Re: [PATCH net-next v5 4/4] net: add per-netns sysctl for devmem autorelease

[Mina Almasry]

On Thu, Oct 23, 2025 at 2:00 PM Bobby Eshleman <bobbyeshleman@xxxxxxxxx> wrote:
>
> From: Bobby Eshleman <bobbyeshleman@xxxxxxxx>
>
> Add a new per-namespace sysctl to control the autorelease
> behavior of devmem dmabuf bindings. The sysctl is found at:
> /proc/sys/net/core/devmem_autorelease
>
> When a binding is created, it inherits the autorelease setting from the
> network namespace of the device to which it's being bound.
>
> If autorelease is enabled (1):
> - Tokens are stored in socket's xarray
> - Tokens are automatically released when socket is closed
>
> If autorelease is disabled (0):
> - Tokens are tracked via uref counter in each net_iov
> - User must manually release tokens via SO_DEVMEM_DONTNEED
> - Lingering tokens are released when dmabuf is unbound
> - This is the new default behavior for better performance
>

Maybe quote the significant better performance in the docs and commit message.

> This allows application developers to choose between automatic cleanup
> (easier, backwards compatible) and manual control (more explicit token
> management, but more performant).
>
> Changes the default to autorelease=0, so that users gain the performance
> benefit by default.
>
> Signed-off-by: Bobby Eshleman <bobbyeshleman@xxxxxxxx>
> ---
>  include/net/netns/core.h   | 1 +
>  net/core/devmem.c          | 2 +-
>  net/core/net_namespace.c   | 1 +
>  net/core/sysctl_net_core.c | 9 +++++++++
>  4 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/netns/core.h b/include/net/netns/core.h
> index 9ef3d70e5e9c..7af5ab0d757b 100644
> --- a/include/net/netns/core.h
> +++ b/include/net/netns/core.h
> @@ -18,6 +18,7 @@ struct netns_core {
>         u8      sysctl_txrehash;
>         u8      sysctl_tstamp_allow_data;
>         u8      sysctl_bypass_prot_mem;
> +       u8      sysctl_devmem_autorelease;
>
>  #ifdef CONFIG_PROC_FS
>         struct prot_inuse __percpu *prot_inuse;
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 8f3199fe0f7b..9cd6d93676f9 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -331,7 +331,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
>                 goto err_free_chunks;
>
>         list_add(&binding->list, &priv->bindings);
> -       binding->autorelease = true;
> +       binding->autorelease = dev_net(dev)->core.sysctl_devmem_autorelease;
>

Do you need to READ_ONCE this and WRITE_ONCE the write site? Or is
that silly for a u8? Maybe better be safe.

Could we not make this an optional netlink argument? I thought that
was a bit nicer than a sysctl.

Needs a doc update.


-- 
Thanks,
Mina