linux-kernel - [PATCH v3 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

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Message-Id: <de386aa864be9158d2f3b344091419ea7c38b2f7.1666599848.git.baolin.wang@linux.alibaba.com>
Date:   Mon, 24 Oct 2022 16:34:21 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     akpm@...ux-foundation.org
Cc:     david@...hat.com, ying.huang@...el.com, ziy@...dia.com,
        shy828301@...il.com, apopple@...dia.com,
        baolin.wang@...ux.alibaba.com, jingshan@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

During THP migration, if THPs are not migrated but they are split and all
subpages are migrated successfully, migrate_pages() will still return the
number of THP pages that were not migrated.  This will confuse the callers
of migrate_pages().  For example, the longterm pinning will failed though
all pages are migrated successfully.

Thus we should return 0 to indicate that all pages are migrated in this
case

Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
Reviewed-by: Alistair Popple <apopple@...dia.com>
Cc: <stable@...r.kernel.org>
---
Changes from v2:
 - Add Fixes tag suggested by Yang Shi and Huang, Ying.
 - Drop 'nr_thp_split' validation suggested by Alistair.
 - Add reviewed tag from Alistair.
 - Update the commit message suggested by Andrew.
Changes from v1:
 - Fix the return value of migrate_pages() instead of fixing the
   callers' validation.
---
 mm/migrate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8e5eb6e..2eb16f8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1582,6 +1582,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	 */
 	list_splice(&ret_pages, from);
 
+	/*
+	 * Return 0 in case all subpages of fail-to-migrate THPs are
+	 * migrated successfully.
+	 */
+	if (list_empty(from))
+		rc = 0;
+
 	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
 	count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
 	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
-- 
1.8.3.1