linux-kernel - [PATCH v2 1/4] mm: migrate: Move the page count validation to the proper place

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Message-Id: <644a666e1e177c57a39a4c37c6e2ca64052b9d7e.1629008158.git.baolin.wang@linux.alibaba.com>
Date:   Sun, 15 Aug 2021 14:23:03 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     akpm@...ux-foundation.org
Cc:     apopple@...dia.com, shy828301@...il.com, willy@...radead.org,
        baolin.wang@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/4] mm: migrate: Move the page count validation to the proper place

We've got the expected count for anonymous page or file page by
expected_page_refs() at the beginning of migrate_page_move_mapping(),
thus we should move the page count validation a little forward to
reduce duplicated code.

Moreover the i_pages lock is not used to guarantee the page refcount
safety in migrate_page_move_mapping(), so we can move the getting page
count out of the i_pages lock. Since now the migration page has
established a migration pte under the page lock now, with the page
refcount freezing validation, to ensure that the page references
meet the migration requirement.

Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 mm/migrate.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index c6eb2a8..8e9282f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -386,11 +386,10 @@ int folio_migrate_mapping(struct address_space *mapping,
 	int expected_count = expected_page_refs(mapping, &folio->page) + extra_count;
 	long nr = folio_nr_pages(folio);
 
-	if (!mapping) {
-		/* Anonymous page without mapping */
-		if (folio_ref_count(folio) != expected_count)
-			return -EAGAIN;
+	if (folio_ref_count(folio) != expected_count)
+		return -EAGAIN;
 
+	if (!mapping) {
 		/* No turning back from here */
 		newfolio->index = folio->index;
 		newfolio->mapping = folio->mapping;
@@ -404,8 +403,7 @@ int folio_migrate_mapping(struct address_space *mapping,
 	newzone = folio_zone(newfolio);
 
 	xas_lock_irq(&xas);
-	if (folio_ref_count(folio) != expected_count ||
-	    xas_load(&xas) != folio) {
+	if (xas_load(&xas) != folio) {
 		xas_unlock_irq(&xas);
 		return -EAGAIN;
 	}
-- 
1.8.3.1