Re: [PATCH net-next v4 8/9] tap: Keep hdr_len in tap_get_user()

[Akihiko Odaki]

On 2025/01/20 20:24, 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>
---
  drivers/net/tap.c | 17 +++++++----------
  1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 
061c2f27dfc83f5e6d0bea4da0e845cc429b1fd8..7ee2e9ee2a89fd539b087496b92d2f6198266f44
 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -645,6 +645,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void 
*msg_control,
        int err;
        struct virtio_net_hdr vnet_hdr = { 0 };
        int vnet_hdr_len = 0;
+       int hdr_len = 0;
        int copylen = 0;
        int depth;
        bool zerocopy = false;
@@ -672,6 +673,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void 
*msg_control,
                err = -EINVAL;
                if (tap16_to_cpu(q, vnet_hdr.hdr_len) > iov_iter_count(from))
                        goto err;
+               hdr_len = tap16_to_cpu(q, vnet_hdr.hdr_len);
        }
  
  	len = iov_iter_count(from);
@@ -683,11 +685,8 @@ 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;
                linear = copylen;
                i = *from;
@@ -698,11 +697,9 @@ 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;

Similar to previous patch, I don't think this patch is significant
enough to warrant the code churn.

The following patch will require replacing
tap16_to_cpu(q, vnet_hdr.hdr_len)
with
tap16_to_cpu(q->flags, vnet_hdr.hdr_len)

It will make some lines a bit too long. Calling tap16_to_cpu() at 
multiple places is also not good to keep the vnet implementation unified 
as the function inspects vnet_hdr.

This patch is independently too trivial, but I think it is a worthwhile 
cleanup combined with the following patch.