[PATCH RFC v3 12/19] mm: page_reporting: skip redundant zeroing of host-zeroed reported pages

["Michael S. Tsirkin" <mst@redhat.com>]

When a guest reports free pages to the hypervisor via the page reporting
framework (used by virtio-balloon and hv_balloon), the host typically
zeros those pages when reclaiming their backing memory.  However, when
those pages are later allocated in the guest, post_alloc_hook()
unconditionally zeros them again if __GFP_ZERO is set.  This
double-zeroing is wasteful, especially for large pages.

Avoid redundant zeroing:

- Add a host_zeroes_pages flag to page_reporting_dev_info, allowing
  drivers to declare that their host zeros reported pages on reclaim.
  A static key (page_reporting_host_zeroes) gates the fast path.

- Add PG_zeroed page flag (sharing PG_private bit) to mark pages
  that have been zeroed by the host.  Set it on reported pages during
  allocation from the buddy in page_del_and_expand().

- Thread the zeroed bool through rmqueue -> prep_new_page ->
  post_alloc_hook, where it skips redundant zeroing for __GFP_ZERO
  allocations.

No driver sets host_zeroes_pages yet; a follow-up patch to
virtio_balloon is needed to opt in.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh
---
 include/linux/mm.h             | 28 +++++++++++++++++
 include/linux/page-flags.h     | 12 ++++++-
 include/linux/page_reporting.h |  3 ++
 mm/compaction.c                |  5 +--
 mm/internal.h                  |  2 +-
 mm/page_alloc.c                | 57 ++++++++++++++++++++++------------
 mm/page_reporting.c            | 14 ++++++++-
 mm/page_reporting.h            | 12 +++++++
 8 files changed, 108 insertions(+), 25 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 541d36e5e420..821034dd33d1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4817,6 +4817,34 @@ static inline bool user_alloc_needs_zeroing(void)
 				   &init_on_alloc);
 }
 
+/**
+ * __page_test_clear_zeroed - test and clear the zeroed marker.
+ * @page: the page to test.
+ *
+ * Returns true if the page was zeroed by the host, and clears
+ * the marker. Caller must have exclusive access to @page.
+ */
+static inline bool __page_test_clear_zeroed(struct page *page)
+{
+	if (PageZeroed(page)) {
+		__ClearPageZeroed(page);
+		return true;
+	}
+	return false;
+}
+
+/**
+ * folio_test_clear_zeroed - test and clear the zeroed marker.
+ * @folio: the folio to test.
+ *
+ * Returns true if the folio was zeroed by the host, and clears
+ * the marker.  Callers can skip their own zeroing.
+ */
+static inline bool folio_test_clear_zeroed(struct folio *folio)
+{
+	return __page_test_clear_zeroed(&folio->page);
+}
+
 int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *status);
 int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status);
 int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f7a0e4af0c73..aa0de99247d4 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -135,6 +135,8 @@ enum pageflags {
 	PG_swapcache = PG_owner_priv_1, /* Swap page: swp_entry_t in private */
 	/* Some filesystems */
 	PG_checked = PG_owner_priv_1,
+	/* Page contents are known to be zero */
+	PG_zeroed = PG_private,
 
 	/*
 	 * Depending on the way an anonymous folio can be mapped into a page
@@ -679,6 +681,13 @@ FOLIO_TEST_CLEAR_FLAG_FALSE(young)
 FOLIO_FLAG_FALSE(idle)
 #endif
 
+/*
+ * PageZeroed() tracks pages known to be zero.  The allocator
+ * uses this to skip redundant zeroing in post_alloc_hook().
+ */
+__PAGEFLAG(Zeroed, zeroed, PF_NO_COMPOUND)
+#define __PG_ZEROED (1UL << PG_zeroed)
+
 /*
  * PageReported() is used to track reported free pages within the Buddy
  * allocator. We can use the non-atomic version of the test and set
@@ -1207,9 +1216,10 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  *
  * __PG_HWPOISON is exceptional because it needs to be kept beyond page's
  * alloc-free cycle to prevent from reusing the page.
+ * __PG_ZEROED survives alloc-free cycles to track known-zero pages.
  */
 #define PAGE_FLAGS_CHECK_AT_PREP	\
-	((PAGEFLAGS_MASK & ~__PG_HWPOISON) | LRU_GEN_MASK | LRU_REFS_MASK)
+	((PAGEFLAGS_MASK & ~(__PG_HWPOISON | __PG_ZEROED)) | LRU_GEN_MASK | LRU_REFS_MASK)
 
 /*
  * Flags stored in the second page of a compound page.  They may overlap
diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
index fe648dfa3a7c..10faadfeb4fb 100644
--- a/include/linux/page_reporting.h
+++ b/include/linux/page_reporting.h
@@ -13,6 +13,9 @@ struct page_reporting_dev_info {
 	int (*report)(struct page_reporting_dev_info *prdev,
 		      struct scatterlist *sg, unsigned int nents);
 
+	/* If true, host zeros reported pages on reclaim */
+	bool host_zeroes_pages;
+
 	/* work struct for processing reports */
 	struct delayed_work work;
 
diff --git a/mm/compaction.c b/mm/compaction.c
index 82f2914962f5..3d9ae727a98a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -82,7 +82,8 @@ static inline bool is_via_compact_memory(int order) { return false; }
 
 static struct page *mark_allocated_noprof(struct page *page, unsigned int order, gfp_t gfp_flags)
 {
-	post_alloc_hook(page, order, __GFP_MOVABLE, USER_ADDR_NONE)
+	post_alloc_hook(page, order, __GFP_MOVABLE, false, USER_ADDR_NONE);
+	set_page_refcounted(page);
 	return page;
 }
 #define mark_allocated(...)	alloc_hooks(mark_allocated_noprof(__VA_ARGS__))
@@ -1831,7 +1832,7 @@ static struct folio *compaction_alloc_noprof(struct folio *src, unsigned long da
 		set_page_private(&freepage[size], start_order);
 	}
 	dst = (struct folio *)freepage;
-	post_alloc_hook(&dst->page, order, __GFP_MOVABLE, USER_ADDR_NONE);
+	post_alloc_hook(&dst->page, order, __GFP_MOVABLE, false, USER_ADDR_NONE);
 	set_page_refcounted(&dst->page);
 	if (order)
 		prep_compound_page(&dst->page, order);
diff --git a/mm/internal.h b/mm/internal.h
index 0b9c0bd133d3..4c33249e03f0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -889,7 +889,7 @@ static inline void prep_compound_tail(struct page *head, int tail_idx)
 }
 
 void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flags,
-		     unsigned long user_addr);
+		     bool zeroed, unsigned long user_addr);
 extern bool free_pages_prepare(struct page *page, unsigned int order);
 
 extern int user_min_free_kbytes;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 211e9e32b91d..2098d569d80c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1774,6 +1774,7 @@ static __always_inline void page_del_and_expand(struct zone *zone,
 	bool was_reported = page_reported(page);
 
 	__del_page_from_free_list(page, zone, high, migratetype);
+
 	nr_pages -= expand(zone, page, low, high, migratetype, was_reported);
 	account_freepages(zone, -nr_pages, migratetype);
 }
@@ -1846,8 +1847,10 @@ static inline bool should_skip_init(gfp_t flags)
 	return (flags & __GFP_SKIP_ZERO);
 }
 
+
 inline void post_alloc_hook(struct page *page, unsigned int order,
-				gfp_t gfp_flags, unsigned long user_addr)
+				gfp_t gfp_flags, bool zeroed,
+				unsigned long user_addr)
 {
 	bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) &&
 			!should_skip_init(gfp_flags);
@@ -1856,6 +1859,14 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 
 	set_page_private(page, 0);
 
+	/*
+	 * If the page is zeroed, skip memory initialization.
+	 * We still need to handle tag zeroing separately since the host
+	 * does not know about memory tags.
+	 */
+	if (zeroed && init && !zero_tags)
+		init = false;
+
 	arch_alloc_page(page, order);
 	debug_pagealloc_map_pages(page, 1 << order);
 
@@ -1913,13 +1924,13 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 }
 
 static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
-							unsigned int alloc_flags,
-							unsigned long user_addr)
+			  unsigned int alloc_flags, bool zeroed,
+			  unsigned long user_addr)
 {
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);
 
-	post_alloc_hook(page, order, gfp_flags, user_addr);
+	post_alloc_hook(page, order, gfp_flags, zeroed, user_addr);
 
 	/*
 	 * page is set pfmemalloc when ALLOC_NO_WATERMARKS was necessary to
@@ -3261,7 +3272,7 @@ static inline void zone_statistics(struct zone *preferred_zone, struct zone *z,
 static __always_inline
 struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
 			   unsigned int order, unsigned int alloc_flags,
-			   int migratetype)
+			   int migratetype, bool *zeroed)
 {
 	struct page *page;
 	unsigned long flags;
@@ -3296,6 +3307,7 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
 			}
 		}
 		spin_unlock_irqrestore(&zone->lock, flags);
+		*zeroed = __page_test_clear_zeroed(page);
 	} while (check_new_pages(page, order));
 
 	__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
@@ -3357,10 +3369,9 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order)
 /* Remove page from the per-cpu list, caller must protect the list */
 static inline
 struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
-			int migratetype,
-			unsigned int alloc_flags,
+			int migratetype, unsigned int alloc_flags,
 			struct per_cpu_pages *pcp,
-			struct list_head *list)
+			struct list_head *list, bool *zeroed)
 {
 	struct page *page;
 
@@ -3381,6 +3392,7 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 		page = list_first_entry(list, struct page, pcp_list);
 		list_del(&page->pcp_list);
 		pcp->count -= 1 << order;
+		*zeroed = __page_test_clear_zeroed(page);
 	} while (check_new_pages(page, order));
 
 	return page;
@@ -3389,7 +3401,8 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 /* Lock and remove page from the per-cpu list */
 static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 			struct zone *zone, unsigned int order,
-			int migratetype, unsigned int alloc_flags)
+			int migratetype, unsigned int alloc_flags,
+			bool *zeroed)
 {
 	struct per_cpu_pages *pcp;
 	struct list_head *list;
@@ -3408,7 +3421,8 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 	 */
 	pcp->free_count >>= 1;
 	list = &pcp->lists[order_to_pindex(migratetype, order)];
-	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
+	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags,
+				 pcp, list, zeroed);
 	pcp_spin_unlock(pcp, UP_flags);
 	if (page) {
 		__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
@@ -3433,19 +3447,19 @@ static inline
 struct page *rmqueue(struct zone *preferred_zone,
 			struct zone *zone, unsigned int order,
 			gfp_t gfp_flags, unsigned int alloc_flags,
-			int migratetype)
+			int migratetype, bool *zeroed)
 {
 	struct page *page;
 
 	if (likely(pcp_allowed_order(order))) {
 		page = rmqueue_pcplist(preferred_zone, zone, order,
-				       migratetype, alloc_flags);
+				       migratetype, alloc_flags, zeroed);
 		if (likely(page))
 			goto out;
 	}
 
 	page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
-							migratetype);
+			     migratetype, zeroed);
 
 out:
 	/* Separate test+clear to avoid unnecessary atomics */
@@ -3836,6 +3850,7 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 	struct pglist_data *last_pgdat = NULL;
 	bool last_pgdat_dirty_ok = false;
 	bool no_fallback;
+	bool zeroed;
 	bool skip_kswapd_nodes = nr_online_nodes > 1;
 	bool skipped_kswapd_nodes = false;
 
@@ -3980,10 +3995,11 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 
 try_this_zone:
 		page = rmqueue(zonelist_zone(ac->preferred_zoneref), zone, order,
-				gfp_mask, alloc_flags, ac->migratetype);
+					gfp_mask, alloc_flags, ac->migratetype,
+					&zeroed);
 		if (page) {
 			prep_new_page(page, order, gfp_mask, alloc_flags,
-				      ac->user_addr);
+				      zeroed, ac->user_addr);
 
 			/*
 			 * If this is a high-order atomic allocation then check
@@ -4218,7 +4234,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 
 	/* Prep a captured page if available */
 	if (page)
-		prep_new_page(page, order, gfp_mask, alloc_flags,
+		prep_new_page(page, order, gfp_mask, alloc_flags, false,
 			      ac->user_addr);
 
 	/* Try get a page from the freelist if available */
@@ -5193,6 +5209,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
 	/* Attempt the batch allocation */
 	pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
 	while (nr_populated < nr_pages) {
+		bool zeroed = false;
 
 		/* Skip existing pages */
 		if (page_array[nr_populated]) {
@@ -5201,7 +5218,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
 		}
 
 		page = __rmqueue_pcplist(zone, 0, ac.migratetype, alloc_flags,
-								pcp, pcp_list);
+					 pcp, pcp_list, &zeroed);
 		if (unlikely(!page)) {
 			/* Try and allocate at least one page */
 			if (!nr_account) {
@@ -5212,7 +5229,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
 		}
 		nr_account++;
 
-		prep_new_page(page, 0, gfp, 0, USER_ADDR_NONE);
+		prep_new_page(page, 0, gfp, 0, zeroed, USER_ADDR_NONE);
 		set_page_refcounted(page);
 		page_array[nr_populated++] = page;
 	}
@@ -6938,7 +6955,7 @@ static void split_free_frozen_pages(struct list_head *list, gfp_t gfp_mask)
 		list_for_each_entry_safe(page, next, &list[order], lru) {
 			int i;
 
-			post_alloc_hook(page, order, gfp_mask, USER_ADDR_NONE);
+			post_alloc_hook(page, order, gfp_mask, false, USER_ADDR_NONE);
 			if (!order)
 				continue;
 
@@ -7144,7 +7161,7 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
 		struct page *head = pfn_to_page(start);
 
 		check_new_pages(head, order);
-		prep_new_page(head, order, gfp_mask, 0, USER_ADDR_NONE);
+		prep_new_page(head, order, gfp_mask, 0, false, USER_ADDR_NONE);
 	} else {
 		ret = -EINVAL;
 		WARN(true, "PFN range: requested [%lu, %lu), allocated [%lu, %lu)\n",
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index f0042d5743af..6177d2413743 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -50,6 +50,8 @@ EXPORT_SYMBOL_GPL(page_reporting_order);
 #define PAGE_REPORTING_DELAY	(2 * HZ)
 static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
 
+DEFINE_STATIC_KEY_FALSE(page_reporting_host_zeroes);
+
 enum {
 	PAGE_REPORTING_IDLE = 0,
 	PAGE_REPORTING_REQUESTED,
@@ -129,8 +131,11 @@ page_reporting_drain(struct page_reporting_dev_info *prdev,
 		 * report on the new larger page when we make our way
 		 * up to that higher order.
 		 */
-		if (PageBuddy(page) && buddy_order(page) == order)
+		if (PageBuddy(page) && buddy_order(page) == order) {
 			__SetPageReported(page);
+			if (page_reporting_host_zeroes_pages())
+				__SetPageZeroed(page);
+		}
 	} while ((sg = sg_next(sg)));
 
 	/* reinitialize scatterlist now that it is empty */
@@ -386,6 +391,10 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
 	/* Assign device to allow notifications */
 	rcu_assign_pointer(pr_dev_info, prdev);
 
+	/* enable zeroed page optimization if host zeroes reported pages */
+	if (prdev->host_zeroes_pages)
+		static_branch_enable(&page_reporting_host_zeroes);
+
 	/* enable page reporting notification */
 	if (!static_key_enabled(&page_reporting_enabled)) {
 		static_branch_enable(&page_reporting_enabled);
@@ -410,6 +419,9 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev)
 
 		/* Flush any existing work, and lock it out */
 		cancel_delayed_work_sync(&prdev->work);
+
+		if (prdev->host_zeroes_pages)
+			static_branch_disable(&page_reporting_host_zeroes);
 	}
 
 	mutex_unlock(&page_reporting_mutex);
diff --git a/mm/page_reporting.h b/mm/page_reporting.h
index c51dbc228b94..736ea7b37e9e 100644
--- a/mm/page_reporting.h
+++ b/mm/page_reporting.h
@@ -15,6 +15,13 @@ DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
 extern unsigned int page_reporting_order;
 void __page_reporting_notify(void);
 
+DECLARE_STATIC_KEY_FALSE(page_reporting_host_zeroes);
+
+static inline bool page_reporting_host_zeroes_pages(void)
+{
+	return static_branch_unlikely(&page_reporting_host_zeroes);
+}
+
 static inline bool page_reported(struct page *page)
 {
 	return static_branch_unlikely(&page_reporting_enabled) &&
@@ -46,6 +53,11 @@ static inline void page_reporting_notify_free(unsigned int order)
 #else /* CONFIG_PAGE_REPORTING */
 #define page_reported(_page)	false
 
+static inline bool page_reporting_host_zeroes_pages(void)
+{
+	return false;
+}
+
 static inline void page_reporting_notify_free(unsigned int order)
 {
 }
-- 
MST