ovl: inherit SB_NOSEC flag from upperdir

Since the stacking of regular file operations [1], the overlayfs edition of
write_iter() is called when writing regular files.

Since then, xattr lookup is needed on every write since file_remove_privs()
is called from ovl_write_iter(), which would become the performance
bottleneck when writing small chunks of data. In my test case,
file_remove_privs() would consume ~15% CPU when running fstime of unixbench
(the workload is repeadly writing 1 KB to the same file) [2].

Inherit the SB_NOSEC flag from upperdir. Since then xattr lookup would be
done only once on the first write. Unixbench fstime gets a ~20% performance
gain with this patch.

[1] https://lore.kernel.org/lkml/20180606150905.GC9426@magnolia/T/
[2] https://www.spinics.net/lists/linux-unionfs/msg07153.html

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -1100,6 +1100,18 @@ static int ovl_get_upper(struct super_block *sb, struct ovl_fs *ofs,
 	upper_mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
 	ofs->upper_mnt = upper_mnt;
 
+	/*
+	 * Inherit SB_NOSEC flag from upperdir.
+	 *
+	 * This optimization changes behavior when a security related attribute
+	 * (suid/sgid/security.*) is changed on an underlying layer.  This is
+	 * okay because we don't yet have guarantees in that case, but it will
+	 * need careful treatment once we want to honour changes to underlying
+	 * filesystems.
+	 */
+	if (upper_mnt->mnt_sb->s_flags & SB_NOSEC)
+		sb->s_flags |= SB_NOSEC;
+
 	if (ovl_inuse_trylock(ofs->upper_mnt->mnt_root)) {
 		ofs->upperdir_locked = true;
 	} else {