Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get_user()

[Akihiko Odaki]

On 2025/02/06 6:21, Willem de Bruijn wrote:
Akihiko Odaki wrote:
hdr_len is repeatedly used so keep it in a local variable.
Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>

@@ -682,11 +683,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void 
*msg_control,
        if (msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
                struct iov_iter i;
  
-		copylen = vnet_hdr.hdr_len ?
-                       tap16_to_cpu(q, vnet_hdr.hdr_len) : GOODCOPY_LEN;
-               if (copylen > good_linear)
-                       copylen = good_linear;
-               else if (copylen < ETH_HLEN)
+               copylen = min(hdr_len ? hdr_len : GOODCOPY_LEN, good_linear);
+               if (copylen < ETH_HLEN)
                        copylen = ETH_HLEN;

I forgot earlier: this can also use single line statement

     copylen = max(copylen, ETH_HLEN);

And perhaps easiest to follow is

     copylen = hdr_len ?: GOODCOPY_LEN;
     copylen = min(copylen, good_linear);
     copylen = max(copylen, ETH_HLEN);

I introduced the min() usage as it now neatly fits in a line, but I 
found even clamp() fits so I'll use it in the next version:
copylen = clamp(hdr_len ?: GOODCOPY_LEN, ETH_HLEN, good_linear);

Please tell me if you prefer hdr_len ?: GOODCOPY_LEN in a separate line:
copylen = hdr_len ?: GOODCOPY_LEN;
copylen = clamp(copylen, ETH_HLEN, good_linear);


                linear = copylen;
                i = *from;
@@ -697,11 +695,9 @@ static ssize_t tap_get_user(struct tap_queue *q, void 
*msg_control,
  
  	if (!zerocopy) {
                copylen = len;
-               linear = tap16_to_cpu(q, vnet_hdr.hdr_len);
-               if (linear > good_linear)
-                       linear = good_linear;
-               else if (linear < ETH_HLEN)
-                       linear = ETH_HLEN;
+               linear = min(hdr_len, good_linear);
+               if (copylen < ETH_HLEN)
+                       copylen = ETH_HLEN;> > Same


I realized I mistakenly replaced linear with copylen here. Using clamp() 
will remove redundant variable references and fix the bug.