[PATCH 1/1] mm/hugetlb.c: teach follow_hugetlb_page() to handle FOLL_NOWAIT

[Andrea Arcangeli]

hugetlb needs the same fix as faultin_nopage (which was applied in
96312e61282ae3f6537a562625706498cbc75594) or KVM hangs because it
thinks the mmap_sem was already released by hugetlb_fault() if it
returned VM_FAULT_RETRY, but it wasn't in the FOLL_NOWAIT case.

Fixes: ce53053ce378 ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Tested-by: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx>
Reported-by: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx>
---
 mm/hugetlb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e37efd5d8318..b3622d7888c8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4301,7 +4301,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				break;
 			}
 			if (ret & VM_FAULT_RETRY) {
-				if (nonblocking)
+				if (nonblocking &&
+				    !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
 					*nonblocking = 0;
 				*nr_pages = 0;
 				/*