Re: [PATCH] mm/x86/pat: Only untrack the pfn range if unmap region

[Yan Zhao <yan.y.zhao@intel.com>]

On Fri, Jul 19, 2024 at 10:28:09AM +0200, David Hildenbrand wrote:
> On 19.07.24 01:18, Yan Zhao wrote:
> > On Thu, Jul 18, 2024 at 10:03:01AM -0400, Peter Xu wrote:
> > > On Thu, Jul 18, 2024 at 09:50:31AM +0800, Yan Zhao wrote:
> > > > Ok. Then if we have two sets of pfns, then we can
> > > > 1. Call remap_pfn_range() in mmap() for pfn set 1.
> > > 
> > > I don't think this will work..  At least from the current implementation,
> > > remap_pfn_range() will only reserve the memtype if the range covers the
> > > whole vma.
> > Hmm, by referring to pfn set 1 and pfn set 2, I mean that they're both
> > covering the entire vma, but at different times.
> > 
> > To make it more accurately:
> > 
> > Consider this hypothetical scenario (not the same as what's implemented in
> > vfio-pci, but seems plausible):
> > 
> > Suppose we have a vma covering only one page, then
> > (1) Initially, the vma is mapped to pfn1, with remap_pfn_range().
> > (2) Subsequently, unmap_single_vma() is invoked to unmap the entire VMA.
> > (3) The driver then maps the entire vma to pfn2 in fault handler
> > 
> > Given this context, my questions are:
> > 1. How can we reserve the memory type for pfn2? Should we call
> >     track_pfn_remap() in mmap() in advance?
> > 2. How do we untrack the memory type for pfn1 and pfn2, considering they
> >     belong to the same VMA but mutual exclusively and not concurrently?
> 
> Do we really have to support such changing PFNs in a VMA? Are there existing
> use cases that would rely on that?
I don't know. But it's not disallowed.

> Would it be a problem if we would merge the mmap+track_pfn_remap, such that
> such a special VMA can only ever belong to a single PFN range, that is
> fixed, but PFNs can be faulted in lazily?
If we allow a fixed PFN range to be faulted in lazily to different physical
PFNs, looks we can't rely on track_pfn_remap() in mmap() and untrack_pfn()
in unmap_vmas() to reserve/untrack memtypes.