Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

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Hi Matthew,

On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
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.

Please add an explanation to the changelog for why it's safe to pull
this out from under the i_pages lock.

Sure. In folio_migrate_mapping(), we are sure that the migration page was isolated from lru list and locked, so I think there are no race to get the page count without i_pages lock. Please correct me if I missed something else. Thanks.


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

diff --git a/mm/migrate.c b/mm/migrate.c
index 239b238..5559571 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






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