On Fri, 31 Jul 2026 18:43:40 +0200 Paolo Bonzini <[email protected]> wrote:
> This ensures that KVM or VFIO correctly see a writable PTE when > they request one. Otherwise, a guest write to an unpopulated > PTE from a mapping backed by a DRM GEM BO triggers a VM exit > with EFAULT. > > The code actually is simpler, because the same logic already > applied to the hugepage mapping case using vmf_insert_pfn_pmd(). > > Reported-by: Sergio Lopez <[email protected]> > Link: https://lore.kernel.org/kvm/[email protected]/ > Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in > mmap") > Signed-off-by: Paolo Bonzini <[email protected]> Reviewed-by: Boris Brezillon <[email protected]> Glad some MM expert looked at this, because we had this partial "get rid of pfn_mkwrite()" workaround pending [1], which apparently never made it to drm-misc-fixes for some reason. [1]https://lore.kernel.org/dri-devel/[email protected]/ > --- > drivers/gpu/drm/drm_gem_shmem_helper.c | 38 ++++++++++++++------------ > 1 file changed, 20 insertions(+), 18 deletions(-) > > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c > b/drivers/gpu/drm/drm_gem_shmem_helper.c > index c989459eb215..33a14f558276 100644 > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c > @@ -589,11 +589,25 @@ static void drm_gem_shmem_record_mkwrite(struct > vm_fault *vmf) > folio_mark_dirty(page_folio(shmem->pages[page_offset])); > } > > +/* > + * Because the vm_ops have a .pfn_mkwrite() callback, vma_set_page_prot() > + * has cleared the write bit from vma->vm_page_prot. vmf_insert_pfn() > + * would install a read-only entry even for a write fault, relying on a > + * second fault to reach .pfn_mkwrite() and upgrade it, but that second > + * fault never happens for fixup_user_fault() callers that directly > + * walk the page tables with follow_pfnmap_start(). To ensure that > + * they don't see the read-only entry, pass FAULT_FLAG_WRITE info down > + * to install a writable entry right away. Because .pfn_mkwrite() is > + * not invoked, record the write afterwards. > + */ > static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order, > unsigned long pfn) > { > + bool write = vmf->flags & FAULT_FLAG_WRITE; > + vm_fault_t ret = VM_FAULT_FALLBACK; > + > if (!order) { > - return vmf_insert_pfn(vmf->vma, vmf->address, pfn); > + ret = vmf_insert_pfn_mkwrite(vmf->vma, vmf->address, pfn, > write); > #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP > } else if (order == PMD_ORDER) { > unsigned long paddr = pfn << PAGE_SHIFT; > @@ -601,27 +615,15 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, > unsigned int order, > > if (aligned && > folio_test_pmd_mappable(page_folio(pfn_to_page(pfn)))) { > - vm_fault_t ret; > - > pfn &= PMD_MASK >> PAGE_SHIFT; > - > - /* Unlike PTEs which are automatically upgraded to > - * writeable entries, the PMD upgrades go through > - * .huge_fault(). Make sure we pass the "write" info > - * along in that case. > - * This also means we have to record the write fault > - * here, instead of in .pfn_mkwrite(). > - */ > - ret = vmf_insert_pfn_pmd(vmf, pfn, > - vmf->flags & FAULT_FLAG_WRITE); > - if (ret == VM_FAULT_NOPAGE && (vmf->flags & > FAULT_FLAG_WRITE)) > - drm_gem_shmem_record_mkwrite(vmf); > - > - return ret; > + ret = vmf_insert_pfn_pmd(vmf, pfn, write); > } > #endif > } > - return VM_FAULT_FALLBACK; > + > + if (ret == VM_FAULT_NOPAGE && write) > + drm_gem_shmem_record_mkwrite(vmf); > + return ret; > } > > static vm_fault_t drm_gem_shmem_any_fault(struct vm_fault *vmf, unsigned int > order)
