bcachefs: Optimize bch2_dirent_name_bytes

Avoids doing a full strnlen for getting the length of the name of a
dirent entry.

Given the fact that the name of dirents is stored at the end of the
bkey's value, and we know the length of that in u64s, we can find the
last u64 and figure out how many NUL bytes are at the end of the string.

On little endian systems this ends up being the leading zeros of the
last u64, whereas on big endian systems this ends up being the trailing
zeros of the last u64.
We can take that value in bits and divide it by 8 to get the number of
NUL bytes at the end.

There is no endian-fixup or other compatibility here as this is string
data interpreted as a u64.
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/dirent.c

@@ -15,10 +15,18 @@
 
 static unsigned bch2_dirent_name_bytes(struct bkey_s_c_dirent d)
 {
-	unsigned len = bkey_val_bytes(d.k) -
-		offsetof(struct bch_dirent, d_name);
-
-	return strnlen(d.v->d_name, len);
+	unsigned bkey_u64s = bkey_val_u64s(d.k);
+	unsigned bkey_bytes = bkey_u64s * sizeof(u64);
+	u64 last_u64 = ((u64*)d.v)[bkey_u64s - 1];
+#if CPU_BIG_ENDIAN
+	unsigned trailing_nuls = last_u64 ? __builtin_ctzll(last_u64) / 8 : 64 / 8;
+#else
+	unsigned trailing_nuls = last_u64 ? __builtin_clzll(last_u64) / 8 : 64 / 8;
+#endif
+
+	return bkey_bytes -
+		offsetof(struct bch_dirent, d_name) -
+		trailing_nuls;
 }
 
 struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
@@ -113,6 +121,11 @@ int bch2_dirent_invalid(const struct bch_fs *c, struct bkey_s_c k,
 		return -BCH_ERR_invalid_bkey;
 	}
 
+	if (d_name.len != strnlen(d_name.name, d_name.len)) {
+		prt_printf(err, "dirent has stray data after name's NUL");
+		return -BCH_ERR_invalid_bkey;
+	}
+
 	if (d_name.len == 1 && !memcmp(d_name.name, ".", 1)) {
 		prt_printf(err, "invalid name");
 		return -BCH_ERR_invalid_bkey;