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

Yan Zhao
2024-07-18T23:20:29Z

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?

Thanks
Yan
>
> > 2. Export track_pfn_remap() and call track_pfn_remap() in mmap() for pfn
> > set 2.
> > 3. Unmap and call vmf_insert_pfn() in the fault handler to map pfn set 2.
>
> IMO this should be the similar case of MMIO being disabled on the bar,
> where we can use track_pfn_remap() to register the whole vma in mmap()
> first. Then in this case if you prefer proactive injection of partial of
> the pfn mappings, one can do that via vmf_insert_pfn*() in mmap() after the
> memtype registered.
>
> >
> > However, I'm not sure if we can properly untrack both pfn sets 1 and 2.
>
> Untrack should so far only happen per-vma, AFAIU, so "set 1+2" need to be
> done together as they belong to the same vma.
>
> >
> > By exporting untrack_pfn() too? Or, you'll leave VFIO to use ioremap/iounmap()
> > (or the variants) to reserve memtype by itself?
>
> Thanks,
>
> --
> Peter Xu
>