[PATCH] mm/gup: allow CMA migration to propagate errors back to caller

[zhong jiang <zhongjiang@huawei.com>]

check_and_migrate_cma_pages() was recording the result of
__get_user_pages_locked() in an unsigned "nr_pages" variable. Because
__get_user_pages_locked() returns a signed value that can include
negative errno values, this had the effect of hiding errors.

Change check_and_migrate_cma_pages() implementation so that it
uses a signed variable instead, and propagates the results back
to the caller just as other gup internal functions do.

This was discovered with the help of unsigned_lesser_than_zero.cocci.

Suggested-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
---
 mm/gup.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 8f236a3..c2b3e11 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
 	bool drain_allow = true;
 	bool migrate_allow = true;
 	LIST_HEAD(cma_page_list);
+	long ret = nr_pages;
 
 check_again:
 	for (i = 0; i < nr_pages;) {
@@ -1504,17 +1505,18 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
 		 * again migrating any new CMA pages which we failed to isolate
 		 * earlier.
 		 */
-		nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
+		ret = __get_user_pages_locked(tsk, mm, start, nr_pages,
 						   pages, vmas, NULL,
 						   gup_flags);
 
-		if ((nr_pages > 0) && migrate_allow) {
+		if ((ret > 0) && migrate_allow) {
+			nr_pages = ret;
 			drain_allow = true;
 			goto check_again;
 		}
 	}
 
-	return nr_pages;
+	return ret;
 }
 #else
 static long check_and_migrate_cma_pages(struct task_struct *tsk,
-- 
1.7.12.4