mm/gup_benchmark: take the mmap lock around GUP

To be safe against concurrent changes to the VMA tree, we must take the
mmap lock around GUP operations (excluding the GUP-fast family of
operations, which will take the mmap lock by themselves if necessary).

This code is only for testing, and it's only reachable by root through
debugfs, so this doesn't really have any impact; however, if we want to
add lockdep asserts into the GUP path, we need to have clean locking here.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Michel Lespinasse <walken@google.com>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://lkml.kernel.org/r/CAG48ez3SG6ngZLtasxJ6LABpOnqCz5-QHqb0B4k44TQ8F9n6+w@mail.gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup_benchmark.c

@@ -72,6 +72,8 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 	int nr;
 	struct page **pages;
 	int ret = 0;
+	bool needs_mmap_lock =
+		cmd != GUP_FAST_BENCHMARK && cmd != PIN_FAST_BENCHMARK;
 
 	if (gup->size > ULONG_MAX)
 		return -EINVAL;
@@ -81,6 +83,11 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 	if (!pages)
 		return -ENOMEM;
 
+	if (needs_mmap_lock && mmap_read_lock_killable(current->mm)) {
+		ret = -EINTR;
+		goto free_pages;
+	}
+
 	i = 0;
 	nr = gup->nr_pages_per_call;
 	start_time = ktime_get();
@@ -120,9 +127,8 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 					    pages + i, NULL);
 			break;
 		default:
-			kvfree(pages);
 			ret = -EINVAL;
-			goto out;
+			goto unlock;
 		}
 
 		if (nr <= 0)
@@ -150,8 +156,11 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 	end_time = ktime_get();
 	gup->put_delta_usec = ktime_us_delta(end_time, start_time);
 
+unlock:
+	if (needs_mmap_lock)
+		mmap_read_unlock(current->mm);
+free_pages:
 	kvfree(pages);
-out:
 	return ret;
 }