+ mm-page_alloc-check-the-correct-buddy-if-it-is-a-starting-block.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/page_alloc: check the correct buddy if it is a starting block
has been added to the -mm mm-new branch.  Its filename is
     mm-page_alloc-check-the-correct-buddy-if-it-is-a-starting-block.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-check-the-correct-buddy-if-it-is-a-starting-block.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Wei Yang <richard.weiyang@gmail.com>
Subject: mm/page_alloc: check the correct buddy if it is a starting block
Date: Fri, 5 Sep 2025 14:03:58 +0000

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>
---

 mm/page_alloc.c |   25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-check-the-correct-buddy-if-it-is-a-starting-block
+++ a/mm/page_alloc.c
@@ -2090,9 +2090,10 @@ static inline void toggle_pageblock_isol
 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_isola
 	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) {
_

Patches currently in -mm which might be from richard.weiyang@gmail.com are

mm-rmap-do-__folio_mod_stat-in-__folio_add_rmap.patch
selftests-mm-do-check_huge_anon-with-a-number-been-passed-in.patch
mm-rmap-not-necessary-to-mask-off-folio_pages_mapped.patch
mm-rmap-use-folio_large_nr_pages-when-we-are-sure-it-is-a-large-folio.patch
selftests-mm-put-general-ksm-operation-into-vm_util.patch
selftests-mm-test-that-rmap-behave-as-expected.patch
mm-khugepaged-use-list_xxx-helper-to-improve-readability.patch
mm-page_alloc-use-xxx_pageblock_isolate-for-better-reading.patch
mm-pageblock-flags-remove-pb_migratetype_bits-pb_migrate_end.patch
mm-page_alloc-find_large_buddy-from-start_pfn-aligned-order.patch
mm-page_alloc-find_large_buddy-from-start_pfn-aligned-order-v2.patch
mm-page_alloc-check-the-correct-buddy-if-it-is-a-starting-block.patch