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

David Hildenbrand
2024-07-22T09:18:03Z

On 22.07.24 08:43, Yan Zhao 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 don't know. But it's not disallowed.

Okay, but we don't know of any such use case.

What we do have is a single VMA, whereby within that VMA we place
various different PFN ranges. (randomly looking at
drivers/video/fbdev/smscufx.c)

These wouldn't have triggered VM_PAT code.

--
Cheers,

David / dhildenb