mm/page_alloc: check the correct buddy if it is a starting block

find_large_buddy() search buddy based on start_pfn, which maybe different
from page's pfn, e.g.  when page is not pageblock aligned, because
prep_move_freepages_block() always align start_pfn to pageblock.

This means when we found a starting block at start_pfn, it may check on
the wrong page theoretically.  And not split the free page as it is
supposed to, causing a freelist migratetype mismatch.

The good news is the page passed to __move_freepages_block_isolate() has
only two possible cases:

  * page is pageblock aligned
  * page is __first_valid_page() of this block

So it is safe for the first case, and it won't get a buddy larger than
pageblock for the second case.

To fix the issue, check the returned pfn of find_large_buddy() to decide
whether to split the free page:

  1. if it is not a PageBuddy pfn, no split;
  2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
  3. if it is a PageBuddy pfn with order > pageblock_order, start_pfn is
     either in the starting block or tail block, split the PageBuddy at
     pageblock_order level.

Link: https://lkml.kernel.org/r/20250905140358.28849-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: RichardWeiYang

mm/page_alloc.c

@@ -2090,9 +2090,10 @@ static inline void toggle_pageblock_isolate(struct page *page, bool isolate)
 static bool __move_freepages_block_isolate(struct zone *zone,
 		struct page *page, bool isolate)
 {
-	unsigned long start_pfn, pfn;
+	unsigned long start_pfn, buddy_pfn;
 	int from_mt;
 	int to_mt;
+	struct page *buddy;
 
 	if (isolate == get_pageblock_isolate(page)) {
 		VM_WARN_ONCE(1, "%s a pageblock that is already in that state",
@@ -2107,29 +2108,19 @@ static bool __move_freepages_block_isolate(struct zone *zone,
 	if (pageblock_order == MAX_PAGE_ORDER)
 		goto move;
 
-	/* We're a tail block in a larger buddy */
-	pfn = find_large_buddy(start_pfn);
-	if (pfn != start_pfn) {
-		struct page *buddy = pfn_to_page(pfn);
+	buddy_pfn = find_large_buddy(start_pfn);
+	buddy = pfn_to_page(buddy_pfn);
+	/* We're a part of a larger buddy */
+	if (PageBuddy(buddy) && buddy_order(buddy) > pageblock_order) {
 		int order = buddy_order(buddy);
 
 		del_page_from_free_list(buddy, zone, order,
-					get_pfnblock_migratetype(buddy, pfn));
+					get_pfnblock_migratetype(buddy, buddy_pfn));
 		toggle_pageblock_isolate(page, isolate);
-		split_large_buddy(zone, buddy, pfn, order, FPI_NONE);
+		split_large_buddy(zone, buddy, buddy_pfn, order, FPI_NONE);
 		return true;
 	}
 
-	/* We're the starting block of a larger buddy */
-	if (PageBuddy(page) && buddy_order(page) > pageblock_order) {
-		int order = buddy_order(page);
-
-		del_page_from_free_list(page, zone, order,
-					get_pfnblock_migratetype(page, pfn));
-		toggle_pageblock_isolate(page, isolate);
-		split_large_buddy(zone, page, pfn, order, FPI_NONE);
-		return true;
-	}
 move:
 	/* Use MIGRATETYPE_MASK to get non-isolate migratetype */
 	if (isolate) {