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

David Hildenbrand
2024-07-19T08:28:15Z

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?

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?

--
Cheers,

David / dhildenb