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. > 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? If we allow a fixed PFN range to be faulted in lazily to different physical PFNs, looks we can't rely on track_pfn_remap() in mmap() and untrack_pfn() in unmap_vmas() to reserve/untrack memtypes.
