Re: [RFC PATCH 1/3] mm: support hugetlb free page reporting

[Mike Kravetz]

On 12/21/20 11:46 PM, Liang Li wrote:
> Free page reporting only supports buddy pages, it can't report the
> free pages reserved for hugetlbfs case. On the other hand, hugetlbfs
> is a good choice for a system with a huge amount of RAM, because it
> can help to reduce the memory management overhead and improve system
> performance.
> This patch add the support for reporting hugepages in the free list
> of hugetlb, it canbe used by virtio_balloon driver for memory
> overcommit and pre zero out free pages for speeding up memory population.

My apologies as I do not follow virtio_balloon driver.  Comments from
the hugetlb perspective.

> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -41,6 +41,7 @@
>  #include <linux/node.h>
>  #include <linux/userfaultfd_k.h>
>  #include <linux/page_owner.h>
> +#include "page_reporting.h"
>  #include "internal.h"
>  
>  int hugetlb_max_hstate __read_mostly;
> @@ -1028,6 +1029,11 @@ static void enqueue_huge_page(struct hstate *h, struct page *page)
>  	list_move(&page->lru, &h->hugepage_freelists[nid]);
>  	h->free_huge_pages++;
>  	h->free_huge_pages_node[nid]++;
> +	if (hugepage_reported(page)) {
> +		__ClearPageReported(page);
> +		pr_info("%s, free_huge_pages=%ld\n", __func__, h->free_huge_pages);
> +	}
> +	hugepage_reporting_notify_free(h->order);
>  }
>  
>  static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
> @@ -5531,6 +5537,29 @@ follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int fla
>  	return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);
>  }
>  
> +bool isolate_free_huge_page(struct page *page, struct hstate *h, int nid)

Looks like this always returns true.  Should it be type void?

> +{
> +	bool ret = true;
> +
> +	VM_BUG_ON_PAGE(!PageHead(page), page);
> +
> +	list_move(&page->lru, &h->hugepage_activelist);
> +	set_page_refcounted(page);
> +	h->free_huge_pages--;
> +	h->free_huge_pages_node[nid]--;
> +
> +	return ret;
> +}
> +

...

> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 20ec3fb1afc4..15d4b5372df8 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -7,6 +7,7 @@
>  #include <linux/delay.h>
>  #include <linux/scatterlist.h>
>  #include <linux/sched.h>
> +#include <linux/hugetlb.h>
>  
>  #include "page_reporting.h"
>  #include "internal.h"
> @@ -16,6 +17,10 @@ static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
>  int page_report_mini_order = pageblock_order;
>  unsigned long page_report_batch_size = 32 * 1024 * 1024;
>  
> +static struct page_reporting_dev_info __rcu *hgpr_dev_info __read_mostly;
> +int hugepage_report_mini_order = pageblock_order;
> +unsigned long hugepage_report_batch_size = 64 * 1024 * 1024;
> +
>  enum {
>  	PAGE_REPORTING_IDLE = 0,
>  	PAGE_REPORTING_REQUESTED,
> @@ -67,6 +72,24 @@ void __page_reporting_notify(void)
>  	rcu_read_unlock();
>  }
>  
> +/* notify prdev of free hugepage reporting request */
> +void __hugepage_reporting_notify(void)
> +{
> +	struct page_reporting_dev_info *prdev;
> +
> +	/*
> +	 * We use RCU to protect the pr_dev_info pointer. In almost all
> +	 * cases this should be present, however in the unlikely case of
> +	 * a shutdown this will be NULL and we should exit.
> +	 */
> +	rcu_read_lock();
> +	prdev = rcu_dereference(hgpr_dev_info);
> +	if (likely(prdev))
> +		__page_reporting_request(prdev);
> +
> +	rcu_read_unlock();
> +}
> +
>  static void
>  page_reporting_drain(struct page_reporting_dev_info *prdev,
>  		     struct scatterlist *sgl, unsigned int nents, bool reported)
> @@ -103,6 +126,213 @@ page_reporting_drain(struct page_reporting_dev_info *prdev,
>  	sg_init_table(sgl, nents);
>  }
>  
> +static void
> +hugepage_reporting_drain(struct page_reporting_dev_info *prdev,
> +			 struct hstate *h, struct scatterlist *sgl,
> +			 unsigned int nents, bool reported)
> +{
> +	struct scatterlist *sg = sgl;
> +
> +	/*
> +	 * Drain the now reported pages back into their respective
> +	 * free lists/areas. We assume at least one page is populated.
> +	 */
> +	do {
> +		struct page *page = sg_page(sg);
> +
> +		putback_isolate_huge_page(h, page);
> +
> +		/* If the pages were not reported due to error skip flagging */
> +		if (!reported)
> +			continue;
> +
> +		__SetPageReported(page);
> +	} while ((sg = sg_next(sg)));
> +
> +	/* reinitialize scatterlist now that it is empty */
> +	sg_init_table(sgl, nents);
> +}
> +
> +/*
> + * The page reporting cycle consists of 4 stages, fill, report, drain, and
> + * idle. We will cycle through the first 3 stages until we cannot obtain a
> + * full scatterlist of pages, in that case we will switch to idle.
> + */

As mentioned, I am not familiar with virtio_balloon and the overall design.
So, some of this does not make sense to me.

> +static int
> +hugepage_reporting_cycle(struct page_reporting_dev_info *prdev,
> +			 struct hstate *h, unsigned int nid,
> +			 struct scatterlist *sgl, unsigned int *offset)
> +{
> +	struct list_head *list = &h->hugepage_freelists[nid];
> +	unsigned int page_len = PAGE_SIZE << h->order;
> +	struct page *page, *next;
> +	long budget;
> +	int ret = 0, scan_cnt = 0;
> +
> +	/*
> +	 * Perform early check, if free area is empty there is
> +	 * nothing to process so we can skip this free_list.
> +	 */
> +	if (list_empty(list))
> +		return ret;

Do note that not all entries on the hugetlb free lists are free.  Reserved
entries are also on the free list.  The actual number of free entries is
'h->free_huge_pages - h->resv_huge_pages'.
Is the intention to process reserved pages as well as free pages?

> +
> +	spin_lock_irq(&hugetlb_lock);
> +
> +	if (huge_page_order(h) > MAX_ORDER)
> +		budget = HUGEPAGE_REPORTING_CAPACITY;
> +	else
> +		budget = HUGEPAGE_REPORTING_CAPACITY * 32;
> +
> +	/* loop through free list adding unreported pages to sg list */
> +	list_for_each_entry_safe(page, next, list, lru) {
> +		/* We are going to skip over the reported pages. */
> +		if (PageReported(page)) {
> +			if (++scan_cnt >= MAX_SCAN_NUM) {
> +				ret = scan_cnt;
> +				break;
> +			}
> +			continue;
> +		}
> +
> +		/*
> +		 * If we fully consumed our budget then update our
> +		 * state to indicate that we are requesting additional
> +		 * processing and exit this list.
> +		 */
> +		if (budget < 0) {
> +			atomic_set(&prdev->state, PAGE_REPORTING_REQUESTED);
> +			next = page;
> +			break;
> +		}
> +
> +		/* Attempt to pull page from list and place in scatterlist */
> +		if (*offset) {
> +			isolate_free_huge_page(page, h, nid);

Once a hugetlb page is isolated, it can not be used and applications that
depend on hugetlb pages can start to fail.
I assume that is acceptable/expected behavior.  Correct?
On some systems, hugetlb pages are a precious resource and the sysadmin
carefully configures the number needed by applications.  Removing a hugetlb
page (even for a very short period of time) could cause serious application
failure.

My apologies if that is a stupid question.  I really have no knowledge of
this area.

> +			/* Add page to scatter list */
> +			--(*offset);
> +			sg_set_page(&sgl[*offset], page, page_len, 0);
> +
> +			continue;
> +		}
> +
> +		/*
> +		 * Make the first non-processed page in the free list
> +		 * the new head of the free list before we release the
> +		 * zone lock.
> +		 */
> +		if (&page->lru != list && !list_is_first(&page->lru, list))
> +			list_rotate_to_front(&page->lru, list);
> +
> +		/* release lock before waiting on report processing */
> +		spin_unlock_irq(&hugetlb_lock);
> +
> +		/* begin processing pages in local list */
> +		ret = prdev->report(prdev, sgl, HUGEPAGE_REPORTING_CAPACITY);
> +
> +		/* reset offset since the full list was reported */
> +		*offset = HUGEPAGE_REPORTING_CAPACITY;
> +
> +		/* update budget to reflect call to report function */
> +		budget--;
> +
> +		/* reacquire zone lock and resume processing */
> +		spin_lock_irq(&hugetlb_lock);
> +
> +		/* flush reported pages from the sg list */
> +		hugepage_reporting_drain(prdev, h, sgl,
> +					 HUGEPAGE_REPORTING_CAPACITY, !ret);
> +
> +		/*
> +		 * Reset next to first entry, the old next isn't valid
> +		 * since we dropped the lock to report the pages
> +		 */
> +		next = list_first_entry(list, struct page, lru);
> +
> +		/* exit on error */
> +		if (ret)
> +			break;
> +	}
> +
> +	/* Rotate any leftover pages to the head of the freelist */
> +	if (&next->lru != list && !list_is_first(&next->lru, list))
> +		list_rotate_to_front(&next->lru, list);
> +
> +	spin_unlock_irq(&hugetlb_lock);
> +
> +	return ret;
> +}

-- 
Mike Kravetz