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

Yan Zhao
2024-07-22T06:50:48Z

On Fri, Jul 19, 2024 at 10:13:33AM -0400, Peter Xu wrote:
> 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:
Previously with remap_pfn_range() being able to be called in fault handlers,
this flexibility is doable. i.e. reserve in the fault handler and untrack
in unmap_single_vma().

>
> 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().
Hmm, in the hypothetical scenario, (vma->vm_end - vma->vm_start) == PAGE_SIZE.