Re: [PATCH] vfio: Request THP-aligned mmap for device fds

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Jun 19, 2026 at 05:11:50PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 18, 2026 at 12:28:05PM -0300, Jason Gunthorpe wrote:
> > On Thu, Jun 18, 2026 at 03:55:58PM +0100, Lorenzo Stoakes wrote:
> > > Can't we figure this out from what the driver tells us when it invokes an
> > > mmap_prepare action?
> > 
> > VFIO installs the pages via fault handler so there is not a naturally
> > existing way to pass in the pfn?
> 
> Is there an advantage to doing it this way?  I understand why we (eg)
> demand-page pagecache, that's obvious.  But I've never really understood
> the advantage to taking page faults for PFNMAP areas where we don't
> really do anything, just figure out which PFN needs to be installed.
> It defers page table allocation, I suppose.

VFIO has a model where the mapping can come and go, so it makes the
entire VMA SIGBUS from time to time. The only way to do this currently
is with faulting.

The mm also had races around populating the mmap in the mmap callback
and using zap on the inode, faulting avoids those too. Lorenzo may
have fixed that with the new interface though

Jason