[PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

["Zach O'Keefe" <zokeefe@google.com>]

During collapse, in a few places we check to see if a given small page
has any unaccounted references.  If the refcount on the page doesn't
match our expectations, it must be there is an unknown user concurrently
interested in the page, and so it's not safe to move the contents
elsewhere. However, the unaccounted pins are likely an ephemeral state.

In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
collapse may succeed on retry.

Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>

---
 mm/khugepaged.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e23619bfecc4..fa38cae240b9 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2712,6 +2712,7 @@ static int madvise_collapse_errno(enum scan_result r)
 	case SCAN_CGROUP_CHARGE_FAIL:
 		return -EBUSY;
 	/* Resource temporary unavailable - trying again might succeed */
+	case SCAN_PAGE_COUNT:
 	case SCAN_PAGE_LOCK:
 	case SCAN_PAGE_LRU:
 	case SCAN_DEL_PAGE_LRU:
-- 
2.39.1.405.gd4c25cc71f-goog