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

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 22, 2024 at 02:49:12PM +0800, Yan Zhao wrote:
> 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().

AFAICT, remap_pfn_range() should never be allowed to be called in a fault
handler..  So IMO it's not "it was allowed before", but we did it wrong
from when we used it in fault path: remap_pfn_range() changes VMA flags
since the 1st day, and that requires a writable lock, while fault paths
only hold it read..

I think it's just that the per-vma lock was added a few years ago (then
some lock attestations on vma lock v.s. vma flag changes), and until then
we found this issue.

-- 
Peter Xu