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

[Peter Xu <peterx@redhat.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 share the same question with David.  I don't think we support that, and I
don't know whether we should, either.

Such flexibility already will break with current PAT design.  See:

untrack_pfn:
	if (!paddr && !size) {
		if (get_pat_info(vma, &paddr, NULL))
			return;
		size = vma->vm_end - vma->vm_start;
	}
	free_pfn_range(paddr, size);  <---- assumes PFNs to be continuous

So untrack_pfn() already assumed the pfn being continuous.  I think it
means pfns cannot be randomly faulted in, but determined when mmap().

Thanks,

-- 
Peter Xu