Re: [PATCH 1/3] mm: migrate: Fix the return value of migrate_pages()

As Zi Yan pointed out, the syscall move_pages() can return a non-migrated
number larger than the number of pages the users tried to migrate, when a
THP page is failed to migrate. This is confusing for users.

Since other migration scenarios do not care about the actual non-migrated
number of pages except the memory compaction migration which will fix in
following patch. Thus we can change the return value to return the number
of {normal page, THP, hugetlb} instead to avoid this issue, and the number
of THP splits will be considered as the number of non-migrated THP, no matter
how many subpages of the THP are migrated successfully. Meanwhile we should
still keep the migration counters using the number of normal pages.

Co-developed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/migrate.c | 63 +++++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index a11e948..9aafdab 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -1428,7 +1428,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  * @mode:		The migration mode that specifies the constraints for
  *			page migration, if any.
  * @reason:		The reason for page migration.
- * @ret_succeeded:	Set to the number of pages migrated successfully if
+ * @ret_succeeded:	Set to the number of normal pages migrated successfully if
  *			the caller passes a non-NULL pointer.
  *
  * The function returns after 10 attempts or if no pages are movable any more

@@ -1436,7 +1436,9 @@ static inline int try_split_thp(struct page *page, struct page **page2,
  * It is caller's responsibility to call putback_movable_pages() to return pages
  * to the LRU or free list only if ret != 0.
  *
- * Returns the number of pages that were not migrated, or an error code.
+ * Returns the number of {normal page, THP} that were not migrated, or an error code.
+ * The number of THP splits will be considered as the number of non-migrated THP,
+ * no matter how many subpages of the THP are migrated successfully.
  */
 int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		free_page_t put_new_page, unsigned long private,

@@ -1445,6 +1447,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	int retry = 1;
 	int thp_retry = 1;
 	int nr_failed = 0;
+	int nr_failed_pages = 0;
 	int nr_succeeded = 0;
 	int nr_thp_succeeded = 0;
 	int nr_thp_failed = 0;

@@ -1456,13 +1459,16 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	int swapwrite = current->flags & PF_SWAPWRITE;
 	int rc, nr_subpages;
 	LIST_HEAD(ret_pages);
+	LIST_HEAD(thp_split_pages);
 	bool nosplit = (reason == MR_NUMA_MISPLACED);
+	bool no_subpage_counting = false;
 
 	trace_mm_migrate_pages_start(mode, reason);
 
 	if (!swapwrite)
 		current->flags |= PF_SWAPWRITE;
 
+thp_subpage_migration:
 	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
 		retry = 0;
 		thp_retry = 0;

@@ -1511,18 +1517,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 			case -ENOSYS:
 				/* THP migration is unsupported */
 				if (is_thp) {
-					if (!try_split_thp(page, &page2, from)) {
+					nr_thp_failed++;
+					if (!try_split_thp(page, &page2, &thp_split_pages)) {