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 B3L0LZL/ 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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 Thu, Nov 20, 2025 at 10:49:59AM +0100, David Hildenbrand (Red Hat) wrote: > On 11/20/25 10:35, Lorenzo Stoakes wrote: > > On Thu, Nov 20, 2025 at 10:16:26AM +0100, David Hildenbrand (Red Hat) wrote: > > > On 11/20/25 10:04, Lorenzo Stoakes wrote: > > > > 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: > > Yeah, I am all confused about expansion. The example code looks like all it > wants to do is move a VM_PFNMAP mapping. > > if (mremap(iov[i].iov_base, 0, iov[i].iov_len, > MREMAP_FIXED | MREMAP_MAYMOVE, cur) == MAP_FAILED) { > goto err; > } > > I guess the expansion is because of iov[i].iov_len is bigger than the > original VMA? > > Is that maybe a bug in QEMU or why are we even expanding here? We're going from size 0 to iov[i].iov_len, which is saying 'please make a copy of this VMA at a new address'. There's never any moving, as input size is 0 :) It's a cute corner case way of using mremap(). We're basically asking for a _copy_. But you can't get a copy of a VM_DONTEXPAND/VM_PFNMAP because you need to invoke mmap_prepare (or legacy mmap) to get something sensible and you are bypassing that on expansion, even if it's a 'clone' style expansion. > > -- > Cheers > > David > Cheers, Lorenzo