[RFC 1/3] mm, migrate: Check return value of try_to_unmap()

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

During the page migration, try_to_unmap() is called to replace the
page table entries with the migration entries.  Now its return value
is ignored, that is generally OK in most cases.  But in theory, it's
possible that try_to_unmap() return false (failure) for the page
migration after arch_unmap_one() is called in unmap code.  Even if
without arch_unmap_one(), it makes code more robust for the future
code changing to check the return value.

Known issue: I don't know what is the appropriate error code for
try_to_unmap() failure.  Whether EIO is OK?

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
---
 mm/migrate.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 3900044cfaa6..981f8374a6ef 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1116,8 +1116,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		/* Establish migration ptes */
 		VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
 				page);
-		try_to_unmap(page,
-			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+		if (!try_to_unmap(page,
+			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS)) {
+			rc = -EIO;
+			goto out_unlock_both;
+		}
 		page_was_mapped = 1;
 	}
 
@@ -1337,8 +1340,11 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		goto put_anon;
 
 	if (page_mapped(hpage)) {
-		try_to_unmap(hpage,
-			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+		if (!try_to_unmap(hpage,
+			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS)) {
+			rc = -EIO;
+			goto unlock_both;
+		}
 		page_was_mapped = 1;
 	}
 
@@ -1349,6 +1355,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		remove_migration_ptes(hpage,
 			rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage, false);
 
+unlock_both:
 	unlock_page(new_hpage);
 
 put_anon:
@@ -2558,8 +2565,7 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 			continue;
 
 		if (page_mapped(page)) {
-			try_to_unmap(page, flags);
-			if (page_mapped(page))
+			if (!try_to_unmap(page, flags))
 				goto restore;
 		}
 
-- 
2.25.0