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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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(). > > untrack_pfn: > if (!paddr && !size) { > if (get_pat_info(vma, &paddr, NULL)) > return; > size = vma->vm_end - vma->vm_start; > } > free_pfn_range(paddr, size); <---- assumes PFNs to be continuous > > So untrack_pfn() already assumed the pfn being continuous. I think it > means pfns cannot be randomly faulted in, but determined when mmap(). Hmm, in the hypothetical scenario, (vma->vm_end - vma->vm_start) == PAGE_SIZE.