linux-kernel - Re: [PATCH 4/5] vfio: Introduce vfio_device_ops.get_unmapped

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Message-ID: <20250617194621.GA1575786@nvidia.com>
Date: Tue, 17 Jun 2025 16:46:21 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: kernel test robot <lkp@...el.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kvm@...r.kernel.org,
	oe-kbuild-all@...ts.linux.dev,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Zi Yan <ziy@...dia.com>, Alex Mastro <amastro@...com>,
	David Hildenbrand <david@...hat.com>,
	Nico Pache <npache@...hat.com>
Subject: Re: [PATCH 4/5] vfio: Introduce vfio_device_ops.get_unmapped_area
 hook

On Tue, Jun 17, 2025 at 03:39:19PM -0400, Peter Xu wrote:
> On Tue, Jun 17, 2025 at 12:47:35PM -0400, Peter Xu wrote:
> > On Tue, Jun 17, 2025 at 12:41:57PM -0300, Jason Gunthorpe wrote:
> > > On Tue, Jun 17, 2025 at 11:39:07AM -0400, Peter Xu wrote:
> > > >  
> > > > +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> > > >  static unsigned long vfio_device_get_unmapped_area(struct file *file,
> > > >                                                    unsigned long addr,
> > > >                                                    unsigned long len,
> > > > @@ -1370,6 +1371,7 @@ static unsigned long vfio_device_get_unmapped_area(struct file *file,
> > > >         return device->ops->get_unmapped_area(device, file, addr, len,
> > > >                                               pgoff, flags);
> > > >  }
> > > > +#endif
> > > >  
> > > >  const struct file_operations vfio_device_fops = {
> > > >         .owner          = THIS_MODULE,
> > > > @@ -1380,7 +1382,9 @@ const struct file_operations vfio_device_fops = {
> > > >         .unlocked_ioctl = vfio_device_fops_unl_ioctl,
> > > >         .compat_ioctl   = compat_ptr_ioctl,
> > > >         .mmap           = vfio_device_fops_mmap,
> > > > +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> > > >         .get_unmapped_area = vfio_device_get_unmapped_area,
> > > > +#endif
> > > >  };
> > > 
> > > IMHO this also seems like something the core code should be dealing
> > > with and not putting weird ifdefs in drivers.
> > 
> > It may depend on whether we want to still do the fallbacks to
> > mm_get_unmapped_area().  I get your point in the other email but not yet
> > get a chance to reply.  I'll try that out to see how it looks and reply
> > there.
> 
> I just noticed this is unfortunate and special; I yet don't see a way to
> avoid the fallback here.
> 
> Note that this is the vfio_device's fallback, even if the new helper
> (whatever we name it..) could do fallback internally, vfio_device still
> would need to be accessible to mm_get_unmapped_area() to make this config
> build pass.

I don't understand this remark?

get_unmapped_area is not conditional on CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP?

Some new mm_get_unmapped_area_aligned() should not be conditional on
CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP? (This is Lorenzo's and Liam's remark)

So what is VFIO doing that requires CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP?

Jason