[PATCH v2] tun: avoid high-order page allocation for packet header

[Tahsin Erdogan]

When GSO is not enabled and a packet is transmitted via writev(), all
payload is treated as header which requires a contiguous memory allocation.
This allocation request is harder to satisfy, and may even fail if there is
enough fragmentation.

Note that sendmsg() code path limits the linear copy length, so this change
makes writev() and sendmsg() more consistent.

Signed-off-by: Tahsin Erdogan <trdgn@xxxxxxxxxx>
---
v2: replace linear == 0 with !linear
v1: https://lore.kernel.org/all/20230726030936.1587269-1-trdgn@xxxxxxxxxx/
 drivers/net/tun.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..4c57804f4cbd 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1523,7 +1523,7 @@ static struct sk_buff *tun_alloc_skb(struct tun_file *tfile,
 	int err;
 
 	/* Under a page?  Don't bother with paged skb. */
-	if (prepad + len < PAGE_SIZE || !linear)
+	if (prepad + len < PAGE_SIZE)
 		linear = len;
 
 	skb = sock_alloc_send_pskb(sk, prepad + linear, len - linear, noblock,
@@ -1838,6 +1838,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			 */
 			zerocopy = false;
 		} else {
+			if (!linear)
+				linear = min_t(size_t, good_linear, copylen);
+
 			skb = tun_alloc_skb(tfile, align, copylen, linear,
 					    noblock);
 		}
-- 
2.41.0