mm/hugetlb.c: teach follow_hugetlb_page() to handle FOLL_NOWAIT

hugetlb needs the same fix as faultin_nopage (which was applied in
commit 96312e6 ("mm/gup.c: teach get_user_pages_unlocked to handle
FOLL_NOWAIT")) 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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: ce53053 ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: Andrea Arcangeli <[email protected]>
Tested-by: "Dr. David Alan Gilbert" <[email protected]>
Reported-by: "Dr. David Alan Gilbert" <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: aagit

mm/hugetlb.c

@@ -4268,7 +4268,8 @@ 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;
 				/*