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

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

[Stanislav Fomichev]

devmem test fails on NIPA. Most likely we get skb(s) with readable
frags (why?) but the failure manifests as an OOM. The OOM happens
because ncdevmem spams the following message:

  recvmsg ret=-1
  recvmsg: Bad address

As of today, ncdevmem can't deal with various reasons of EFAULT:
- falling back to regular recvmsg for non-devmem skbs
- increasing ctrl_data size (can't happen with ncdevmem's large buffer)

Exit (cleanly) with error when recvmsg returns EFAULT. This should at
least cause the test to cleanup its state.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 tools/testing/selftests/drivers/net/hw/ncdevmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/drivers/net/hw/ncdevmem.c b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
index 8dc9511d046f..c0a22938bed2 100644
--- a/tools/testing/selftests/drivers/net/hw/ncdevmem.c
+++ b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
@@ -945,6 +945,10 @@ static int do_server(struct memory_buffer *mem)
 			continue;
 		if (ret < 0) {
 			perror("recvmsg");
+			if (errno == EFAULT) {
+				pr_err("received EFAULT, won't recover");
+				goto err_close_client;
+			}
 			continue;
 		}
 		if (ret == 0) {
-- 
2.51.0

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

[Mina Almasry]

On Thu, Sep 4, 2025 at 11:27 AM Stanislav Fomichev <sdf@fomichev.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.

> but the failure manifests as an OOM. The OOM happens
> because ncdevmem spams the following message:
>
>   recvmsg ret=-1
>   recvmsg: Bad address
>
> As of today, ncdevmem can't deal with various reasons of EFAULT:
> - falling back to regular recvmsg for non-devmem skbs
> - increasing ctrl_data size (can't happen with ncdevmem's large buffer)
>
> Exit (cleanly) with error when recvmsg returns EFAULT. This should at
> least cause the test to cleanup its state.
>
> Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>

Either way, change looks good to me.

Reviewed-by: Mina Almasry <almasrymina@google.com>

-- 
Thanks,
Mina

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

[Stanislav Fomichev]

On 09/04, Mina Almasry wrote:
> On Thu, Sep 4, 2025 at 11:27 AM Stanislav Fomichev <sdf@fomichev.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..

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

[Mina Almasry]

On Fri, Sep 5, 2025 at 8:22 AM Stanislav Fomichev <stfomichev@gmail.com> wrote:
>
> On 09/04, Mina Almasry wrote:
> > On Thu, Sep 4, 2025 at 11:27 AM Stanislav Fomichev <sdf@fomichev.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

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

[Stanislav Fomichev]

On 09/05, Mina Almasry wrote:
> On Fri, Sep 5, 2025 at 8:22 AM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> >
> > On 09/04, Mina Almasry wrote:
> > > On Thu, Sep 4, 2025 at 11:27 AM Stanislav Fomichev <sdf@fomichev.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.

SG! Exporting new errnos seems to be ok because we are dealing with a
new devmem mode? I think the bug we have on the fbnic is that we don't
properly set skb->unreadable, so it's completely unrelated to this,
but still feels like it might help with future debugging..

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu,  4 Sep 2025 11:27:10 -0700 you wrote:
> devmem test fails on NIPA. Most likely we get skb(s) with readable
> frags (why?) but the failure manifests as an OOM. The OOM happens
> because ncdevmem spams the following message:
> 
>   recvmsg ret=-1
>   recvmsg: Bad address
> 
> [...]

Here is the summary with links:
  - [net-next] selftests: ncdevmem: don't retry EFAULT
    https://git.kernel.org/netdev/net-next/c/8c0b9ed2401b

You are awesome, thank you!
-- 
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