[PATCH v2] fs: warn in case userspace lied about modprobe return

["Luis R. Rodriguez" <mcgrof@kernel.org>]

kmod <= v19 was broken -- it could return 0 to modprobe calls,
incorrectly assuming that a kernel module was built-in, whereas in
reality the module was just forming in the kernel. The reason for this
is an incorrect userspace heuristics. A userspace kmod fix is available
for it [0], however should userspace break again we could go on with
an failed get_fs_type() which is hard to debug as the request_module()
is detected as returning 0. The first suspect would be that there is
something worth with the kernel's module loader and obviously in this
case that is not the issue.

Since these issues are painful to debug complain when we know userspace
has outright lied to us.

[0] http://git.kernel.org/cgit/utils/kernel/kmod/kmod.git/commit/libkmod/libkmod-module.c?id=fd44a98ae2eb5eb32161088954ab21e58e19dfc4

Suggested-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jessica Yu <jeyu@redhat.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---

This v2 addresses the ordering issue Jessica pointed out.

 fs/filesystems.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index cac75547d35c..8b99955e3504 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -275,8 +275,10 @@ struct file_system_type *get_fs_type(const char *name)
 	int len = dot ? dot - name : strlen(name);
 
 	fs = __get_fs_type(name, len);
-	if (!fs && (request_module("fs-%.*s", len, name) == 0))
+	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
 		fs = __get_fs_type(name, len);
+		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
+	}
 
 	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
 		put_filesystem(fs);
-- 
2.11.0