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

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Message-ID: <aFHJh7sKO9CBaLHV@x1.local>
Date: Tue, 17 Jun 2025 16:01:11 -0400
From: Peter Xu <peterx@...hat.com>
To: Jason Gunthorpe <jgg@...dia.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 04:46:21PM -0300, Jason Gunthorpe wrote:
> > 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)

Yes, this will be addressed.

> 
> So what is VFIO doing that requires CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP?

It's the fallback part for vfio device, not vfio_pci device.  vfio_pci
device doesn't need this special treatment after moving to the new helper
because that hides everything.  vfio_device still needs it.

So, we have two ops that need to be touched to support this:

        vfio_device_fops
        vfio_pci_ops 

For the 1st one's vfio_device_fops.get_unmapped_area(), it'll need its own
fallback which must be mm_get_unmapped_area() to keep the old behavior, and
that was defined only if CONFIG_MMU.

IOW, if one day file_operations.get_unmapped_area() would allow some other
retval to be able to fallback to the default (mm_get_unmapped_area()), then
we don't need this special ifdef.  But now it's not ready for that..

-- 
Peter Xu