[patch 04/24] mm/hugetlb.c: teach follow_hugetlb_page() to handle FOLL_NOWAIT

[patch 04/24] mm/hugetlb.c: teach follow_hugetlb_page() to handle FOLL_NOWAIT

[akpm]

From: Andrea Arcangeli <aarcange@redhat.com>
Subject: mm/hugetlb.c: teach follow_hugetlb_page() to handle FOLL_NOWAIT

hugetlb needs the same fix as faultin_nopage (which was applied in
96312e61282ae ("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/20190109020203.26669-2-aarcange@redhat.com
Fixes: ce53053ce378 ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Reported-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/hugetlb.c~mm-hugetlbc-teach-follow_hugetlb_page-to-handle-foll_nowait
+++ a/mm/hugetlb.c
@@ -4268,7 +4268,8 @@ long follow_hugetlb_page(struct mm_struc
 				break;
 			}
 			if (ret & VM_FAULT_RETRY) {
-				if (nonblocking)
+				if (nonblocking &&
+				    !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
 					*nonblocking = 0;
 				*nr_pages = 0;
 				/*
_