[PATCH 2/2] mm: split free page with properly free memory accounting and without race

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

In isolate_single_pageblock(), free pages are checked without holding zone
lock, but they can go away in split_free_page() when zone lock is held.
Check the free page and its order again in split_free_page() when zone lock
is held. Recheck the page if the free page is gone under zone lock.

In addition, in split_free_page(), the free page was deleted from the page
list without changing free page accounting. Add the missing free page
accounting code.

Fix the type of order parameter in split_free_page().
Link: https://lore.kernel.org/lkml/20220525103621.987185e2ca0079f7b97b856d@linux-foundation.org/

Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
Reported-by: Doug Berger <opendmb@gmail.com>
Link: https://lore.kernel.org/linux-mm/c3932a6f-77fe-29f7-0c29-fe6b1c67ab7b@gmail.com/
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/internal.h       |  4 ++--
 mm/page_alloc.c     | 24 ++++++++++++++++++++----
 mm/page_isolation.c | 10 +++++++---
 3 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 20e0a990da40..7cf12a15475b 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -374,8 +374,8 @@ extern void *memmap_alloc(phys_addr_t size, phys_addr_t align,
 			  phys_addr_t min_addr,
 			  int nid, bool exact_nid);
 
-void split_free_page(struct page *free_page,
-				int order, unsigned long split_pfn_offset);
+int split_free_page(struct page *free_page,
+			unsigned int order, unsigned long split_pfn_offset);
 
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 355bd017b185..2717d6dede99 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1112,30 +1112,44 @@ static inline void __free_one_page(struct page *page,
  * @order:		the order of the page
  * @split_pfn_offset:	split offset within the page
  *
+ * Return -ENOENT if the free page is changed, otherwise 0
+ *
  * It is used when the free page crosses two pageblocks with different migratetypes
  * at split_pfn_offset within the page. The split free page will be put into
  * separate migratetype lists afterwards. Otherwise, the function achieves
  * nothing.
  */
-void split_free_page(struct page *free_page,
-				int order, unsigned long split_pfn_offset)
+int split_free_page(struct page *free_page,
+			unsigned int order, unsigned long split_pfn_offset)
 {
 	struct zone *zone = page_zone(free_page);
 	unsigned long free_page_pfn = page_to_pfn(free_page);
 	unsigned long pfn;
 	unsigned long flags;
 	int free_page_order;
+	int mt;
+	int ret = 0;
 
 	if (split_pfn_offset == 0)
-		return;
+		return ret;
 
 	spin_lock_irqsave(&zone->lock, flags);
+
+	if (!PageBuddy(free_page) || buddy_order(free_page) != order) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	mt = get_pageblock_migratetype(free_page);
+	if (likely(!is_migrate_isolate(mt)))
+		__mod_zone_freepage_state(zone, -(1UL << order), mt);
+
 	del_page_from_free_list(free_page, zone, order);
 	for (pfn = free_page_pfn;
 	     pfn < free_page_pfn + (1UL << order);) {
 		int mt = get_pfnblock_migratetype(pfn_to_page(pfn), pfn);
 
-		free_page_order = min_t(int,
+		free_page_order = min_t(unsigned int,
 					pfn ? __ffs(pfn) : order,
 					__fls(split_pfn_offset));
 		__free_one_page(pfn_to_page(pfn), pfn, zone, free_page_order,
@@ -1146,7 +1160,9 @@ void split_free_page(struct page *free_page,
 		if (split_pfn_offset == 0)
 			split_pfn_offset = (1UL << order) - (pfn - free_page_pfn);
 	}
+out:
 	spin_unlock_irqrestore(&zone->lock, flags);
+	return ret;
 }
 /*
  * A bad page could be due to a number of fields. Instead of multiple branches,
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index fbd820b21292..6021f8444b5a 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -371,9 +371,13 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
 		if (PageBuddy(page)) {
 			int order = buddy_order(page);
 
-			if (pfn + (1UL << order) > boundary_pfn)
-				split_free_page(page, order, boundary_pfn - pfn);
-			pfn += (1UL << order);
+			if (pfn + (1UL << order) > boundary_pfn) {
+				/* free page changed before split, check it again */
+				if (split_free_page(page, order, boundary_pfn - pfn))
+					continue;
+			}
+
+			pfn += 1UL << order;
 			continue;
 		}
 		/*
-- 
2.35.1