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

[Jason Gunthorpe]

On Thu, Jun 18, 2026 at 04:04:06PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 18, 2026 at 03:55:58PM +0100, Lorenzo Stoakes wrote:
> > On Wed, Jun 17, 2026 at 04:29:28PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Jun 17, 2026 at 07:34:06PM +0100, Matthew Wilcox wrote:
> > >
> > > > I don't see this as being something that drivers should be involved with
> > > > at all.  The MM should be able to get this right without any hints from
> > > > the file-provider.  Yes, that means I also want to get rid of the setting
> > > > of get_unmapped_area in ext4/xfs/other filesystems.
> > > >
> > > > Looking at generic_get_unmapped_area_topdown(), I think we can do this by
> > > > making an additional call to vm_unmapped_area() before the existing two,
> > > > setting info.align_mask and info.align_offset appropriately.
> > > >
> > > > Now, what's "appropriately"?  I think it's based on length (>= PMD_SIZE,
> > > > then >= PUD_SIZE), but we should also take CONTPTE architectures into
> > > > account.
> > >
> > > The info.align_mask and info.align_offset do need information from the
> > > driver based on what it intends to map into the VMA that is being
> > > created.
> 
> What you're saying is that offset 0 of the opened file might correspond
> to a PFN that is not aligned in any way?  I had assumed that when trying
> to do the mapping of (2MB+4KiB to 64MB), that the offset specified to
> mmap was 2MB+4KiB.  But you seem to be saying that the offset in that
> case would be 0 and someone needs to know that it corresponds to a PFN
> that is misaligned?

I do expect that the pgoff space is usually aligned to the pfn space,
most drivers do that or could be improved to do that. There will be
some off cases, but maybe we don't care, and VFIO should be fine.

That is certainly an easier place to start.

Jason