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

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Message-ID: <CAHS8izNR8OSZA-F8d9s4GeRSJsnJKwW1nVc+wv01h8yo=V=6fQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 13:27:24 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Stanislav Fomichev <stfomichev@...il.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 Fri, Sep 5, 2025 at 8:22 AM Stanislav Fomichev <stfomichev@...il.com> wrote:
>
> 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..

Hmm, the other code paths overwrite the error to EFAULT; I don't know
if that's significant in some way. But seems fine to me, I don't see
why not do this, other than maybe potentional confusion with recvmsg
returning an error not documented here:

https://linux.die.net/man/2/recvmsg

But that seems a minor point.


-- 
Thanks,
Mina