Re: [PATCH v2 2/6] drm/shmem_helper: use vmf_insert_pfn_mkwrite()

[Boris Brezillon]

On Tue,  4 Aug 2026 14:05:24 +0200
Paolo Bonzini <pbonzini@xxxxxxxxxx> 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 <slp@xxxxxxxxxx>
> Link: https://lore.kernel.org/kvm/20260729072044.25796-1-slp@xxxxxxxxxx/
> Tested-by: Sergio Lopez <slp@xxxxxxxxxx>
> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in mmap")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  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..c81be3e97317 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);

Actually, if we're making the drm_gem_shmem_record_mkwrite() call
unconditional (for PTE and PMD updates) in that path, can't we drop the
drm_gem_shmem_pfn_mkwrite() call living in drm_gem_shmem_pfn_mkwrite()?

Also, I'm not even sure we can end up with write=true for PTE updates,
because our pfn_mkwrite implementation returns zero, not VM_FAULT_ERROR
or VM_FAULT_NOPAGE. This means the default RO -> RW PTE upgrade
implemented in finish_mkwrite_fault() [1] will take place. If we really
want out try_insert_pfn() to be called for those RO -> RW updgrades, we
need to call try_insert_pfn() from drm_gem_shmem_pfn_mkwrite().

[1]https://elixir.bootlin.com/linux/v7.2-rc5/source/mm/memory.c#L4021