[PATCH v4] tun: avoid high-order page allocation for packet header

[Tahsin Erdogan <trdgn@amazon.com>]

When gso.hdr_len is zero and a packet is transmitted via write() or
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 write()/writev() and sendmsg() paths more consistent.

Signed-off-by: Tahsin Erdogan <trdgn@amazon.com>
---
v4: updated commit message address comments from Willem
v3: rebase to latest net-next
v2: replace linear == 0 with !linear
v1: https://lore.kernel.org/all/20230726030936.1587269-1-trdgn@amazon.com/
 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 973b2fc74de3..62106464f1b9 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;
 
 	if (len - linear > MAX_SKB_FRAGS * (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
@@ -1840,6 +1840,9 @@ 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