[PATCH] lib/test_hmm: fail dmirror_fault() when the mirrored mm is gone

[Stanislav Kinsburskii]

dmirror_fault() is called from the dmirror_read() and dmirror_write()
retry loops after dmirror_do_read() or dmirror_do_write() finds a missing
device page table entry.

If the mirrored mm has already exited, mmget_not_zero() fails. The current
code returns 0 in that case, which tells the caller that faulting succeeded
even though no page was faulted and no device page table entry was installed.
The caller then retries the same address, hits -ENOENT again, and can loop
forever without making progress.

Return -EFAULT instead, so the ioctl fails when the mirrored mm is no
longer faultable.

Fixes: b2ef9f5a5cb37 ("mm/hmm/test: add selftest driver for HMM")
Signed-off-by: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
---
 lib/test_hmm.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index c4adbf98fac7..45c0cb992218 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -407,7 +407,7 @@ static int dmirror_fault(struct dmirror *dmirror, unsigned long start,
 
 	/* Since the mm is for the mirrored process, get a reference first. */
 	if (!mmget_not_zero(mm))
-		return 0;
+		return -EFAULT;
 
 	for (addr = start; addr < end; addr = range.end) {
 		range.start = addr;