Subject: Re: [net-next v2 3/4] eventpoll: Add epoll ioctl for epoll_params

From: Joe Damato <[email-masked]>
Date: Wed, 24 Jan 2024 20:27:46 -0800

On Wed, Jan 24, 2024 at 09:46:23PM -0500, Willem de Bruijn wrote:
> Joe Damato wrote:
> > Add an ioctl for getting and setting epoll_params. User programs can use
> > this ioctl to get and set the busy poll usec time or packet budget
> > params for a specific epoll context.
> > 
> > Signed-off-by: Joe Damato <[email-masked]>
> 
> Please be sure to include the lists and people suggested by
> `get_maintainer.pl -f fs/eventpoll.c`.

Thanks - I must have done something wrong when trying to get the maintainer
list.

Should I resend this v2? Not sure what the appropriate thing to do is in
this case. My apologies.

> Adding ioctls is generally discouraged.
> 
> As this affects the behavior of epoll_wait, should this just be a
> flag to (a new variant of) epoll_wait?

I have no strong preference either way. It seems to me that adding a new
system call is a fairly significant change vs adding an ioctl, but I am
open to whatever is preferred by the maintainers.

I have no idea who would need to weigh-in to make this decision.

> Speaking from some experience with adding epoll_pwait2. I initially
> there added a stateful change that would affect wait behavior. The
> sensible feedback as the time was to just change the behavior of the
> syscall it affected. Even if that requires a syscall (which is not
> that different from an ioctl, if better defined).
> 
> The discussion in that thread may be informative to decide on API:
> https://lwn.net/ml/linux-kernel/[email-masked]/

Interesting thread, thanks for sending.

> Agreed on the overall principle that it is preferable to be able to
> enable busypolling selectively. We already do for SO_BUSY_POLL and
> sysctl busy_read.

Thanks for taking a look and providing feedback.

>
> > ---
> >  .../userspace-api/ioctl/ioctl-number.rst      |  1 +
> >  fs/eventpoll.c                                | 47 +++++++++++++++++++
> >  include/uapi/linux/eventpoll.h                | 12 +++++
> >  3 files changed, 60 insertions(+)
> > 
> > diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > index 457e16f06e04..b33918232f78 100644
> > --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> > +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > @@ -309,6 +309,7 @@ Code  Seq#    Include File                                           Comments
> >  0x89  0B-DF  linux/sockios.h
> >  0x89  E0-EF  linux/sockios.h                                         SIOCPROTOPRIVATE range
> >  0x89  F0-FF  linux/sockios.h                                         SIOCDEVPRIVATE range
> > +0x8A  00-1F  linux/eventpoll.h
> >  0x8B  all    linux/wireless.h
> >  0x8C  00-3F                                                          WiNRADiO driver
> >                                                                       <http://www.winradio.com.au/>
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index 40bd97477b91..c1ee0fe01da1 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -6,6 +6,8 @@
> >   *  Davide Libenzi <[email-masked]>
> >   */
> >  
> > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > +
> >  #include <linux/init.h>
> >  #include <linux/kernel.h>
> >  #include <linux/sched/signal.h>
> > @@ -869,6 +871,49 @@ static void ep_clear_and_put(struct eventpoll *ep)
> >  		ep_free(ep);
> >  }
> >  
> > +static long ep_eventpoll_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > +{
> > +	int ret;
> > +	struct eventpoll *ep;
> > +	struct epoll_params epoll_params;
> > +	void __user *uarg = (void __user *) arg;
> > +
> > +	if (!is_file_epoll(file))
> > +		return -EINVAL;
> > +
> > +	ep = file->private_data;
> > +
> > +	switch (cmd) {
> > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > +	case EPIOCSPARAMS:
> > +		if (copy_from_user(&epoll_params, uarg, sizeof(epoll_params)))
> > +			return -EFAULT;
> > +
> > +		if (epoll_params.busy_poll_budget > NAPI_POLL_WEIGHT)
> > +			pr_err("busy poll budget %u exceeds suggested maximum %u\n",
> > +					epoll_params.busy_poll_budget, NAPI_POLL_WEIGHT);
> > +
> > +		ep->busy_poll_usecs = epoll_params.busy_poll_usecs;
> > +		ep->busy_poll_budget = epoll_params.busy_poll_budget;
> > +		return 0;
> > +
> > +	case EPIOCGPARAMS:
> > +		memset(&epoll_params, 0, sizeof(epoll_params));
> > +		epoll_params.busy_poll_usecs = ep->busy_poll_usecs;
> > +		epoll_params.busy_poll_budget = ep->busy_poll_budget;
> > +		if (copy_to_user(uarg, &epoll_params, sizeof(epoll_params)))
> > +			return -EFAULT;
> > +
> > +		return 0;
> > +#endif
> > +	default:
> > +		ret = -EINVAL;
> > +		break;
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> >  static int ep_eventpoll_release(struct inode *inode, struct file *file)
> >  {
> >  	struct eventpoll *ep = file->private_data;
> > @@ -975,6 +1020,8 @@ static const struct file_operations eventpoll_fops = {
> >  	.release	= ep_eventpoll_release,
> >  	.poll		= ep_eventpoll_poll,
> >  	.llseek		= noop_llseek,
> > +	.unlocked_ioctl	= ep_eventpoll_ioctl,
> > +	.compat_ioctl   = compat_ptr_ioctl,
> >  };
> >  
> >  /*
> > diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
> > index cfbcc4cc49ac..8eb0fdbce995 100644
> > --- a/include/uapi/linux/eventpoll.h
> > +++ b/include/uapi/linux/eventpoll.h
> > @@ -85,4 +85,16 @@ struct epoll_event {
> >  	__u64 data;
> >  } EPOLL_PACKED;
> >  
> > +struct epoll_params {
> > +	u64 busy_poll_usecs;
> > +	u16 busy_poll_budget;
> > +
> > +	/* for future fields */
> > +	u8 data[118];
> > +} EPOLL_PACKED;
> > +
> > +#define EPOLL_IOC_TYPE 0x8A
> > +#define EPIOCSPARAMS _IOW(EPOLL_IOC_TYPE, 0x01, struct epoll_params)
> > +#define EPIOCGPARAMS _IOR(EPOLL_IOC_TYPE, 0x02, struct epoll_params)
> > +
> >  #endif /* _UAPI_LINUX_EVENTPOLL_H */
> > -- 
> > 2.25.1
> > 
> 
>