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

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Message-ID: <aFH5oO1M1_TZz4NF@x1.local>
Date: Tue, 17 Jun 2025 19:26:24 -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 08:00:30PM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 17, 2025 at 04:01:11PM -0400, Peter Xu wrote:
> 
> > > 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.
> 
> OK, CONFIG_MMU makes a little bit of sense
> 
> > 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..
> 
> That can't be fixed with a config, the logic in vfio_device_fops has
> to be 
> 
> if (!device->ops->get_unmapped_area()
>    return .. do_default thing..
> 
> return device->ops->get_unmapped()
> 
> Has nothing to do with CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP, there are
> more device->ops that just PCI.

IMHO CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP doesn't imply anything PCI specific
either, it only says an arch supports PFNMAP in larger than PAGE_SIZE.
IIUC it doesn't necessarily need to be PCI.

So here in this case, get_unmapped_area() will only be customized if the
kernel is compiled with any possible huge mapping on pfnmaps.  Otherwise
the customized hook isn't needed.

> 
> If you do the API with an align/order argument then the default
> behavior should happen when passing PAGE_SIZE.

This should indeed also work.

I'll wait for comments in the other threads.  So far I didn't yet add the
"order" parameter or anything like it.  If we would like to have the
parameter, I can use it here to avoid the ifdef with PAGE_SIZE / PAGE_SHIFT
/ .... when repost.

Said that, I don't think I understand at all the use of get_unmapped_area()
for !MMU use case.

Thanks,

-- 
Peter Xu