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 wZxNbBnp 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Hi Vivek, thanks for the patch. > > > > In general though, let's please not make a fundamental change to mremap() > > behaviour in late -rc6. Late in cycle/during merge window we're really only > > interested in existing series, series that are less involved than this. > > > > On Wed, Nov 19, 2025 at 09:35:46PM -0800, Vivek Kasireddy wrote: > > > When mremap is used to create a new mapping, we should not return > > > -EFAULT for VMAs with VM_DONTEXPAND or VM_PFNMAP flags set because > > > the old VMA would neither be expanded nor shrunk in this case. This > > > > I guess you're trying to be succinct here and 'clone' each input VMA using > > the 0 source size input. > > > > However this can't work. > > > > This operation is not equivalent to an mmap(). It may seem to be for > > ordinary mappings but in practice it isn't: > > > > (syscall) > > -> do_mremap() > > -> mremap_at() > > -> expand_vma() > > -> move_vma() > > -> copy_vma_and_data() > > -> copy_vma() > > > > Essentially copying the properties of the VMA to the new region. > > > > But this doesn't work for PFN map. > > > > At _no point_ are you invoking the original f_op->mmap or > > f_op->mmap_prepare handler. > > > > And these handles for PFN maps set up page tables, because PFN maps > > literally do not exist as VMAs which have properties independent of their > > page tables like this. > > vfio-pci is a bit different, though, as it uses > vmf_insert_pfn()/vmf_insert_pfn_pmd()/vmf_insert_pfn_pud() at fault time to > insert PFNs, not at mmap time using remap_pfn_range() and friends. > > (see vfio_pci_mmap_page_fault() ) It sets VM_DONTEXPAND but is fine with being expanded? :) That sounds like a bug there: vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP); Drop the VM_DONTEXPAND then? But on the other hand... I see a _whole bunch_ of logic in vfio_pci_core_mmap() that just wouldn't be executed on expansion (and the requested case here is 100% an expand due to not invoking mmap callbacks...) So we'd still be in a broken state here surely if we allowed it? > > Now, I have to idea if that is the main use case we want to target here and > how we could handle it, just wanted to point it out :) Right, the objections all remain, we absolutely cannot take this patch or anything like it. I guess VM_DONTEXPAND is really 'we do stuff in the (soon to be) mmap_prepare callback that is dependent on region size'. Most notably in this case: phys_len = PAGE_ALIGN(pci_resource_len(pdev, index)); ... if (req_start + req_len > phys_len) return -EINVAL; > > -- > Cheers > > David > I wouldn't be entirely opposed to a _new system call_ explicitly for cloning an existing memory region that explicitly invokes an mmap with equivalent parameters for VM_DONTEXPAND/VM_PFNMAP cases/does a copy for ordinary cases. But I'm not interested in doing that for mremap(ptr, 0, new_size, ...). An alternative would be to have a new VMA callback for expansion where the driver could explicitly do these checks :) but not sure if worth it. Cheers, Lorenzo