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 gOA8ov8Y 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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, Nov 21, 2025 at 07:16:39PM +0900, Akihiko Odaki wrote: > On 2025/11/21 18:10, Lorenzo Stoakes wrote: > > It doesn't almost do what we want at all. All the drivers known VM_PFNMAP and > > VM_DONTEXPAND will _not_ be mremap()'d so unless you have a time machine I don't > > know about we can't in any way take the existence of this callback to be > > meaningful here :) > > Correct me if I'm wrong, but looking at check_prep_vma(), VM_PFNMAP is > checked only if MREMAP_DONTUNMAP is set or expansion is requested. Actually it is the case that a straight move is permitted you're right. So virtual alignment not a concern then. I should make the logic clearer on this. Metadata state + any assumption about multiple mappings remains problematic. > > So it is already possible to move and shrink VM_PFNMAP, and if you need to > e.g., assert alignment requirements or synchronize metadata. Well no re: metadata, because you're _moving_ an existing mapping so the metadata is still valid. And generally anything regarding span will still be functional if you map less of the span (keep in mind you can munmap() parts of a mapping at any time you like, so this is impliled). The problem remains with a copy/expansion and having correct page table state. And again, drivers are all implemented on the assumption that things behave as they do right now. Also as I said I"m not interested in us making mremap() _even more of a complicated mess_ by supporting further weird edge case behaviours. Most people are not aware of this cloning behaviour so adding in additional very specific behaviour here is only going to lead to confusion. > > That said... > > > > > > > > > So my idea is simple: change copy_vma_and_data() to copy the page table > > > without clearing the old page table entries if !old_len && (vma->vm_flags & > > > VM_PFNMAP). > > > > No, absolutely not. > > > > I already went over the reasons, but to highlight: > > > > - There may be alignment requirements that are no longer fulfilled. We can drop this, I was mistaken as it turns out we do in fact allow moves in this case (patch will be incoming to make this clearer in the mremap code :) > > > > - There may be metadata associated with the VMA that no longer exists in the > > copied VMA. > > > > - There may be some requirement that only one mapping exists at a time of the > > given range. > > Obviously what I suggested goes against "the only one mapping exists at a > time" so, taking that into account, I agree that it will not work. Well also the metadata remains a problem. A typical usage is a reference count... > > > > Like I said, I may look into adding some _new_ kernel functionality that gives > > you what you want. I will cc you and Vivek if/when I put something forward. > > Thank you. No problem :) in the meantime you can explore using the /proc/%pid/... to do something similar in userspace, I pointed out some ideas in the other thread. > > Regards, > Akihiko Odaki Cheers, Lorenzo