linux-kernel - [PATCH v2 2/5] mm: khugepaged: use the number of pages in the folio to check the reference count

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Message-Id: <9ea49262308de28957596cc6e8edc2d3a4f54659.1724140601.git.baolin.wang@linux.alibaba.com>
Date: Tue, 20 Aug 2024 17:49:14 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: akpm@...ux-foundation.org
Cc: hughd@...gle.com,
	willy@...radead.org,
	david@...hat.com,
	21cnbao@...il.com,
	ryan.roberts@....com,
	shy828301@...il.com,
	ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/5] mm: khugepaged: use the number of pages in the folio to check the reference count

Use the number of pages in the folio to check the reference count as
preparation for supporting shmem mTHP collapse.

Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
Acked-by: David Hildenbrand <david@...hat.com>
---
 mm/khugepaged.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 76f05215ee87..e015c94eba09 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1988,9 +1988,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		VM_BUG_ON_FOLIO(folio != xa_load(xas.xa, index), folio);
 
 		/*
-		 * We control three references to the folio:
+		 * We control 2 + nr_pages references to the folio:
 		 *  - we hold a pin on it;
-		 *  - one reference from page cache;
+		 *  - nr_pages reference from page cache;
 		 *  - one from lru_isolate_folio;
 		 * If those are the only references, then any new usage
 		 * of the folio will have to fetch it from the page
@@ -1998,7 +1998,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		 * truncate, so any new usage will be blocked until we
 		 * unlock folio after collapse/during rollback.
 		 */
-		if (folio_ref_count(folio) != 3) {
+		if (folio_ref_count(folio) != 2 + folio_nr_pages(folio)) {
 			result = SCAN_PAGE_COUNT;
 			xas_unlock_irq(&xas);
 			folio_putback_lru(folio);
@@ -2181,7 +2181,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		folio_clear_active(folio);
 		folio_clear_unevictable(folio);
 		folio_unlock(folio);
-		folio_put_refs(folio, 3);
+		folio_put_refs(folio, 2 + folio_nr_pages(folio));
 	}
 
 	goto out;
-- 
2.39.3