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

[Lorenzo Stoakes <ljs@kernel.org>]

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.
>
> > > Not in favour of that, fear it'll be seen as a new go-faster stripe.  Ask
> > > somebody how many free pints they want and they may veer rather towards the
> > > upper bound :)
> >
> > I think you need something, otherwise we will be aligning VMAs that
> > never have anything larger than a 2M THP to 1GB boundaries, doesn't
> > seem good.
>
> If Lorenzo's willing to move the mmap_prepare call earlier, to before we
> allocate the vma then we can do it that way ...

Hmm, I wonder if we could defer the get_unmapped_area stuff until later.

Taking a quick look it's actually not totally insane to do that... Let me fiddle
around with it...

Thanks, Lorenzo