Re: [PATCH 1/3] drm/gem-shmem: Map pages in mmap fault handler

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jan 27, 2026 at 02:16:36PM +0100, Thomas Zimmermann wrote:
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -553,17 +553,18 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
>  static bool drm_gem_shmem_try_map_pmd(struct vm_fault *vmf, unsigned long addr,
>  				      struct page *page)
>  {
> -#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> -	unsigned long pfn = page_to_pfn(page);
> -	unsigned long paddr = pfn << PAGE_SHIFT;
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	phys_addr_t paddr = page_to_phys(page);
>  	bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
>  
> -	if (aligned &&
> -	    pmd_none(*vmf->pmd) &&
> -	    folio_test_pmd_mappable(page_folio(page))) {
> -		pfn &= PMD_MASK >> PAGE_SHIFT;
> -		if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
> -			return true;
> +	if (aligned && pmd_none(*vmf->pmd)) {
> +		struct folio *folio = page_folio(page);
> +
> +		if (folio_test_pmd_mappable(folio)) {
> +			/* Read-only mapping; split upon write fault */
> +			if (vmf_insert_folio_pmd(vmf, folio, false) == VM_FAULT_NOPAGE)
> +				return true;
> +		}
>  	}

It would feel more natural to me to have this function return vm_fault_t
instead of a bool.  What do you think?

> +	pgoff_t page_offset = vmf->pgoff - vma->vm_pgoff; /* page offset within VMA */
> +	struct page *page = pages[page_offset];
> +	vm_fault_t ret;

[...]

> -	if (drm_gem_shmem_try_map_pmd(vmf, vmf->address, pages[page_offset])) {
> -		ret = VM_FAULT_NOPAGE;
> -		goto out;
> +	page = pages[page_offset];

Can page_offset have changed at this point?

> +	if (drm_gem_shmem_try_map_pmd(vmf, vmf->address, page)) {
> +		ret = VM_FAULT_NOPAGE;
> +	} else {
> +		struct folio *folio = page_folio(page);
> +
> +		get_page(page);

folio_get(folio);

> -	vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> +	vm_flags_mod(vma, VM_DONTEXPAND | VM_DONTDUMP, VM_PFNMAP);

Just so you know, this is going to change observable behaviour, maybe
in a good way.  For example, we'll now be able to do O_DIRECT I/O to
this range and ptrace accesses.  I think everything will be fine, but
VM_PFNMAP does prohibit some things which are now allowed.