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

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 22, 2024 at 11:17:57AM +0200, David Hildenbrand wrote:
> 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.

Right, it looks like VM_PAT is only applying to a whole-vma mapping, even
though I don't know how that was designed..

I wished vma->vm_pgoff was for storing the base PFN for VM_SHARED too: now
it only works like that for CoW mappings in remap_pfn_range_notrack(), then
it looks like VM_SHARED users of remap_pfn_range() can reuse vm_pgoff, and
I think VFIO does reuse it at least..

I am a bit confused on why Linux made that different for VM_SHARED,
probably since b3b9c2932c32 ("mm, x86, pat: rework linear pfn-mmap
tracking").  I wished vm_pgoff was always used for internal maintenance
(even for VM_SHARED) so this issue should be easier to solve.

Maybe we can still re-define vm_pgoff for VM_SHARED pfnmaps? The caller
should always be able to encode information in vm_private_data anyway.
But I think that might break OOT users..

-- 
Peter Xu