netdev - Re: [PATCH net-next] selftests: ncdevmem: don't retry EFAULT

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Message-ID: <aLsAP-UgtINNwIMJ@mini-arch>
Date: Fri, 5 Sep 2025 08:22:39 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Stanislav Fomichev <sdf@...ichev.me>, netdev@...r.kernel.org,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, andrew+netdev@...n.ch, shuah@...nel.org,
	joe@...a.to, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] selftests: ncdevmem: don't retry EFAULT

On 09/04, Mina Almasry wrote:
> On Thu, Sep 4, 2025 at 11:27 AM Stanislav Fomichev <sdf@...ichev.me> wrote:
> >
> > devmem test fails on NIPA. Most likely we get skb(s) with readable
> > frags (why?)
> 
> I would expect if we get readable frags that the frags land in the
> host buffer we provide ncdevmem and we actually hit this error:
> 
> ```
>   1                 if (!is_devmem) {
>   0                         pr_err("flow steering error");
>   1                         goto err_close_client;
>   2                 }
> ```
> 
> which as it says, should be root caused in a flow steering error. I
> don't know what would cause an EFAULT off the top of my head.

Yea, I don't understand what happens :-( I'm thinking of doing the
following as well:

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 40b774b4f587..0c18a8c7965f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2820,7 +2820,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
 							 used);
 				if (err <= 0) {
 					if (!copied)
-						copied = -EFAULT;
+						copied = err;
 
 					break;
 				}

Should give us more info for the devmem case... LMK if you don't like
it. If I don't hear from you in a couple of days, I'll send it out..