Re: [PATCH] drm/shmem-helper: Add huge page fault handler

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 23 Sep 2025 11:56:34 +0200
Loïc Molinari <loic.molinari@collabora.com> wrote:

> This gives the mm subsystem the ability to propose the insertion of a
> PMD or PUD mapping for the faulting address.
> 
> If the virtual address provided from userspace with mmap() using the
> address hint parameter is aligned to a huge page size, if the GEM
> object is backed by a tmpfs mount point using Transparent Hugepage and
> if the shmem backing store manages to allocate enough contiguous
> physical pages to fit within a huge page, the CPU mapping will then
> benefit from significantly increased memcpy() performance. For
> instance, when these conditions are met on a system with 2 MiB huge
> pages, a (fresh) aligned copy of 2 MiB would raise a single page fault
> instead of 4096.
> 
> Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 51 ++++++++++++++++++++++++--
>  1 file changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 50594cf8e17c..30aa0d72093b 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -573,7 +573,8 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
>  
> -static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> +static vm_fault_t drm_gem_shmem_huge_fault(struct vm_fault *vmf,
> +					   unsigned int order)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct drm_gem_object *obj = vma->vm_private_data;
> @@ -582,6 +583,7 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
>  	vm_fault_t ret;
>  	struct page *page;
>  	pgoff_t page_offset;
> +	unsigned long pfn, paddr;
>  
>  	/* We don't use vmf->pgoff since that has the fake offset */
>  	page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> @@ -592,17 +594,55 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
>  	    drm_WARN_ON_ONCE(obj->dev, !shmem->pages) ||
>  	    shmem->madv < 0) {
>  		ret = VM_FAULT_SIGBUS;
> -	} else {
> -		page = shmem->pages[page_offset];
> +		goto out;
> +	}
> +
> +	page = shmem->pages[page_offset];
> +	pfn = page_to_pfn(page);
> +
> +	switch (order) {
> +	case 0:
> +		ret = vmf_insert_pfn(vma, vmf->address, pfn);
> +		break;
> +
> +#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> +	case PMD_ORDER:
> +		paddr = pfn << PAGE_SHIFT;
> +		if (((vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK)) &&
> +		    (folio_order(page_folio(page)) == PMD_ORDER))
> +			ret = vmf_insert_pfn_pmd(
> +				    vmf, pfn & (PMD_MASK >> PAGE_SHIFT), false);
> +		else
> +			ret = VM_FAULT_FALLBACK;
> +		break;
> +#endif
> +
> +#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
> +	case PUD_ORDER:
> +		paddr = pfn << PAGE_SHIFT;
> +		if (((vmf->address & ~PUD_MASK) == (paddr & ~PUD_MASK)) &&
> +		    (folio_order(page_folio(page)) == PUD_ORDER))
> +			ret = vmf_insert_pfn_pud(
> +				    vmf, pfn & (PUD_MASK >> PAGE_SHIFT), false);
> +		else
> +			ret = VM_FAULT_FALLBACK;
> +		break;
> +#endif
>  
> -		ret = vmf_insert_pfn(vma, vmf->address, page_to_pfn(page));
> +	default:

Not sure about the error, but we should have something like

		ret = VM_FAULT_FALLBACK;
		break;

because otherwise ret is uninitialized. We probably want a WARN_ON()
too, because this function is not supposed to be called with a
non-PTE/PUD/PMD order.

The rest looks good to me, so once this is addressed, you can add

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

>  	}
>  
> + out:
>  	dma_resv_unlock(shmem->base.resv);
>  
>  	return ret;
>  }
>  
> +static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> +{
> +	return drm_gem_shmem_huge_fault(vmf, 0);
> +}
> +
>  static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
>  {
>  	struct drm_gem_object *obj = vma->vm_private_data;
> @@ -639,6 +679,9 @@ static void drm_gem_shmem_vm_close(struct vm_area_struct *vma)
>  
>  const struct vm_operations_struct drm_gem_shmem_vm_ops = {
>  	.fault = drm_gem_shmem_fault,
> +#if defined(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP) || defined(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP)
> +	.huge_fault = drm_gem_shmem_huge_fault,
> +#endif
>  	.open = drm_gem_shmem_vm_open,
>  	.close = drm_gem_shmem_vm_close,
>  };