mm / virtio: Provide support for unused page reporting

From: Alexander Duyck <[email protected]>

In order to pave the way for free page reporting in virtualized environments we will need a way to get pages out of the free lists and identify those pages after they have been returned. To accomplish this, this patch adds the concept of a Reported Buddy, which is essentially meant to just be the Uptodate flag used in conjunction with the Buddy page type.

It adds a set of pointers we shall call "boundary" which represents the upper boundary between the unreported and reported pages. The general idea is that in order for a page to cross from one side of the boundary to the other it will need to go through the reporting process. Ultimately a free_list has been fully processed when the boundary has been moved from the tail all they way up to occupying the first entry in the list.

Doing this we should be able to make certain that we keep the reported pages as one contiguous block in each free list. This will allow us to efficiently manipulate the free lists whenever we need to go in and start sending reports to the hypervisor that there are new pages that have been freed and are no longer in use.

An added advantage to this approach is that we should be reducing the overall memory footprint of the guest as it will be more likely to recycle warm pages versus trying to allocate the reported pages that were likely evicted from the guest memory.

Since we will only be reporting one zone at a time we keep the boundary limited to being defined for just the zone we are currently reporting pages from. Doing this we can keep the number of additional pointers needed quite small. To flag that the boundaries are in place we use a single bit in the zone to indicate that reporting and the boundaries are active.

The determination of when to start reporting is based on the tracking of the number of free pages in a given area versus the number of reported pages in that area. We keep track of the number of reported pages per free_area in a separate zone specific area. We do this to avoid modifying the free_area structure as this can lead to false sharing for the highest order with the zone lock which leads to a noticeable performance degradation.

Signed-off-by Alexander Duyck <[email protected]>

---
 include/linux/mmzone.h         |   40 +++++
 include/linux/page-flags.h     |   11 +
 include/linux/page_reporting.h |  138 ++++++++++++++++++
 mm/Kconfig                     |    5 +
 mm/Makefile                    |    1 
 mm/memory_hotplug.c            |    1 
 mm/page_alloc.c                |  136 ++++++++++++++++++
 mm/page_reporting.c            |  299 ++++++++++++++++++++++++++++++++++++++++
 8 files changed, 623 insertions(+), 8 deletions(-)
 create mode 100644 include/linux/page_reporting.h
 create mode 100644 mm/page_reporting.c

include/linux/mmzone.h +37 -3

index f0c68b6b6154..4e6692380deb 100644
@@ -460,6 +460,14 @@ struct zone {
 	seqlock_t		span_seqlock;
 #endif
 
+#ifdef CONFIG_PAGE_REPORTING
+	/*
+	 * Pointer to reported page tracking statistics array. The size of
+	 * the array is MAX_ORDER - PAGE_REPORTING_MIN_ORDER. NULL when
+	 * unused page reporting is not present.
+	 */
+	unsigned long		*reported_pages;
+#endif
 	int initialized;
 
 	/* Write-intensive fields used from the page allocator */

@@ -535,6 +543,14 @@ enum zone_flags {
 	ZONE_BOOSTED_WATERMARK,		/* zone recently boosted watermarks.
 					 * Cleared when kswapd is woken.
 					 */
+	ZONE_PAGE_REPORTING_REQUESTED,	/* zone enabled page reporting and has
+					 * requested flushing the data out of
+					 * higher order pages.
+					 */
+	ZONE_PAGE_REPORTING_ACTIVE,	/* zone enabled page reporting and is
+					 * activly flushing the data out of
+					 * higher order pages.
+					 */
 };
 
 static inline unsigned long zone_managed_pages(struct zone *zone)

@@ -755,6 +771,8 @@ static inline bool pgdat_is_empty(pg_data_t *pgdat)
 	return !pgdat->node_start_pfn && !pgdat->node_spanned_pages;
 }
 
+#include <linux/page_reporting.h>
+
 /* Used for pages not on another list */
 static inline void add_to_free_list(struct page *page, struct zone *zone,
 				    unsigned int order, int migratetype)

@@ -769,10 +787,16 @@ static inline void add_to_free_list(struct page *page, struct zone *zone,
 static inline void add_to_free_list_tail(struct page *page, struct zone *zone,
 					 unsigned int order, int migratetype)
 {
-	struct free_area *area = &zone->free_area[order];
+	struct list_head *tail = get_unreported_tail(zone, order, migratetype);
 
-	list_add_tail(&page->lru, &area->free_list[migratetype]);
-	area->nr_free++;
+	/*
+	 * To prevent the unreported pages from being interleaved with the
+	 * reported ones while we are actively processing pages we will use
+	 * the head of the reported pages to determine the tail of the free
+	 * list.
+	 */
+	list_add_tail(&page->lru, tail);
+	zone->free_area[order].nr_free++;
 }
 
 /* Used for pages which are on another list */

@@ -781,12 +805,22 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 {
 	struct free_area *area = &zone->free_area[order];
 
+	/*
+	 * Clear Hinted flag, if present, to avoid placing reported pages
+	 * at the top of the free_list. It is cheaper to just process this
+	 * page again than to walk around a page that is already reported.
+	 */
+	clear_page_reported(page, zone);
+
 	list_move(&page->lru, &area->free_list[migratetype]);
 }
 
 static inline void del_page_from_free_list(struct page *page, struct zone *zone,
 					   unsigned int order)
 {
+	/* Clear Reported flag, if present, before resetting page type */
+	clear_page_reported(page, zone);
+
 	list_del(&page->lru);
 	__ClearPageBuddy(page);
 	set_page_private(page, 0);

include/linux/page-flags.h +11 -0

index f91cb8898ff0..759a3b3956f2 100644
@@ -163,6 +163,9 @@ enum pageflags {
 
 	/* non-lru isolated movable page */
 	PG_isolated = PG_reclaim,
+
+	/* Buddy pages. Used to track which pages have been reported */
+	PG_reported = PG_uptodate,
 };
 
 #ifndef __GENERATING_BOUNDS_H

@@ -432,6 +435,14 @@ static inline bool set_hwpoison_free_buddy_page(struct page *page)
 #endif
 
 /*
+ * PageReported() is used to track reported free pages within the Buddy
+ * allocator. We can use the non-atomic version of the test and set
+ * operations as both should be shielded with the zone lock to prevent
+ * any possible races on the setting or clearing of the bit.
+ */
+__PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
+
+/*
  * On an anonymous page mapped into a user virtual memory area,
  * page->mapping points to its anon_vma, not to a struct address_space;
  * with the PAGE_MAPPING_ANON bit set to distinguish it.  See rmap.h.

include/linux/page_reporting.h new file +138 -0

new file mode 100644
index 000000000000..498bde6ea764
@@ -0,0 +1,138 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PAGE_REPORTING_H
+#define _LINUX_PAGE_REPORTING_H
+
+#include <linux/mmzone.h>
+#include <linux/jump_label.h>
+#include <linux/pageblock-flags.h>
+#include <asm/pgtable_types.h>
+
+#define PAGE_REPORTING_MIN_ORDER	pageblock_order
+#define PAGE_REPORTING_HWM		32
+
+#ifdef CONFIG_PAGE_REPORTING
+struct page_reporting_dev_info {
+	/* function that alters pages to make them "reported" */
+	void (*report)(struct page_reporting_dev_info *phdev,
+		       unsigned int nents);
+
+	/* scatterlist containing pages to be processed */
+	struct scatterlist *sg;
+
+	/*
+	 * Upper limit on the number of pages that the react function
+	 * expects to be placed into the batch list to be processed.
+	 */
+	unsigned long capacity;
+
+	/* work struct for processing reports */
+	struct delayed_work work;
+
+	/*
+	 * The number of zones requesting reporting, plus one additional if
+	 * processing thread is active.
+	 */
+	atomic_t refcnt;
+};
+
+extern struct static_key page_reporting_notify_enabled;
+
+/* Boundary functions */
+struct list_head *__page_reporting_get_boundary(unsigned int order,
+						int migratetype);
+void page_reporting_del_from_boundary(struct page *page, struct zone *zone);
+void page_reporting_add_to_boundary(struct page *page, struct zone *zone,
+				    int migratetype);
+
+/* Hinted page accessors, defined in page_alloc.c */
+struct page *get_unreported_page(struct zone *zone, unsigned int order,
+				 int migratetype);
+void put_reported_page(struct zone *zone, struct page *page);
+
+void __page_reporting_request(struct zone *zone);
+void __page_reporting_free_stats(struct zone *zone);
+
+/* Tear-down and bring-up for page reporting devices */
+void page_reporting_shutdown(struct page_reporting_dev_info *phdev);
+int page_reporting_startup(struct page_reporting_dev_info *phdev);
+#endif /* CONFIG_PAGE_REPORTING */
+
+static inline struct list_head *
+get_unreported_tail(struct zone *zone, unsigned int order, int migratetype)
+{
+#ifdef CONFIG_PAGE_REPORTING
+	if (order >= PAGE_REPORTING_MIN_ORDER &&
+	    test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
+		return __page_reporting_get_boundary(order, migratetype);
+#endif
+	return &zone->free_area[order].free_list[migratetype];
+}
+
+static inline void clear_page_reported(struct page *page,
+				     struct zone *zone)
+{
+#ifdef CONFIG_PAGE_REPORTING
+	if (likely(!PageReported(page)))
+		return;
+
+	/* push boundary back if we removed the upper boundary */
+	if (test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
+		page_reporting_del_from_boundary(page, zone);
+
+	__ClearPageReported(page);
+
+	/* page_private will contain the page order, so just use it directly */
+	zone->reported_pages[page_private(page) - PAGE_REPORTING_MIN_ORDER]--;
+#endif
+}
+
+/* Free reported_pages and reset reported page tracking count to 0 */
+static inline void page_reporting_reset(struct zone *zone)
+{
+#ifdef CONFIG_PAGE_REPORTING
+	if (zone->reported_pages)
+		__page_reporting_free_stats(zone);
+#endif
+}
+
+/**
+ * page_reporting_notify_free - Free page notification to start page processing
+ * @zone: Pointer to current zone of last page processed
+ * @order: Order of last page added to zone
+ *
+ * This function is meant to act as a screener for __page_reporting_request
+ * which will determine if a give zone has crossed over the high-water mark
+ * that will justify us beginning page treatment. If we have crossed that
+ * threshold then it will start the process of pulling some pages and
+ * placing them in the batch list for treatment.
+ */
+static inline void page_reporting_notify_free(struct zone *zone, int order)
+{
+#ifdef CONFIG_PAGE_REPORTING
+	unsigned long nr_reported;
+
+	/* Called from hot path in __free_one_page() */
+	if (!static_key_false(&page_reporting_notify_enabled))
+		return;
+
+	/* Limit notifications only to higher order pages */
+	if (order < PAGE_REPORTING_MIN_ORDER)
+		return;
+
+	/* Do not bother with tests if we have already requested reporting */
+	if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
+		return;
+
+	/* If reported_pages is not populated, assume 0 */
+	nr_reported = zone->reported_pages ?
+		    zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER] : 0;
+
+	/* Only request it if we have enough to begin the page reporting */
+	if (zone->free_area[order].nr_free < nr_reported + PAGE_REPORTING_HWM)
+		return;
+
+	/* This is slow, but should be called very rarely */
+	__page_reporting_request(zone);
+#endif
+}
+#endif /*_LINUX_PAGE_REPORTING_H */

mm/Kconfig +5 -0

index 56cec636a1fc..f5c68bba522f 100644
@@ -237,6 +237,11 @@ config COMPACTION
           [email protected].
 
 #
+# support for unused page reporting
+config PAGE_REPORTING
+	bool
+
+#
 # support for page migration
 #
 config MIGRATION

mm/Makefile +1 -0

index d0b295c3b764..1e17ba0ed2f0 100644
@@ -105,3 +105,4 @@ obj-$(CONFIG_PERCPU_STATS) += percpu-stats.o
 obj-$(CONFIG_ZONE_DEVICE) += memremap.o
 obj-$(CONFIG_HMM_MIRROR) += hmm.o
 obj-$(CONFIG_MEMFD_CREATE) += memfd.o
+obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o

mm/memory_hotplug.c +1 -0

index 9a82e12bd0e7..3acd2c3e53b3 100644
@@ -1608,6 +1608,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	if (!populated_zone(zone)) {
 		zone_pcp_reset(zone);
 		build_all_zonelists(NULL);
+		page_reporting_reset(zone);
 	} else
 		zone_pcp_update(zone);
 

mm/page_alloc.c +100 -5

index 71aadc7d5ff6..69b848e5b83f 100644
@@ -68,6 +68,7 @@
 #include <linux/lockdep.h>
 #include <linux/nmi.h>
 #include <linux/psi.h>
+#include <linux/page_reporting.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>

@@ -915,7 +916,7 @@ static inline struct capture_control *task_capc(struct zone *zone)
 static inline void __free_one_page(struct page *page,
 		unsigned long pfn,
 		struct zone *zone, unsigned int order,
-		int migratetype)
+		int migratetype, bool reported)
 {
 	struct capture_control *capc = task_capc(zone);
 	unsigned long uninitialized_var(buddy_pfn);

@@ -990,11 +991,20 @@ static inline void __free_one_page(struct page *page,
 done_merging:
 	set_page_order(page, order);
 
-	if (is_shuffle_order(order) ? shuffle_add_to_tail() :
-	    buddy_merge_likely(pfn, buddy_pfn, page, order))
+	if (reported ||
+	    (is_shuffle_order(order) ? shuffle_add_to_tail() :
+	     buddy_merge_likely(pfn, buddy_pfn, page, order)))
 		add_to_free_list_tail(page, zone, order, migratetype);
 	else
 		add_to_free_list(page, zone, order, migratetype);
+
+	/*
+	 * No need to notify on a reported page as the total count of
+	 * unreported pages will not have increased since we have essentially
+	 * merged the reported page with one or more unreported pages.
+	 */
+	if (!reported)
+		page_reporting_notify_free(zone, order);
 }
 
 /*

@@ -1305,7 +1315,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 		if (unlikely(isolated_pageblocks))
 			mt = get_pageblock_migratetype(page);
 
-		__free_one_page(page, page_to_pfn(page), zone, 0, mt);
+		__free_one_page(page, page_to_pfn(page), zone, 0, mt, false);
 		trace_mm_page_pcpu_drain(page, 0, mt);
 	}
 	spin_unlock(&zone->lock);

@@ -1321,7 +1331,7 @@ static void free_one_page(struct zone *zone,
 		is_migrate_isolate(migratetype))) {
 		migratetype = get_pfnblock_migratetype(page, pfn);
 	}
-	__free_one_page(page, pfn, zone, order, migratetype);
+	__free_one_page(page, pfn, zone, order, migratetype, false);
 	spin_unlock(&zone->lock);
 }
 

@@ -2183,6 +2193,122 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
 	return NULL;
 }
 
+#ifdef CONFIG_PAGE_REPORTING
+/**
+ * get_unreported_page - Pull an unreported page from the free_list
+ * @zone: Zone to draw pages from
+ * @order: Order to draw pages from
+ * @mt: Migratetype to draw pages from
+ *
+ * This function will obtain a page from the free list. It will start by
+ * attempting to pull from the tail of the free list and if that is already
+ * reported on it will instead pull the head if that is unreported.
+ *
+ * The page will have the migrate type and order stored in the page
+ * metadata. While being processed the page will not be avaialble for
+ * allocation.
+ *
+ * Return: page pointer if raw page found, otherwise NULL
+ */
+struct page *get_unreported_page(struct zone *zone, unsigned int order, int mt)
+{
+	struct list_head *tail = get_unreported_tail(zone, order, mt);
+	struct free_area *area = &(zone->free_area[order]);
+	struct list_head *list = &area->free_list[mt];
+	struct page *page;
+
+	/* zone lock should be held when this function is called */
+	lockdep_assert_held(&zone->lock);
+
+	/* Find a page of the appropriate size in the preferred list */
+	page = list_last_entry(tail, struct page, lru);
+	list_for_each_entry_from_reverse(page, list, lru) {
+		/* If we entered this loop then the "raw" list isn't empty */
+
+		/* If the page is reported try the head of the list */
+		if (PageReported(page)) {
+			page = list_first_entry(list, struct page, lru);
+
+			/*
+			 * If both the head and tail are reported then reset
+			 * the boundary so that we read as an empty list
+			 * next time and bail out.
+			 */
+			if (PageReported(page)) {
+				page_reporting_add_to_boundary(page, zone, mt);
+				break;
+			}
+		}
+
+		del_page_from_free_list(page, zone, order);
+
+		/* record migratetype and order within page */
+		set_pcppage_migratetype(page, mt);
+		set_page_private(page, order);
+
+		/*
+		 * Page will not be available for allocation while we are
+		 * processing it so update the freepage state.
+		 */
+		__mod_zone_freepage_state(zone, -(1 << order), mt);
+
+		return page;
+	}
+
+	return NULL;
+}
+
+/**
+ * put_reported_page - Return a now-reported page back where we got it
+ * @zone: Zone to return pages to
+ * @page: Page that was reported
+ *
+ * This function will pull the migratetype and order information out
+ * of the page and attempt to return it where it found it. If the page
+ * is added to the free list without changes we will mark it as being
+ * reported.
+ */
+void put_reported_page(struct zone *zone, struct page *page)
+{
+	unsigned int order, mt;
+	unsigned long pfn;
+
+	/* zone lock should be held when this function is called */
+	lockdep_assert_held(&zone->lock);
+
+	mt = get_pcppage_migratetype(page);
+	pfn = page_to_pfn(page);

mm/page_alloc.c +21 -0

+
+	/*
+	 * If page was comingled with another page we cannot consider
+	 * the result to be "reported" since part of the page hasn't been.
+	 * In this case we will simply exit and not update the "reported"
+	 * state. Instead just treat the result as a unreported page.
+	 */
+	if (!PageBuddy(page) || page_order(page) != order)
+		return;
+
+	/* update areated page accounting */
+	zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]++;
+
+	/* update boundary of new migratetype and record it */
+	page_reporting_add_to_boundary(page, zone, mt);
+
+	/* flag page as reported */
+	__SetPageReported(page);
+}
+#endif /* CONFIG_PAGE_REPORTING */
+
 /*
  * This array describes the order lists are fallen back to when
  * the free lists for the desirable migrate type are depleted

mm/page_reporting.c new file +276 -0

new file mode 100644
index 000000000000..971138205ae5
@@ -0,0 +1,299 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mm.h>
+#include <linux/mmzone.h>
+#include <linux/page-isolation.h>
+#include <linux/gfp.h>
+#include <linux/export.h>
+#include <linux/delay.h>
+#include <linux/slab.h>
+#include <linux/scatterlist.h>
+#include "internal.h"
+
+static struct page_reporting_dev_info __rcu *ph_dev_info __read_mostly;
+struct static_key page_reporting_notify_enabled;
+
+struct list_head *boundary[MAX_ORDER - PAGE_REPORTING_MIN_ORDER][MIGRATE_TYPES];
+
+static void page_reporting_reset_boundary(struct zone *zone, unsigned int order,
+					  unsigned int migratetype)
+{
+	boundary[order - PAGE_REPORTING_MIN_ORDER][migratetype] =
+			&zone->free_area[order].free_list[migratetype];
+}
+
+#define for_each_reporting_migratetype_order(_order, _type) \
+	for (_order = MAX_ORDER; _order-- != PAGE_REPORTING_MIN_ORDER;) \
+		for (_type = MIGRATE_TYPES; _type--;)
+
+static int page_reporting_populate_metadata(struct zone *zone)
+{
+	unsigned int order, mt;
+
+	/*
+	 * We need to make sure we have somewhere to store the tracking
+	 * data for how many reported pages are in the zone. To do that
+	 * we need to make certain zone->reported_pages is populated.
+	 */
+	if (!zone->reported_pages) {
+		zone->reported_pages =
+			kcalloc(MAX_ORDER - PAGE_REPORTING_MIN_ORDER,
+				sizeof(unsigned long),
+				GFP_KERNEL);
+		if (!zone->reported_pages)
+			return -ENOMEM;
+	}
+
+	/* Update boundary data to reflect the zone we are currently working */
+	for_each_reporting_migratetype_order(order, mt)
+		page_reporting_reset_boundary(zone, order, mt);
+
+	return 0;
+}
+
+struct list_head *__page_reporting_get_boundary(unsigned int order,
+						int migratetype)
+{
+	return boundary[order - PAGE_REPORTING_MIN_ORDER][migratetype];
+}
+
+void page_reporting_del_from_boundary(struct page *page, struct zone *zone)
+{
+	unsigned int order = page_private(page) - PAGE_REPORTING_MIN_ORDER;
+	int mt = get_pcppage_migratetype(page);
+	struct list_head **tail = &boundary[order][mt];
+
+	if (*tail == &page->lru)
+		*tail = page->lru.next;
+}
+
+void page_reporting_add_to_boundary(struct page *page, struct zone *zone,
+				    int migratetype)
+{
+	unsigned int order = page_private(page) - PAGE_REPORTING_MIN_ORDER;
+	struct list_head **tail = &boundary[order][migratetype];
+
+	*tail = &page->lru;
+}
+
+static unsigned int page_reporting_fill(struct zone *zone,
+					struct page_reporting_dev_info *phdev)
+{
+	struct scatterlist *sg = phdev->sg;
+	unsigned int order, mt, count = 0;
+
+	sg_init_table(phdev->sg, phdev->capacity);
+
+	for_each_reporting_migratetype_order(order, mt) {
+		struct page *page;
+
+		/*
+		 * Pull pages from free list until we have drained
+		 * it or we have reached capacity.
+		 */
+		while ((page = get_unreported_page(zone, order, mt))) {
+			sg_set_page(&sg[count], page, PAGE_SIZE << order, 0);
+
+			if (++count == phdev->capacity)
+				return count;
+		}
+	}
+
+	/* mark end of scatterlist due to underflow */
+	if (count)
+		sg_mark_end(&sg[count - 1]);
+
+	/*
+	 * If there are no longer enough free pages to fully populate
+	 * the scatterlist, then we can just shut it down for this zone.
+	 */
+	__clear_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags);
+	atomic_dec(&phdev->refcnt);
+
+	return count;
+}
+
+static void page_reporting_drain(struct zone *zone,
+				 struct page_reporting_dev_info *phdev)
+{
+	struct scatterlist *sg = phdev->sg;
+
+	/*
+	 * Drain the now reported pages back into their respective
+	 * free lists/areas. We assume at least one page is populated.
+	 */
+	do {
+		put_reported_page(zone, sg_page(sg));
+	} while (!sg_is_last(sg++));
+}
+
+/*
+ * The page reporting cycle consists of 4 stages, fill, report, drain, and idle.
+ * We will cycle through the first 3 stages until we fail to obtain any
+ * pages, in that case we will switch to idle.
+ */
+static void page_reporting_cycle(struct zone *zone,
+				 struct page_reporting_dev_info *phdev)
+{
+	/*
+	 * Guarantee boundaries and stats are populated before we
+	 * start placing reported pages in the zone.
+	 */
+	if (page_reporting_populate_metadata(zone))
+		return;
+
+	spin_lock(&zone->lock);
+
+	/* set bit indicating boundaries are present */
+	__set_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags);
+
+	do {
+		/* Pull pages out of allocator into a scaterlist */
+		unsigned int nents = page_reporting_fill(zone, phdev);
+
+		/* no pages were acquired, give up */
+		if (!nents)
+			break;
+
+		spin_unlock(&zone->lock);
+
+		/* begin processing pages in local list */
+		phdev->report(phdev, nents);
+
+		spin_lock(&zone->lock);
+
+		/*
+		 * We should have a scatterlist of pages that have been
+		 * processed. Return them to their original free lists.
+		 */
+		page_reporting_drain(zone, phdev);
+
+		/* keep pulling pages till there are none to pull */
+	} while (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags));
+
+	/* processing of the zone is complete, we can disable boundaries */
+	__clear_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags);
+
+	spin_unlock(&zone->lock);
+}
+
+static void page_reporting_process(struct work_struct *work)
+{
+	struct delayed_work *d_work = to_delayed_work(work);
+	struct page_reporting_dev_info *phdev =
+		container_of(d_work, struct page_reporting_dev_info, work);
+	struct zone *zone = first_online_pgdat()->node_zones;
+
+	do {
+		if (test_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags))
+			page_reporting_cycle(zone, phdev);
+
+		/*
+		 * Move to next zone, if at the end of the list
+		 * test to see if we can just go into idle.
+		 */
+		zone = next_zone(zone);
+		if (zone)
+			continue;
+		zone = first_online_pgdat()->node_zones;
+
+		/*
+		 * As long as refcnt has not reached zero there are still
+		 * zones to be processed.
+		 */
+	} while (atomic_read(&phdev->refcnt));
+}
+
+/* request page reporting on this zone */
+void __page_reporting_request(struct zone *zone)
+{
+	struct page_reporting_dev_info *phdev;
+
+	rcu_read_lock();
+
+	/*
+	 * We use RCU to protect the ph_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.
+	 */
+	phdev = rcu_dereference(ph_dev_info);
+	if (unlikely(!phdev))
+		return;
+
+	/*
+	 * We can use separate test and set operations here as there
+	 * is nothing else that can set or clear this bit while we are
+	 * holding the zone lock. The advantage to doing it this way is
+	 * that we don't have to dirty the cacheline unless we are
+	 * changing the value.
+	 */
+	__set_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags);
+
+	/*
+	 * Delay the start of work to allow a sizable queue to
+	 * build. For now we are limiting this to running no more
+	 * than 10 times per second.
+	 */
+	if (!atomic_fetch_inc(&phdev->refcnt))
+		schedule_delayed_work(&phdev->work, HZ / 10);
+
+	rcu_read_unlock();
+}
+
+void __page_reporting_free_stats(struct zone *zone)
+{
+	/* free reported_page statisitics */
+	kfree(zone->reported_pages);
+	zone->reported_pages = NULL;
+}
+
+void page_reporting_shutdown(struct page_reporting_dev_info *phdev)
+{
+	if (rcu_access_pointer(ph_dev_info) != phdev)
+		return;
+
+	/* Disable page reporting notification */
+	static_key_slow_dec(&page_reporting_notify_enabled);
+	RCU_INIT_POINTER(ph_dev_info, NULL);
+	synchronize_rcu();
+
+	/* Flush any existing work, and lock it out */
+	cancel_delayed_work_sync(&phdev->work);
+
+	/* Free scatterlist */
+	kfree(phdev->sg);
+	phdev->sg = NULL;
+}
+EXPORT_SYMBOL_GPL(page_reporting_shutdown);
+
+int page_reporting_startup(struct page_reporting_dev_info *phdev)
+{
+	struct zone *zone;
+
+	/* nothing to do if already in use */
+	if (rcu_access_pointer(ph_dev_info))
+		return -EBUSY;
+
+	/* allocate scatterlist to store pages being reported on */

mm/page_reporting.c new file +13 -0

+	for_each_populated_zone(zone) {
+		spin_lock(&zone->lock);
+		__page_reporting_request(zone);
+		spin_unlock(&zone->lock);
+	}
+
+	/* enable page reporting notification */
+	static_key_slow_inc(&page_reporting_notify_enabled);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(page_reporting_startup);
+

Quoted 55 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

In order to pave the way for free page reporting in virtualized environments we will need a way to get pages out of the free lists and identify those pages after they have been returned. To accomplish this, this patch adds the concept of a Reported Buddy, which is essentially meant to just be the Uptodate flag used in conjunction with the Buddy page type.

It adds a set of pointers we shall call "boundary" which represents the upper boundary between the unreported and reported pages. The general idea is that in order for a page to cross from one side of the boundary to the other it will need to go through the reporting process. Ultimately a free_list has been fully processed when the boundary has been moved from the tail all they way up to occupying the first entry in the list.

Doing this we should be able to make certain that we keep the reported pages as one contiguous block in each free list. This will allow us to efficiently manipulate the free lists whenever we need to go in and start sending reports to the hypervisor that there are new pages that have been freed and are no longer in use.

An added advantage to this approach is that we should be reducing the overall memory footprint of the guest as it will be more likely to recycle warm pages versus trying to allocate the reported pages that were likely evicted from the guest memory.

Since we will only be reporting one zone at a time we keep the boundary limited to being defined for just the zone we are currently reporting pages from. Doing this we can keep the number of additional pointers needed quite small. To flag that the boundaries are in place we use a single bit in the zone to indicate that reporting and the boundaries are active.

The determination of when to start reporting is based on the tracking of the number of free pages in a given area versus the number of reported pages in that area. We keep track of the number of reported pages per free_area in a separate zone specific area. We do this to avoid modifying the free_area structure as this can lead to false sharing for the highest order with the zone lock which leads to a noticeable performance degradation.

Signed-off-by Alexander Duyck <[email protected]>

---
 include/linux/mmzone.h         |   40 +++++
 include/linux/page-flags.h     |   11 +
 include/linux/page_reporting.h |  138 ++++++++++++++++++
 mm/Kconfig                     |    5 +
 mm/Makefile                    |    1
 mm/memory_hotplug.c            |    1
 mm/page_alloc.c                |  136 ++++++++++++++++++
 mm/page_reporting.c            |  299 ++++++++++++++++++++++++++++++++++++++++
 8 files changed, 623 insertions(+), 8 deletions(-)
 create mode 100644 include/linux/page_reporting.h
 create mode 100644 mm/page_reporting.c

Quoted 158 lines

mm/page_alloc.c +107 -5

index 71aadc7d5ff6..69b848e5b83f 100644
@@ -68,6 +68,7 @@
 #include <linux/lockdep.h>
 #include <linux/nmi.h>
 #include <linux/psi.h>
+#include <linux/page_reporting.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>

@@ -915,7 +916,7 @@ static inline struct capture_control *task_capc(struct zone *zone)
 static inline void __free_one_page(struct page *page,
              unsigned long pfn,
              struct zone *zone, unsigned int order,
-             int migratetype)
+             int migratetype, bool reported)
 {
      struct capture_control *capc = task_capc(zone);
      unsigned long uninitialized_var(buddy_pfn);

@@ -990,11 +991,20 @@ static inline void __free_one_page(struct page *page,
 done_merging:
      set_page_order(page, order);
 
-     if (is_shuffle_order(order) ? shuffle_add_to_tail() :
-         buddy_merge_likely(pfn, buddy_pfn, page, order))
+     if (reported ||
+         (is_shuffle_order(order) ? shuffle_add_to_tail() :
+          buddy_merge_likely(pfn, buddy_pfn, page, order)))
              add_to_free_list_tail(page, zone, order, migratetype);
      else
              add_to_free_list(page, zone, order, migratetype);
+
+     /*
+      * No need to notify on a reported page as the total count of
+      * unreported pages will not have increased since we have essentially
+      * merged the reported page with one or more unreported pages.
+      */
+     if (!reported)
+             page_reporting_notify_free(zone, order);
 }
 
 /*

@@ -1305,7 +1315,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
              if (unlikely(isolated_pageblocks))
                      mt = get_pageblock_migratetype(page);
 
-             __free_one_page(page, page_to_pfn(page), zone, 0, mt);
+             __free_one_page(page, page_to_pfn(page), zone, 0, mt, false);
              trace_mm_page_pcpu_drain(page, 0, mt);
      }
      spin_unlock(&zone->lock);

@@ -1321,7 +1331,7 @@ static void free_one_page(struct zone *zone,
              is_migrate_isolate(migratetype))) {
              migratetype = get_pfnblock_migratetype(page, pfn);
      }
-     __free_one_page(page, pfn, zone, order, migratetype);
+     __free_one_page(page, pfn, zone, order, migratetype, false);
      spin_unlock(&zone->lock);
 }
 

@@ -2183,6 +2193,122 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
      return NULL;
 }
 
+#ifdef CONFIG_PAGE_REPORTING
+/**
+ * get_unreported_page - Pull an unreported page from the free_list
+ * @zone: Zone to draw pages from
+ * @order: Order to draw pages from
+ * @mt: Migratetype to draw pages from
+ *
+ * This function will obtain a page from the free list. It will start by
+ * attempting to pull from the tail of the free list and if that is already
+ * reported on it will instead pull the head if that is unreported.
+ *
+ * The page will have the migrate type and order stored in the page
+ * metadata. While being processed the page will not be avaialble for
+ * allocation.
+ *
+ * Return: page pointer if raw page found, otherwise NULL
+ */
+struct page *get_unreported_page(struct zone *zone, unsigned int order, int mt)
+{
+     struct list_head *tail = get_unreported_tail(zone, order, mt);
+     struct free_area *area = &(zone->free_area[order]);
+     struct list_head *list = &area->free_list[mt];
+     struct page *page;
+
+     /* zone lock should be held when this function is called */
+     lockdep_assert_held(&zone->lock);
+
+     /* Find a page of the appropriate size in the preferred list */
+     page = list_last_entry(tail, struct page, lru);
+     list_for_each_entry_from_reverse(page, list, lru) {
+             /* If we entered this loop then the "raw" list isn't empty */
+
+             /* If the page is reported try the head of the list */
+             if (PageReported(page)) {
+                     page = list_first_entry(list, struct page, lru);
+
+                     /*
+                      * If both the head and tail are reported then reset
+                      * the boundary so that we read as an empty list
+                      * next time and bail out.
+                      */
+                     if (PageReported(page)) {
+                             page_reporting_add_to_boundary(page, zone, mt);
+                             break;
+                     }
+             }
+
+             del_page_from_free_list(page, zone, order);
+
+             /* record migratetype and order within page */
+             set_pcppage_migratetype(page, mt);
+             set_page_private(page, order);
+
+             /*
+              * Page will not be available for allocation while we are
+              * processing it so update the freepage state.
+              */
+             __mod_zone_freepage_state(zone, -(1 << order), mt);
+
+             return page;
+     }
+
+     return NULL;
+}
+
+/**
+ * put_reported_page - Return a now-reported page back where we got it
+ * @zone: Zone to return pages to
+ * @page: Page that was reported
+ *
+ * This function will pull the migratetype and order information out
+ * of the page and attempt to return it where it found it. If the page
+ * is added to the free list without changes we will mark it as being
+ * reported.
+ */
+void put_reported_page(struct zone *zone, struct page *page)
+{
+     unsigned int order, mt;
+     unsigned long pfn;
+
+     /* zone lock should be held when this function is called */
+     lockdep_assert_held(&zone->lock);
+
+     mt = get_pcppage_migratetype(page);
+     pfn = page_to_pfn(page);
+
+     if (unlikely(has_isolate_pageblock(zone) || is_migrate_isolate(mt))) {
+             mt = get_pfnblock_migratetype(page, pfn);
+             set_pcppage_migratetype(page, mt);
+     }
+
+     order = page_private(page);

mm/page_alloc.c struct page *__rmqueue_smallest(struct zone *zone, unsigned int order, · line 2290

Quoted 3 lines

mm/page_alloc.c +3 -0

+     set_page_private(page, 0);
+
+     __free_one_page(page, pfn, zone, order, mt, true);

I don't think we need to hold the zone lock for fetching migratetype and other information.
We can save some lock held time by acquiring and releasing zone lock before and after __free_one_page() respectively. Isn't?

Quoted 36 lines

mm/page_alloc.c +21 -0

+
+     /*
+      * If page was comingled with another page we cannot consider
+      * the result to be "reported" since part of the page hasn't been.
+      * In this case we will simply exit and not update the "reported"
+      * state. Instead just treat the result as a unreported page.
+      */
+     if (!PageBuddy(page) || page_order(page) != order)
+             return;
+
+     /* update areated page accounting */
+     zone->reported_pages[order - PAGE_REPORTING_MIN_ORDER]++;
+
+     /* update boundary of new migratetype and record it */
+     page_reporting_add_to_boundary(page, zone, mt);
+
+     /* flag page as reported */
+     __SetPageReported(page);
+}
+#endif /* CONFIG_PAGE_REPORTING */
+
 /*
  * This array describes the order lists are fallen back to when
  * the free lists for the desirable migrate type are depleted

mm/page_reporting.c new file +6 -0

new file mode 100644
index 000000000000..971138205ae5
@@ -0,0 +1,299 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mm.h>
+#include <linux/mmzone.h>
+#include <linux/page-isolation.h>
+#include <linux/gfp.h>
+#include <linux/export.h>

Quoted 15 lines

mm/page_reporting.c new file +15 -0

+int page_reporting_startup(struct page_reporting_dev_info *phdev)
+{
+     struct zone *zone;
+
+     /* nothing to do if already in use */
+     if (rcu_access_pointer(ph_dev_info))
+             return -EBUSY;
+
+     /* allocate scatterlist to store pages being reported on */
+     phdev->sg = kcalloc(phdev->capacity, sizeof(*phdev->sg), GFP_KERNEL);
+     if (!phdev->sg)
+             return -ENOMEM;
+
+     /* initialize refcnt and work structures */
+     atomic_set(&phdev->refcnt, 0);

mm/page_reporting.c line 286

Quoted 4 lines

mm/page_reporting.c new file +4 -0

+     INIT_DELAYED_WORK(&phdev->work, &page_reporting_process);
+
+     /* assign device, and begin initial flush of populated zones */
+     rcu_assign_pointer(ph_dev_info, phdev);

Will, it not make sense to do this at the top after rcu_access_pointer check()? Otherwise, there could be a race between two enablers. Am I missing something here?

Quoted 4 lines

mm/page_reporting.c new file +3 -0

+     for_each_populated_zone(zone) {
+             spin_lock(&zone->lock);
+             __page_reporting_request(zone);

mm/page_reporting.c line 299

Quoted 10 lines

mm/page_reporting.c new file +10 -0

+             spin_unlock(&zone->lock);
+     }
+
+     /* enable page reporting notification */
+     static_key_slow_inc(&page_reporting_notify_enabled);
+
+     return 0;
+}
+EXPORT_SYMBOL_GPL(page_reporting_startup);
+

--
Thanks
Nitesh

From: Alexander Duyck <[email protected]>

Add support for the page reporting feature provided by virtio-balloon. Reporting differs from the regular balloon functionality in that is is much less durable than a standard memory balloon. Instead of creating a list of pages that cannot be accessed the pages are only inaccessible while they are being indicated to the virtio interface. Once the interface has acknowledged them they are placed back into their respective free lists and are once again accessible by the guest system.

Signed-off-by Alexander Duyck <[email protected]>

---
 drivers/virtio/Kconfig              |    1 +
 drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 +
 3 files changed, 58 insertions(+)

drivers/virtio/Kconfig +1 -0

index 078615cf2afc..4b2dd8259ff5 100644
@@ -58,6 +58,7 @@ config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO
 	select MEMORY_BALLOON
+	select PAGE_REPORTING
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.

drivers/virtio/virtio_balloon.c +10 -0

index 2c19457ab573..971fe924e34f 100644
@@ -19,6 +19,7 @@
 #include <linux/mount.h>
 #include <linux/magic.h>
 #include <linux/pseudo_fs.h>
+#include <linux/page_reporting.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by

@@ -37,6 +38,9 @@
 #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
 	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
 
+/*  limit on the number of pages that can be on the reporting vq */
+#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
+
 #ifdef CONFIG_BALLOON_COMPACTION
 static struct vfsmount *balloon_mnt;
 #endif

@@ -46,6 +50,7 @@ enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_DEFLATE,
 	VIRTIO_BALLOON_VQ_STATS,
 	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_REPORTING,
 	VIRTIO_BALLOON_VQ_MAX
 };
 

@@ -113,6 +118,10 @@ struct virtio_balloon {
 
 	/* To register a shrinker to shrink memory upon memory pressure */
 	struct shrinker shrinker;
+
+	/* Unused page reporting device */
+	struct virtqueue *reporting_vq;
+	struct page_reporting_dev_info ph_dev_info;
 };
 
 static struct virtio_device_id id_table[] = {

@@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 
+void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,

drivers/virtio/virtio_balloon.c +36 -0

+	virtqueue_kick(vq);
+
+	/* When host has read buffer, this completes via balloon_ack */
+	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
+}
+
 static void set_page_pfns(struct virtio_balloon *vb,
 			  __virtio32 pfns[], struct page *page)
 {

@@ -476,6 +502,7 @@ static int init_vqs(struct virtio_balloon *vb)
 	names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
 	names[VIRTIO_BALLOON_VQ_STATS] = NULL;
 	names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
+	names[VIRTIO_BALLOON_VQ_REPORTING] = NULL;
 
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
 		names[VIRTIO_BALLOON_VQ_STATS] = "stats";

@@ -487,11 +514,19 @@ static int init_vqs(struct virtio_balloon *vb)
 		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
 	}
 
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
+		names[VIRTIO_BALLOON_VQ_REPORTING] = "reporting_vq";
+		callbacks[VIRTIO_BALLOON_VQ_REPORTING] = balloon_ack;
+	}
+
 	err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
 					 vqs, callbacks, names, NULL, NULL);
 	if (err)
 		return err;
 
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
+		vb->reporting_vq = vqs[VIRTIO_BALLOON_VQ_REPORTING];
+
 	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
 	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {

@@ -931,12 +966,30 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		if (err)
 			goto out_del_balloon_wq;
 	}
+
+	vb->ph_dev_info.report = virtballoon_unused_page_report;
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
+		unsigned int capacity;
+
+		capacity = min_t(unsigned int,
+				 virtqueue_get_vring_size(vb->reporting_vq),
+				 VIRTIO_BALLOON_VRING_HINTS_MAX);
+		vb->ph_dev_info.capacity = capacity;
+
+		err = page_reporting_startup(&vb->ph_dev_info);
+		if (err)
+			goto out_unregister_shrinker;
+	}
+
 	virtio_device_ready(vdev);
 
 	if (towards_target(vb))
 		virtballoon_changed(vdev);
 	return 0;
 
+out_unregister_shrinker:
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+		virtio_balloon_unregister_shrinker(vb);
 out_del_balloon_wq:
 	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
 		destroy_workqueue(vb->balloon_wq);

@@ -965,6 +1018,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;
 
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
+		page_reporting_shutdown(&vb->ph_dev_info);
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 		virtio_balloon_unregister_shrinker(vb);
 	spin_lock_irq(&vb->stop_update_lock);

@@ -1034,6 +1089,7 @@ static int virtballoon_validate(struct virtio_device *vdev)
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
 	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
 	VIRTIO_BALLOON_F_PAGE_POISON,
+	VIRTIO_BALLOON_F_REPORTING,
 };
 
 static struct virtio_driver virtio_balloon_driver = {

include/uapi/linux/virtio_balloon.h +1 -0

index a1966cd7b677..19974392d324 100644
@@ -36,6 +36,7 @@
 #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
 #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */
 #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
+#define VIRTIO_BALLOON_F_REPORTING	5 /* Page reporting virtqueue */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12

Quoted 74 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

Add support for the page reporting feature provided by virtio-balloon. Reporting differs from the regular balloon functionality in that is is much less durable than a standard memory balloon. Instead of creating a list of pages that cannot be accessed the pages are only inaccessible while they are being indicated to the virtio interface. Once the interface has acknowledged them they are placed back into their respective free lists and are once again accessible by the guest system.

Signed-off-by Alexander Duyck <[email protected]>

---
 drivers/virtio/Kconfig              |    1 +
 drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 +
 3 files changed, 58 insertions(+)

drivers/virtio/Kconfig +1 -0

index 078615cf2afc..4b2dd8259ff5 100644
@@ -58,6 +58,7 @@ config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO
 	select MEMORY_BALLOON
+	select PAGE_REPORTING
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.

drivers/virtio/virtio_balloon.c +9 -0

index 2c19457ab573..971fe924e34f 100644
@@ -19,6 +19,7 @@
 #include <linux/mount.h>
 #include <linux/magic.h>
 #include <linux/pseudo_fs.h>
+#include <linux/page_reporting.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by

@@ -37,6 +38,9 @@
 #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
 	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
 
+/*  limit on the number of pages that can be on the reporting vq */
+#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
+
 #ifdef CONFIG_BALLOON_COMPACTION
 static struct vfsmount *balloon_mnt;
 #endif

@@ -46,6 +50,7 @@ enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_DEFLATE,
 	VIRTIO_BALLOON_VQ_STATS,
 	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_REPORTING,
 	VIRTIO_BALLOON_VQ_MAX
 };
 

@@ -113,6 +118,10 @@ struct virtio_balloon {
 
 	/* To register a shrinker to shrink memory upon memory pressure */
 	struct shrinker shrinker;
+
+	/* Unused page reporting device */
+	struct virtqueue *reporting_vq;
+	struct page_reporting_dev_info ph_dev_info;
 };
 
 static struct virtio_device_id id_table[] = {

@@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 

drivers/virtio/virtio_balloon.c static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq) · line 174

Quoted 11 lines

drivers/virtio/virtio_balloon.c +11 -0

+void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
+				    unsigned int nents)
+{
+	struct virtio_balloon *vb =
+		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
+	struct virtqueue *vq = vb->reporting_vq;
+	unsigned int unused;
+
+	/* We should always be able to add these buffers to an empty queue. */
+	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
+			    GFP_NOWAIT | __GFP_NOWARN);

I think you should handle allocation failure here. It is a possibility, isn't? Maybe return an error or even disable page hinting/reporting?

Quoted 75 lines · Nitesh Narayan Lal

Quoted 74 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

Add support for the page reporting feature provided by virtio-balloon. Reporting differs from the regular balloon functionality in that is is much less durable than a standard memory balloon. Instead of creating a list of pages that cannot be accessed the pages are only inaccessible while they are being indicated to the virtio interface. Once the interface has acknowledged them they are placed back into their respective free lists and are once again accessible by the guest system.

Signed-off-by Alexander Duyck <[email protected]>

---
 drivers/virtio/Kconfig              |    1 +
 drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 +
 3 files changed, 58 insertions(+)

drivers/virtio/Kconfig +1 -0

index 078615cf2afc..4b2dd8259ff5 100644
@@ -58,6 +58,7 @@ config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO
 	select MEMORY_BALLOON
+	select PAGE_REPORTING
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.

drivers/virtio/virtio_balloon.c +9 -0

index 2c19457ab573..971fe924e34f 100644
@@ -19,6 +19,7 @@
 #include <linux/mount.h>
 #include <linux/magic.h>
 #include <linux/pseudo_fs.h>
+#include <linux/page_reporting.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by

@@ -37,6 +38,9 @@
 #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
 	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
 
+/*  limit on the number of pages that can be on the reporting vq */
+#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
+
 #ifdef CONFIG_BALLOON_COMPACTION
 static struct vfsmount *balloon_mnt;
 #endif

@@ -46,6 +50,7 @@ enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_DEFLATE,
 	VIRTIO_BALLOON_VQ_STATS,
 	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_REPORTING,
 	VIRTIO_BALLOON_VQ_MAX
 };
 

@@ -113,6 +118,10 @@ struct virtio_balloon {
 
 	/* To register a shrinker to shrink memory upon memory pressure */
 	struct shrinker shrinker;
+
+	/* Unused page reporting device */
+	struct virtqueue *reporting_vq;
+	struct page_reporting_dev_info ph_dev_info;
 };
 
 static struct virtio_device_id id_table[] = {

@@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 

Quoted 15 lines

drivers/virtio/virtio_balloon.c static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq) · line 174

Quoted 11 lines

drivers/virtio/virtio_balloon.c +11 -0

+void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
+				    unsigned int nents)
+{
+	struct virtio_balloon *vb =
+		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
+	struct virtqueue *vq = vb->reporting_vq;
+	unsigned int unused;
+
+	/* We should always be able to add these buffers to an empty queue. */
+	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
+			    GFP_NOWAIT | __GFP_NOWARN);

I think you should handle allocation failure here. It is a possibility, isn't? Maybe return an error or even disable page hinting/reporting?

I don't think it is an issue I have to worry about. Specifically I am limiting the size of the scatterlist based on the size of the vq. As such I will never exceed the size and should be able to use it to store the scatterlist directly.

Quoted 92 lines · Alexander Duyck

Quoted 90 lines · Nitesh Narayan Lal

Quoted 75 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

Add support for the page reporting feature provided by virtio-balloon. Reporting differs from the regular balloon functionality in that is is much less durable than a standard memory balloon. Instead of creating a list of pages that cannot be accessed the pages are only inaccessible while they are being indicated to the virtio interface. Once the interface has acknowledged them they are placed back into their respective free lists and are once again accessible by the guest system.

Signed-off-by Alexander Duyck <[email protected]>

---
 drivers/virtio/Kconfig              |    1 +
 drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 +
 3 files changed, 58 insertions(+)

drivers/virtio/Kconfig +1 -0

index 078615cf2afc..4b2dd8259ff5 100644
@@ -58,6 +58,7 @@ config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO
 	select MEMORY_BALLOON
+	select PAGE_REPORTING
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.

drivers/virtio/virtio_balloon.c +10 -0

index 2c19457ab573..971fe924e34f 100644
@@ -19,6 +19,7 @@
 #include <linux/mount.h>
 #include <linux/magic.h>
 #include <linux/pseudo_fs.h>
+#include <linux/page_reporting.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by

@@ -37,6 +38,9 @@
 #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
 	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
 
+/*  limit on the number of pages that can be on the reporting vq */
+#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
+
 #ifdef CONFIG_BALLOON_COMPACTION
 static struct vfsmount *balloon_mnt;
 #endif

@@ -46,6 +50,7 @@ enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_DEFLATE,
 	VIRTIO_BALLOON_VQ_STATS,
 	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_REPORTING,
 	VIRTIO_BALLOON_VQ_MAX
 };
 

@@ -113,6 +118,10 @@ struct virtio_balloon {
 
 	/* To register a shrinker to shrink memory upon memory pressure */
 	struct shrinker shrinker;
+
+	/* Unused page reporting device */
+	struct virtqueue *reporting_vq;
+	struct page_reporting_dev_info ph_dev_info;
 };
 
 static struct virtio_device_id id_table[] = {

@@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 
+void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,

drivers/virtio/virtio_balloon.c static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq) · line 174

Quoted 10 lines

drivers/virtio/virtio_balloon.c +10 -0

+				    unsigned int nents)
+{
+	struct virtio_balloon *vb =
+		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
+	struct virtqueue *vq = vb->reporting_vq;
+	unsigned int unused;
+
+	/* We should always be able to add these buffers to an empty queue. */
+	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
+			    GFP_NOWAIT | __GFP_NOWARN);

I think you should handle allocation failure here. It is a possibility, isn't? Maybe return an error or even disable page hinting/reporting?

Quoted 4 lines

I don't think it is an issue I have to worry about. Specifically I am limiting the size of the scatterlist based on the size of the vq. As such I will never exceed the size and should be able to use it to store the scatterlist directly.

I agree. But it can't hurt to BUG_ON for good measure.

Quoted 95 lines · Michael S. Tsirkin

Quoted 94 lines · Alexander Duyck

Quoted 75 lines · Nitesh Narayan Lal

Quoted 74 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

Add support for the page reporting feature provided by virtio-balloon. Reporting differs from the regular balloon functionality in that is is much less durable than a standard memory balloon. Instead of creating a list of pages that cannot be accessed the pages are only inaccessible while they are being indicated to the virtio interface. Once the interface has acknowledged them they are placed back into their respective free lists and are once again accessible by the guest system.

Signed-off-by Alexander Duyck <[email protected]>

---
 drivers/virtio/Kconfig              |    1 +
 drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 +
 3 files changed, 58 insertions(+)

drivers/virtio/Kconfig +1 -0

index 078615cf2afc..4b2dd8259ff5 100644
@@ -58,6 +58,7 @@ config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO
 	select MEMORY_BALLOON
+	select PAGE_REPORTING
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.

drivers/virtio/virtio_balloon.c +9 -0

index 2c19457ab573..971fe924e34f 100644
@@ -19,6 +19,7 @@
 #include <linux/mount.h>
 #include <linux/magic.h>
 #include <linux/pseudo_fs.h>
+#include <linux/page_reporting.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by

@@ -37,6 +38,9 @@
 #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
 	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
 
+/*  limit on the number of pages that can be on the reporting vq */
+#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
+
 #ifdef CONFIG_BALLOON_COMPACTION
 static struct vfsmount *balloon_mnt;
 #endif

@@ -46,6 +50,7 @@ enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_DEFLATE,
 	VIRTIO_BALLOON_VQ_STATS,
 	VIRTIO_BALLOON_VQ_FREE_PAGE,
+	VIRTIO_BALLOON_VQ_REPORTING,
 	VIRTIO_BALLOON_VQ_MAX
 };
 

@@ -113,6 +118,10 @@ struct virtio_balloon {
 
 	/* To register a shrinker to shrink memory upon memory pressure */
 	struct shrinker shrinker;
+
+	/* Unused page reporting device */
+	struct virtqueue *reporting_vq;
+	struct page_reporting_dev_info ph_dev_info;
 };
 
 static struct virtio_device_id id_table[] = {

@@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 }
 

Quoted 14 lines

drivers/virtio/virtio_balloon.c static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq) · line 174

Quoted 11 lines

drivers/virtio/virtio_balloon.c +11 -0

+void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
+				    unsigned int nents)
+{
+	struct virtio_balloon *vb =
+		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
+	struct virtqueue *vq = vb->reporting_vq;
+	unsigned int unused;
+
+	/* We should always be able to add these buffers to an empty queue. */
+	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
+			    GFP_NOWAIT | __GFP_NOWARN);

I think you should handle allocation failure here. It is a possibility, isn't? Maybe return an error or even disable page hinting/reporting?

I don't think it is an issue I have to worry about. Specifically I am limiting the size of the scatterlist based on the size of the vq. As such I will never exceed the size and should be able to use it to store the scatterlist directly.

Quoted 2 lines

I agree. But it can't hurt to BUG_ON for good measure.

I wouldn't use a BUG_ON as that seems overkill. No need to panic the kernel just because we couldn't report some idle pages.

I can probably do something like:

	if (WARN_ON(err))
		return;

That way the unused page reporting can run to completion still and the fact that we aren't really hinting on the pages would effectively be no different then if we had a direct assigned device or shared memory in the hypervisor.

From: Alexander Duyck <[email protected]>

Add support for what I am referring to as "unused page reporting". Basically the idea is to function very similar to how the balloon works in that we basically end up madvising the page as not being used. However we don't really need to bother with any deflate type logic since the page will be faulted back into the guest when it is read or written to.

This is meant to be a simplification of the existing balloon interface to use for providing hints to what memory needs to be freed. I am assuming this is safe to do as the deflate logic does not actually appear to do very much other than tracking what subpages have been released and which ones haven't.

Signed-off-by Alexander Duyck <[email protected]>

---
 hw/virtio/virtio-balloon.c                      |   46 ++++++++++++++++++++++-
 include/hw/virtio/virtio-balloon.h              |    2 +
 include/standard-headers/linux/virtio_balloon.h |    1 +
 3 files changed, 46 insertions(+), 3 deletions(-)

hw/virtio/virtio-balloon.c +38 -2

index 003b3ebcfdfb..7a30df63bc77 100644
@@ -320,6 +320,40 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
     balloon_stats_change_timer(s, 0);
 }
 
+static void virtio_balloon_handle_report(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
+    VirtQueueElement *elem;
+
+    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
+    	unsigned int i;
+
+        for (i = 0; i < elem->in_num; i++) {
+            void *addr = elem->in_sg[i].iov_base;
+            size_t size = elem->in_sg[i].iov_len;
+            ram_addr_t ram_offset;
+            size_t rb_page_size;
+            RAMBlock *rb;
+
+            if (qemu_balloon_is_inhibited() || dev->poison_val)
+                continue;
+
+            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
+            rb_page_size = qemu_ram_pagesize(rb);
+
+            /* For now we will simply ignore unaligned memory regions */
+            if ((ram_offset | size) & (rb_page_size - 1))
+                continue;
+
+            ram_block_discard_range(rb, ram_offset, size);
+        }
+
+        virtqueue_push(vq, elem, 0);
+        virtio_notify(vdev, vq);
+        g_free(elem);
+    }
+}
+
 static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);

@@ -627,7 +661,8 @@ static size_t virtio_balloon_config_size(VirtIOBalloon *s)
         return sizeof(struct virtio_balloon_config);
     }
     if (virtio_has_feature(features, VIRTIO_BALLOON_F_PAGE_POISON) ||
-        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
+        virtio_has_feature(features, VIRTIO_BALLOON_F_REPORTING)) {
         return sizeof(struct virtio_balloon_config);
     }
     return offsetof(struct virtio_balloon_config, free_page_report_cmd_id);

@@ -715,7 +750,8 @@ static uint64_t virtio_balloon_get_features(VirtIODevice *vdev, uint64_t f,
     VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
     f |= dev->host_features;
     virtio_add_feature(&f, VIRTIO_BALLOON_F_STATS_VQ);
-    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
+        virtio_has_feature(f, VIRTIO_BALLOON_F_REPORTING)) {
         virtio_add_feature(&f, VIRTIO_BALLOON_F_PAGE_POISON);
     }
 

hw/virtio/virtio-balloon.c +2 -0

     if (virtio_has_feature(s->host_features,
                            VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
         s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,

@@ -931,6 +971,8 @@ static Property virtio_balloon_properties[] = {
      */
     DEFINE_PROP_BOOL("qemu-4-0-config-size", VirtIOBalloon,
                      qemu_4_0_config_size, false),
+    DEFINE_PROP_BIT("unused-page-reporting", VirtIOBalloon, host_features,
+                    VIRTIO_BALLOON_F_REPORTING, true),
     DEFINE_PROP_LINK("iothread", VirtIOBalloon, iothread, TYPE_IOTHREAD,
                      IOThread *),
     DEFINE_PROP_END_OF_LIST(),

include/hw/virtio/virtio-balloon.h +1 -1

index 7fe78e5c14d7..db5bf7127112 100644
@@ -42,7 +42,7 @@ enum virtio_balloon_free_page_report_status {
 
 typedef struct VirtIOBalloon {
     VirtIODevice parent_obj;
-    VirtQueue *ivq, *dvq, *svq, *free_page_vq;
+    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *rvq;
     uint32_t free_page_report_status;
     uint32_t num_pages;
     uint32_t actual;

include/standard-headers/linux/virtio_balloon.h +0 -0

index 9375ca2a70de..1c5f6d6f2de6 100644

include/standard-headers/linux/virtio_balloon.h +0 -0

 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
 

Quoted 90 lines · Alexander Duyck

From: Alexander Duyck <[email protected]>

Add support for what I am referring to as "unused page reporting". Basically the idea is to function very similar to how the balloon works in that we basically end up madvising the page as not being used. However we don't really need to bother with any deflate type logic since the page will be faulted back into the guest when it is read or written to.

This is meant to be a simplification of the existing balloon interface to use for providing hints to what memory needs to be freed. I am assuming this is safe to do as the deflate logic does not actually appear to do very much other than tracking what subpages have been released and which ones haven't.

Signed-off-by Alexander Duyck <[email protected]>

---
 hw/virtio/virtio-balloon.c                      |   46 ++++++++++++++++++++++-
 include/hw/virtio/virtio-balloon.h              |    2 +
 include/standard-headers/linux/virtio_balloon.h |    1 +
 3 files changed, 46 insertions(+), 3 deletions(-)

hw/virtio/virtio-balloon.c +38 -2

index 003b3ebcfdfb..7a30df63bc77 100644
@@ -320,6 +320,40 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
     balloon_stats_change_timer(s, 0);
 }
 
+static void virtio_balloon_handle_report(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
+    VirtQueueElement *elem;
+
+    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
+    	unsigned int i;
+
+        for (i = 0; i < elem->in_num; i++) {
+            void *addr = elem->in_sg[i].iov_base;
+            size_t size = elem->in_sg[i].iov_len;
+            ram_addr_t ram_offset;
+            size_t rb_page_size;
+            RAMBlock *rb;
+
+            if (qemu_balloon_is_inhibited() || dev->poison_val)
+                continue;
+
+            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
+            rb_page_size = qemu_ram_pagesize(rb);
+
+            /* For now we will simply ignore unaligned memory regions */
+            if ((ram_offset | size) & (rb_page_size - 1))
+                continue;
+
+            ram_block_discard_range(rb, ram_offset, size);
+        }
+
+        virtqueue_push(vq, elem, 0);
+        virtio_notify(vdev, vq);
+        g_free(elem);
+    }
+}
+
 static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);

@@ -627,7 +661,8 @@ static size_t virtio_balloon_config_size(VirtIOBalloon *s)
         return sizeof(struct virtio_balloon_config);
     }
     if (virtio_has_feature(features, VIRTIO_BALLOON_F_PAGE_POISON) ||
-        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
+        virtio_has_feature(features, VIRTIO_BALLOON_F_REPORTING)) {
         return sizeof(struct virtio_balloon_config);
     }
     return offsetof(struct virtio_balloon_config, free_page_report_cmd_id);

@@ -715,7 +750,8 @@ static uint64_t virtio_balloon_get_features(VirtIODevice *vdev, uint64_t f,
     VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
     f |= dev->host_features;
     virtio_add_feature(&f, VIRTIO_BALLOON_F_STATS_VQ);
-    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
+        virtio_has_feature(f, VIRTIO_BALLOON_F_REPORTING)) {
         virtio_add_feature(&f, VIRTIO_BALLOON_F_PAGE_POISON);
     }
 

@@ -805,6 +841,10 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
     s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
     s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
 

hw/virtio/virtio-balloon.c static void virtio_balloon_device_realize(DeviceState *dev, Error **errp) · line 847

Quoted 4 lines

hw/virtio/virtio-balloon.c +4 -0

+    if (virtio_has_feature(s->host_features, VIRTIO_BALLOON_F_REPORTING)) {
+        s->rvq = virtio_add_queue(vdev, 32, virtio_balloon_handle_report);
+    }
+

This does makes sense. I haven't seen the kernel patch yet, but I am guessing you will use this max_vq size to define the capacity.

Quoted 32 lines

hw/virtio/virtio-balloon.c +2 -0

     if (virtio_has_feature(s->host_features,
                            VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
         s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,

@@ -931,6 +971,8 @@ static Property virtio_balloon_properties[] = {
      */
     DEFINE_PROP_BOOL("qemu-4-0-config-size", VirtIOBalloon,
                      qemu_4_0_config_size, false),
+    DEFINE_PROP_BIT("unused-page-reporting", VirtIOBalloon, host_features,
+                    VIRTIO_BALLOON_F_REPORTING, true),
     DEFINE_PROP_LINK("iothread", VirtIOBalloon, iothread, TYPE_IOTHREAD,
                      IOThread *),
     DEFINE_PROP_END_OF_LIST(),

include/hw/virtio/virtio-balloon.h +1 -1

index 7fe78e5c14d7..db5bf7127112 100644
@@ -42,7 +42,7 @@ enum virtio_balloon_free_page_report_status {
 
 typedef struct VirtIOBalloon {
     VirtIODevice parent_obj;
-    VirtQueue *ivq, *dvq, *svq, *free_page_vq;
+    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *rvq;
     uint32_t free_page_report_status;
     uint32_t num_pages;
     uint32_t actual;

include/standard-headers/linux/virtio_balloon.h +0 -0

index 9375ca2a70de..1c5f6d6f2de6 100644
@@ -36,6 +36,7 @@
 #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
 #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */

include/standard-headers/linux/virtio_balloon.h line 39

Quoted 2 lines

include/standard-headers/linux/virtio_balloon.h +1 -0

 #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
+#define VIRTIO_BALLOON_F_REPORTING	5 /* Page reporting virtqueue */

Do we really need this change? or is this something which is picked from the Linux kernel?
If we do need it, then Cornelia suggested to split off any update to this header into a separate patch, so that it can be replaced by a proper headers update when it is merged.

This series provides an asynchronous means of reporting to a hypervisor that a guest page is no longer in use and can have the data associated with it dropped. To do this I have implemented functionality that allows for what I am referring to as unused page reporting

The functionality for this is fairly simple. When enabled it will allocate statistics to track the number of reported pages in a given free area. When the number of free pages exceeds this value plus a high water value, currently 32, it will begin performing page reporting which consists of pulling pages off of free list and placing them into a scatter list. The scatterlist is then given to the page reporting device and it will perform the required action to make the pages "reported", in the case of virtio-balloon this results in the pages being madvised as MADV_DONTNEED and as such they are forced out of the guest. After this they are placed back on the free list, and an additional bit is added if they are not merged indicating that they are a reported buddy page instead of a standard buddy page. The cycle then repeats with additional non-reported pages being pulled until the free areas all consist of reported pages.

I am leaving a number of things hard-coded such as limiting the lowest order processed to PAGEBLOCK_ORDER, and have left it up to the guest to determine what the limit is on how many pages it wants to allocate to process the hints. The upper limit for this is based on the size of the queue used to store the scatterlist.

My primary testing has just been to verify the memory is being freed after allocation by running memhog 40g on a 40g guest and watching the total free memory via /proc/meminfo on the host. With this I have verified most of the memory is freed after each iteration. As far as performance I have been mainly focusing on the will-it-scale/page_fault1 test running with 16 vcpus. With that I have seen up to a 2% difference between the base kernel without these patches and the patches with virtio-balloon enabled or disabled.

One side effect of these patches is that the guest becomes much more resilient in terms of NUMA locality. With the pages being freed and then reallocated when used it allows for the pages to be much closer to the active thread, and as a result there can be situations where this patch set will out-perform the stock kernel when the guest memory is not local to the guest vCPUs.

Patch 4 is a bit on the large side at about 600 lines of change, however I really didn't see a good way to break it up since each piece feeds into the next. So I couldn't add the statistics by themselves as it didn't really make sense to add them without something that will either read or increment/decrement them, or add the Hinted state without something that would set/unset it. As such I just ended up adding the entire thing as one patch. It makes it a bit bigger but avoids the issues in the previous set where I was referencing things that had not yet been added.

Changes from the RFC:
https://lore.kernel.org/lkml/[email protected]/
Moved aeration requested flag out of aerator and into zone->flags. Moved boundary out of free_area and into local variables for aeration. Moved aeration cycle out of interrupt and into workqueue.
Left nr_free as total pages instead of splitting it between raw and aerated. Combined size and physical address values in virtio ring into one 64b value.

Changes from v1:
https://lore.kernel.org/lkml/[email protected]/
Dropped "waste page treatment" in favor of "page hinting" Renamed files and functions from "aeration" to "page_hinting" Moved from page->lru list to scatterlist
Replaced wait on refcnt in shutdown with RCU and cancel_delayed_work_sync Virtio now uses scatterlist directly instead of intermediate array Moved stats out of free_area, now in separate area and pointed to from zone Merged patch 5 into patch 4 to improve review-ability
Updated various code comments throughout

Changes from v2:
https://lore.kernel.org/lkml/[email protected]/
Dropped "page hinting" in favor of "page reporting" Renamed files from "hinting" to "reporting" Replaced "Hinted" page type with "Reported" page flag Added support for page poisoning while hinting is active Add QEMU patch that implements PAGE_POISON feature

---

Alexander Duyck (6):
mm: Adjust shuffle code to allow for future coalescing
mm: Move set/get_pcppage_migratetype to mmzone.h
mm: Use zone and order instead of free area in free_list manipulators
mm: Introduce Reported pages
virtio-balloon: Pull page poisoning config out of free page hinting
virtio-balloon: Add support for providing unused page reports to host

 drivers/virtio/Kconfig              |    1 
 drivers/virtio/virtio_balloon.c     |   75 ++++++++-
 include/linux/mmzone.h              |  116 ++++++++------
 include/linux/page-flags.h          |   11 +
 include/linux/page_reporting.h      |  138 ++++++++++++++++
 include/uapi/linux/virtio_balloon.h |    1 
 mm/Kconfig                          |    5 +
 mm/Makefile                         |    1 
 mm/internal.h                       |   18 ++
 mm/memory_hotplug.c                 |    1 
 mm/page_alloc.c                     |  238 ++++++++++++++++++++--------
 mm/page_reporting.c                 |  299 +++++++++++++++++++++++++++++++++++
 mm/shuffle.c                        |   24 ---
 mm/shuffle.h                        |   32 ++++
 14 files changed, 821 insertions(+), 139 deletions(-)
 create mode 100644 include/linux/page_reporting.h
 create mode 100644 mm/page_reporting.c

--

Quoted 25 lines · Alexander Duyck

This series provides an asynchronous means of reporting to a hypervisor that a guest page is no longer in use and can have the data associated with it dropped. To do this I have implemented functionality that allows for what I am referring to as unused page reporting

The functionality for this is fairly simple. When enabled it will allocate statistics to track the number of reported pages in a given free area. When the number of free pages exceeds this value plus a high water value, currently 32, it will begin performing page reporting which consists of pulling pages off of free list and placing them into a scatter list. The scatterlist is then given to the page reporting device and it will perform the required action to make the pages "reported", in the case of virtio-balloon this results in the pages being madvised as MADV_DONTNEED and as such they are forced out of the guest. After this they are placed back on the free list, and an additional bit is added if they are not merged indicating that they are a reported buddy page instead of a standard buddy page. The cycle then repeats with additional non-reported pages being pulled until the free areas all consist of reported pages.

I am leaving a number of things hard-coded such as limiting the lowest order processed to PAGEBLOCK_ORDER, and have left it up to the guest to determine what the limit is on how many pages it wants to allocate to process the hints. The upper limit for this is based on the size of the queue used to store the scatterlist.

Quoted 8 lines

My primary testing has just been to verify the memory is being freed after allocation by running memhog 40g on a 40g guest and watching the total free memory via /proc/meminfo on the host. With this I have verified most of the memory is freed after each iteration. As far as performance I have been mainly focusing on the will-it-scale/page_fault1 test running with 16 vcpus. With that I have seen up to a 2% difference between the base kernel without these patches and the patches with virtio-balloon enabled or disabled.

A couple of questions:

- The 2% difference which you have mentioned, is this visible for all the 16 cores or just the 16th core?
- I am assuming that the difference is seen for both "number of process" and "number of threads" launched by page_fault1. Is that right?

Quoted 6 lines

One side effect of these patches is that the guest becomes much more resilient in terms of NUMA locality. With the pages being freed and then reallocated when used it allows for the pages to be much closer to the active thread, and as a result there can be situations where this patch set will out-perform the stock kernel when the guest memory is not local to the guest vCPUs.

Was this the reason because of which you were seeing better results for page_fault1 earlier?

Quoted 34 lines · Nitesh Narayan Lal

Quoted 25 lines · Alexander Duyck

This series provides an asynchronous means of reporting to a hypervisor that a guest page is no longer in use and can have the data associated with it dropped. To do this I have implemented functionality that allows for what I am referring to as unused page reporting

The functionality for this is fairly simple. When enabled it will allocate statistics to track the number of reported pages in a given free area. When the number of free pages exceeds this value plus a high water value, currently 32, it will begin performing page reporting which consists of pulling pages off of free list and placing them into a scatter list. The scatterlist is then given to the page reporting device and it will perform the required action to make the pages "reported", in the case of virtio-balloon this results in the pages being madvised as MADV_DONTNEED and as such they are forced out of the guest. After this they are placed back on the free list, and an additional bit is added if they are not merged indicating that they are a reported buddy page instead of a standard buddy page. The cycle then repeats with additional non-reported pages being pulled until the free areas all consist of reported pages.

I am leaving a number of things hard-coded such as limiting the lowest order processed to PAGEBLOCK_ORDER, and have left it up to the guest to determine what the limit is on how many pages it wants to allocate to process the hints. The upper limit for this is based on the size of the queue used to store the scatterlist.

Quoted 8 lines

My primary testing has just been to verify the memory is being freed after allocation by running memhog 40g on a 40g guest and watching the total free memory via /proc/meminfo on the host. With this I have verified most of the memory is freed after each iteration. As far as performance I have been mainly focusing on the will-it-scale/page_fault1 test running with 16 vcpus. With that I have seen up to a 2% difference between the base kernel without these patches and the patches with virtio-balloon enabled or disabled.

Quoted 6 lines

A couple of questions:

- The 2% difference which you have mentioned, is this visible for all the 16 cores or just the 16th core?
- I am assuming that the difference is seen for both "number of process" and "number of threads" launched by page_fault1. Is that right?

Really, the 2% is bordering on just being noise. Sometimes it is better sometimes it is worse. However I think it is just slight variability in the tests since it doesn't usually form any specific pattern.

I have been able to tighten it down a bit by actually splitting my guest over 2 nodes and pinning the vCPUs so that the nodes in the guest match up to the nodes in the host. Doing that I have seen results where I had less than 1% variability between with the patches and without.

One thing I am looking at now is modifying the page_fault1 test to use THP instead of 4K pages as I suspect there is a fair bit of overhead in accessing the pages 4K at a time vs 2M at a time. I am hoping with that I can put more pressure on the actual change and see if there are any additional spots I should optimize.

Quoted 8 lines

Quoted 6 lines

One side effect of these patches is that the guest becomes much more resilient in terms of NUMA locality. With the pages being freed and then reallocated when used it allows for the pages to be much closer to the active thread, and as a result there can be situations where this patch set will out-perform the stock kernel when the guest memory is not local to the guest vCPUs.

Was this the reason because of which you were seeing better results for page_fault1 earlier?

Yes I am thinking so. What I have found is that in the case where the patches are not applied on the guest it takes a few runs for the numbers to stabilize. What I think was going on is that I was running memhog to initially fill the guest and that was placing all the pages on one node or the other and as such was causing additional variability as the pages were slowly being migrated over to the other node to rebalance the workload. One way I tested it was by trying the unpatched case with a direct- assigned device since that forces it to pin the memory. In that case I was getting bad results consistently as all the memory was forced to come from one node during the pre-allocation process.

Quoted 45 lines · Alexander Duyck

Quoted 34 lines · Nitesh Narayan Lal

Quoted 25 lines · Alexander Duyck

This series provides an asynchronous means of reporting to a hypervisor that a guest page is no longer in use and can have the data associated with it dropped. To do this I have implemented functionality that allows for what I am referring to as unused page reporting

The functionality for this is fairly simple. When enabled it will allocate statistics to track the number of reported pages in a given free area. When the number of free pages exceeds this value plus a high water value, currently 32, it will begin performing page reporting which consists of pulling pages off of free list and placing them into a scatter list. The scatterlist is then given to the page reporting device and it will perform the required action to make the pages "reported", in the case of virtio-balloon this results in the pages being madvised as MADV_DONTNEED and as such they are forced out of the guest. After this they are placed back on the free list, and an additional bit is added if they are not merged indicating that they are a reported buddy page instead of a standard buddy page. The cycle then repeats with additional non-reported pages being pulled until the free areas all consist of reported pages.

I am leaving a number of things hard-coded such as limiting the lowest order processed to PAGEBLOCK_ORDER, and have left it up to the guest to determine what the limit is on how many pages it wants to allocate to process the hints. The upper limit for this is based on the size of the queue used to store the scatterlist.

Quoted 8 lines

My primary testing has just been to verify the memory is being freed after allocation by running memhog 40g on a 40g guest and watching the total free memory via /proc/meminfo on the host. With this I have verified most of the memory is freed after each iteration. As far as performance I have been mainly focusing on the will-it-scale/page_fault1 test running with 16 vcpus. With that I have seen up to a 2% difference between the base kernel without these patches and the patches with virtio-balloon enabled or disabled.

Quoted 6 lines

A couple of questions:

- The 2% difference which you have mentioned, is this visible for all the 16 cores or just the 16th core?
- I am assuming that the difference is seen for both "number of process" and "number of threads" launched by page_fault1. Is that right?

Really, the 2% is bordering on just being noise. Sometimes it is better sometimes it is worse. However I think it is just slight variability in the tests since it doesn't usually form any specific pattern.

Quoted 4 lines

I have been able to tighten it down a bit by actually splitting my guest over 2 nodes and pinning the vCPUs so that the nodes in the guest match up to the nodes in the host. Doing that I have seen results where I had less than 1% variability between with the patches and without.

Interesting. I usually pin the guest to a single NUMA node to avoid this.

Quoted 5 lines

One thing I am looking at now is modifying the page_fault1 test to use THP instead of 4K pages as I suspect there is a fair bit of overhead in accessing the pages 4K at a time vs 2M at a time. I am hoping with that I can put more pressure on the actual change and see if there are any additional spots I should optimize.

+1. Right now I don't think will-it-scale touches all the guest memory. May I know how much memory does will-it-scale/page_fault1, occupies in your case and how much do you get back with your patch-set?

Do you have any plans of running any other benchmarks as well? Just to see the impact on other sub-systems.

Quoted 8 lines

Quoted 8 lines

Quoted 6 lines

One side effect of these patches is that the guest becomes much more resilient in terms of NUMA locality. With the pages being freed and then reallocated when used it allows for the pages to be much closer to the active thread, and as a result there can be situations where this patch set will out-perform the stock kernel when the guest memory is not local to the guest vCPUs.

Was this the reason because of which you were seeing better results for page_fault1 earlier?

Quoted 11 lines

Yes I am thinking so. What I have found is that in the case where the patches are not applied on the guest it takes a few runs for the numbers to stabilize. What I think was going on is that I was running memhog to initially fill the guest and that was placing all the pages on one node or the other and as such was causing additional variability as the pages were slowly being migrated over to the other node to rebalance the workload. One way I tested it was by trying the unpatched case with a direct- assigned device since that forces it to pin the memory. In that case I was getting bad results consistently as all the memory was forced to come from one node during the pre-allocation process.

I have also seen that the page_fault1 values take some time to get stabilize on an unmodified kernel.
What I am wondering here is that if on a single NUMA guest doing the following will give the right/better idea or not:

1. Pin the guest to a single NUMA node.
2. Run memhog so that it touches all the guest memory.
3. Run will-it-scale/page_fault1.

Compare/observe the values for the last core (this is considering the other core values doesn't drastically differ).

Quoted 4 lines · Nitesh Narayan Lal

Quoted 3 lines · Alexander Duyck

Quoted 2 lines · Nitesh Narayan Lal

Quoted 1 line · Alexander Duyck

Quoted 29 lines

Quoted 8 lines

Quoted 8 lines

Quoted 6 lines

One side effect of these patches is that the guest becomes much more resilient in terms of NUMA locality. With the pages being freed and then reallocated when used it allows for the pages to be much closer to the active thread, and as a result there can be situations where this patch set will out-perform the stock kernel when the guest memory is not local to the guest vCPUs.

Was this the reason because of which you were seeing better results for page_fault1 earlier?

Quoted 11 lines

Yes I am thinking so. What I have found is that in the case where the patches are not applied on the guest it takes a few runs for the numbers to stabilize. What I think was going on is that I was running memhog to initially fill the guest and that was placing all the pages on one node or the other and as such was causing additional variability as the pages were slowly being migrated over to the other node to rebalance the workload. One way I tested it was by trying the unpatched case with a direct- assigned device since that forces it to pin the memory. In that case I was getting bad results consistently as all the memory was forced to come from one node during the pre-allocation process.

I have also seen that the page_fault1 values take some time to get stabilize on an unmodified kernel.
What I am wondering here is that if on a single NUMA guest doing the following will give the right/better idea or not:

1. Pin the guest to a single NUMA node.
2. Run memhog so that it touches all the guest memory.
3. Run will-it-scale/page_fault1.

Quoted 2 lines

Compare/observe the values for the last core (this is considering the other core values doesn't drastically differ).

I'll rerun the test with qemu affinitized to one specific socket. It will cut the core/thread count down to 8/16 on my test system. Also I will try with THP and page shuffling enabled.