + * Driver must call it to properly de-allocate a previous enlisted balloon pages
>> + * before definetively releasing it back to the guest system. This function
>> + * tries to remove @n_req_pages from the ballooned pages and return it to the
>> + * caller in the @pages list.
>> + *
>> + * Note that this function may fail to dequeue some pages temporarily empty due
>> + * to compaction isolated pages.
>> + *
>> + * Return: number of pages that were added to the @pages list.
>> + */
>> +int balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
>> +			       struct list_head *pages, int n_req_pages)
> 
> Are we sure this int never overflows? Why not just use u64
> or size_t straight away?

size_t it is.

> 
>> +{
>> +	struct page *page, *tmp;
>> +	unsigned long flags;
>> +	int n_pages = 0;
>> +
>> +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>> +	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
>> +		/*
>> +		 * Block others from accessing the 'page' while we get around
>> +		 * establishing additional references and preparing the 'page'
>> +		 * to be released by the balloon driver.
>> +		 */
>> +		if (!trylock_page(page))
>> +			continue;
>> +
>> +		if (IS_ENABLED(CONFIG_BALLOON_COMPACTION) &&
>> +		    PageIsolated(page)) {
>> +			/* raced with isolation */
>> +			unlock_page(page);
>> +			continue;
>> +		}
>> +		balloon_page_delete(page);
>> +		__count_vm_event(BALLOON_DEFLATE);
>> +		unlock_page(page);
>> +		list_add(&page->lru, pages);
>> +		if (++n_pages >= n_req_pages)
>> +			break;
>> +	}
>> +	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>> +
>> +	return n_pages;
>> +}
>> +EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
>> +
> 
> This looks quite reasonable. In fact virtio can be reworked to use
> this too and then the original one can be dropped.
> 
> Have the time?

Obviously not, but I am willing to make the time. What I cannot “make" is an
approval to send patches for other hypervisors. Let me run a quick check
with our FOSS people here.

Anyhow, I hope it would not prevent the patches from getting to the next
release.