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Subject[PATCH net] tuntap: try not batch packets for devmap
Date
Commit 762c330d670e ("tuntap: add missing xdp flush") tries to fix the
devmap stall caused by missed xdp flush by counting the pending xdp
redirected packets and flush when it exceeds NAPI_POLL_WEIGHT or
MSG_MORE is clear. This may lead BUG() since xdp_do_flush() was
called under process context with preemption enabled. Simply disable
preemption may silent the warning but be not enough since process may
move between different CPUS during a batch which cause xdp_do_flush()
misses some CPU where the process run previously. For -net, fix this
by simply calling xdp_do_flush() immediately after xdp_do_redirect(),
a side effect is that this removes any possibility of batching which
could be addressed in the future.

Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
Fixes: 762c330d670e ("tuntap: add missing xdp flush")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
drivers/net/tun.c | 15 +--------------
1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 17e496b..6a4cd97 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -181,7 +181,6 @@ struct tun_file {
struct tun_struct *detached;
struct ptr_ring tx_ring;
struct xdp_rxq_info xdp_rxq;
- int xdp_pending_pkts;
};

struct tun_flow_entry {
@@ -1666,10 +1665,10 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
case XDP_REDIRECT:
get_page(alloc_frag->page);
alloc_frag->offset += buflen;
- ++tfile->xdp_pending_pkts;
err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
if (err)
goto err_redirect;
+ xdp_do_flush_map();
rcu_read_unlock();
return NULL;
case XDP_TX:
@@ -1988,11 +1987,6 @@ static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)
result = tun_get_user(tun, tfile, NULL, from,
file->f_flags & O_NONBLOCK, false);

- if (tfile->xdp_pending_pkts) {
- tfile->xdp_pending_pkts = 0;
- xdp_do_flush_map();
- }
-
tun_put(tun);
return result;
}
@@ -2330,12 +2324,6 @@ static int tun_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
m->msg_flags & MSG_DONTWAIT,
m->msg_flags & MSG_MORE);

- if (tfile->xdp_pending_pkts >= NAPI_POLL_WEIGHT ||
- !(m->msg_flags & MSG_MORE)) {
- tfile->xdp_pending_pkts = 0;
- xdp_do_flush_map();
- }
-
tun_put(tun);
return ret;
}
@@ -3167,7 +3155,6 @@ static int tun_chr_open(struct inode *inode, struct file * file)
sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);

memset(&tfile->tx_ring, 0, sizeof(tfile->tx_ring));
- tfile->xdp_pending_pkts = 0;

return 0;
}
--
2.7.4
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