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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. Equally VM_DONTEXPAND implies the same kind of semantics. > is particularly useful when trying to create a new VMA using other > existing VMAs that have these flags set, such as the ones associated > with VFIO devices. > > Specifically, there are use-cases where a VMM such as Qemu would > want to map a non-contiguous buffer associated with a VFIO device > in the following way: > > void *start, *cur; > int i; > > start = mmap(NULL, size, PROT_NONE, MAP_SHARED, -1, 0); > if (start == MAP_FAILED) { > return start; > } Err, MAP_SHARED and -1 fd? Doesn't this always fail? It does locally with -EBADFD? > > cur = start; > for (i = 0; i < iov_cnt; i++) { > if (mremap(iov[i].iov_base, 0, iov[i].iov_len, > MREMAP_FIXED | MREMAP_MAYMOVE, cur) == MAP_FAILED) { Trying to cleverly clone a VMA I guess. > goto err; > } > cur += iov[i].iov_len; > } > return start; > > The above code currently works when mapping buffers backed by > shmem (memfd) but fails with -EFAULT when mapping VFIO backed Right, which is expected, as described above, you _are_ expanding here. > buffers because the VMAs associated with iov[i].iov_base addresses > have VM_DONTEXPAND and VM_PFNMAP flags set. Therefore, fix this > issue by not returning -EFAULT when a new mapping is being created. We're not fixing an issue, we're (unfortunately, incorrectly) introducing an entirely new behaviour of permitting the remapping of page tables belonging to VMAs posssesing VM_DONTEXPAND and VM_PFNMAP flags. > > Cc: Andrew Morton > Cc: Liam R. Howlett > Cc: Lorenzo Stoakes > Cc: Vlastimil Babka > Cc: Jann Horn > Cc: Pedro Falcato > Cc: David Hildenbrand > Cc: Akihiko Odaki > Signed-off-by: Vivek Kasireddy > --- > mm/mremap.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index fdb0485ede74..d3868d941f72 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -1736,7 +1736,8 @@ static int check_prep_vma(struct vma_remap_struct *vrm) > if (pgoff + (new_len >> PAGE_SHIFT) < pgoff) > return -EINVAL; > > - if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)) > + if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP) && > + !vrm_implies_new_addr(vrm)) > return -EFAULT; This is just incorrect. I mean firstly, as explained above, we simply do not support this kind of operation and cannot correctly. But also here you're allowing !(MREMAP_FIXED | MREMAP_DONTUNMAP) VMAs to expand VM_DONTEXPAND, VM_PFNMAP VMAs which is really really not right. The missing context here is: if (new_len == old_len) return 0; ... all code below involves an expansion of a VMA ... if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)) return -EFAULT; And making _all_ expands that aren't _definitely_ specifying a new address work correclty. But you are in fact 100% specifying a new address because you're setting input length of 0... So this allows _anybody_ to try to incorrectly expand VM_PFNMAP, VM_DONTEXPAND VMAs. So is wildly incorrect. > > if (!mlock_future_ok(mm, vma->vm_flags, vrm->delta)) > -- > 2.50.1 > > In any case, I don't want to try to support this behaviour, I don't think there's really a sensible way to be sure we can _copy_ page tables correctly, and even though the source size = 0 is a cute trick you _ARE_ expanding, and VM_DONTEXPAND is very clear - do not do this. Additionally, please if trying to make such fundamental changes going forward _always_ provide test code, it's absolutely a requirement. Thanks, Lorenzo