linux-kernel - Re: [PATCH v6 06/25] iommufd/access: Allow access->ops to be NULL for internal use

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Message-ID: <aFwlu7FlfIP85gko@Asurada-Nvidia>
Date: Wed, 25 Jun 2025 09:37:15 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v6 06/25] iommufd/access: Allow access->ops to be NULL
 for internal use

On Wed, Jun 25, 2025 at 03:38:19AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Saturday, June 14, 2025 3:15 PM
> > 
> > +int iommufd_access_notify_unmap(struct io_pagetable *iopt, unsigned long
> > iova,
> > +				unsigned long length)
> >  {
> >  	struct iommufd_ioas *ioas =
> >  		container_of(iopt, struct iommufd_ioas, iopt);
> >  	struct iommufd_access *access;
> >  	unsigned long index;
> > +	int ret = 0;
> > 
> >  	xa_lock(&ioas->iopt.access_list);
> >  	xa_for_each(&ioas->iopt.access_list, index, access) {
> > +		if (!access->ops || !access->ops->unmap) {
> > +			ret = -EBUSY;
> > +			goto unlock;
> > +		}
> 
> then accesses before this one have been notified to unpin the area
> while accesses afterwards are left unnotified.
> 
> in the end the unmap fails but with some side-effect incurred.
> 
> I'm not sure whether this intermediate state may lead to any undesired
> effect later. Just raise it in case you or Jason already thought about it.

That's a good point. When an access blocks the unmap, there is no
unmap happening so no point in notifying devices for ops->unmap.

And, when the function is re-entered, there could be a duplicated
ops->unmap call for those devices that are already notified once?

So, if we play safe, there can be a standalone xa_for_each to dig
for !access->ops->unmap. And it could be a bit cleaner to add an
iommufd_access_has_internal_use() to be called under those rwsems.

> >  			/* Something is not responding to unmap requests.
> > */
> >  			tries++;
> > -			if (WARN_ON(tries > 100))
> > -				return -EDEADLOCK;
> > +			if (WARN_ON(tries > 100)) {
> > +				rc = -EDEADLOCK;
> > +				goto out_unmapped;
> > +			}
> 
> this looks an unrelated fix?

Yea.. let me separate it out.

Thanks
Nicolin