Re: [PATCH v3 5/7] drm/xe/mmio_gem: cache the dummy page per object

On 23/07/2026 17:18, Ilia Levi wrote:
> Currently, when the fault handler provides a dummy page, it
> allocates a new one on every invocation and ties its lifetime to
> the drm_device via drmm_add_action_or_reset(). Concurrent faults
> after hot-unplug therefore accumulate pages that persist until
> device teardown.
> 
> Cache a single dummy page in the xe_mmio_gem object and use dma_resv
> lock to protect its allocation. Free it with the object.
> 
> v2: use dma_resv lock to protect the allocation (Matt Auld)
> 
> Assisted-by: GitHub-Copilot:claude-opus-4.6
> Signed-off-by: Ilia Levi <[email protected]>

Do we anticipate this be a real issue in practice? I think normal BO 
mmap path just allocates a new page without caching? Is that also a concern?

Reviewed-by: Matthew Auld <[email protected]>

> ---
>   drivers/gpu/drm/xe/xe_mmio_gem.c | 27 ++++++++++++++++-----------
>   1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_mmio_gem.c b/drivers/gpu/drm/xe/xe_mmio_gem.c
> index 2838fcb4ff9f..5bd2759fc876 100644
> --- a/drivers/gpu/drm/xe/xe_mmio_gem.c
> +++ b/drivers/gpu/drm/xe/xe_mmio_gem.c
> @@ -5,9 +5,9 @@
>   
>   #include "xe_mmio_gem.h"
>   
> +#include <linux/dma-resv.h>
>   #include <drm/drm_drv.h>
>   #include <drm/drm_gem.h>
> -#include <drm/drm_managed.h>
>   
>   #include "xe_device_types.h"
>   
> @@ -37,6 +37,7 @@ static vm_fault_t xe_mmio_gem_vm_fault(struct vm_fault *);
>   struct xe_mmio_gem {
>   	struct drm_gem_object base;
>   	phys_addr_t phys_addr;
> +	struct page *dummy_page; /* protected by the GEM's dma_resv */
>   };
>   
>   static int xe_mmio_gem_vm_may_split(struct vm_area_struct *area, unsigned long addr)
> @@ -131,6 +132,8 @@ static void xe_mmio_gem_free(struct drm_gem_object *base)
>   {
>   	struct xe_mmio_gem *obj = to_xe_mmio_gem(base);
>   
> +	if (obj->dummy_page)
> +		__free_page(obj->dummy_page);
>   	drm_gem_object_release(base);
>   	kfree(obj);
>   }
> @@ -169,27 +172,29 @@ static int xe_mmio_gem_mmap(struct drm_gem_object *base, struct vm_area_struct *
>   	return 0;
>   }
>   
> -static void xe_mmio_gem_release_dummy_page(struct drm_device *dev, void *res)
> +static int alloc_dummy_page_if_needed(struct drm_gem_object *base)
>   {
> -	__free_page((struct page *)res);
> +	struct xe_mmio_gem *obj = to_xe_mmio_gem(base);
> +
> +	dma_resv_lock(base->resv, NULL);
> +	if (!obj->dummy_page)
> +		obj->dummy_page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +	dma_resv_unlock(base->resv);
> +
> +	return obj->dummy_page ? 0 : -ENOMEM;
>   }
>   
>   static vm_fault_t xe_mmio_gem_vm_fault_dummy_page(struct vm_fault *vmf)
>   {
>   	struct vm_area_struct *vma = vmf->vma;
>   	struct drm_gem_object *base = vma->vm_private_data;
> -	struct drm_device *dev = base->dev;
> -	struct page *page;
> +	struct xe_mmio_gem *obj = to_xe_mmio_gem(base);
>   	unsigned long pfn;
>   
> -	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> -	if (!page)
> -		return VM_FAULT_OOM;
> -
> -	if (drmm_add_action_or_reset(dev, xe_mmio_gem_release_dummy_page, page))
> +	if (alloc_dummy_page_if_needed(base))
>   		return VM_FAULT_OOM;
>   
> -	pfn = page_to_pfn(page);
> +	pfn = page_to_pfn(obj->dummy_page);
>   
>   	return vmf_insert_pfn_prot(vma, vmf->address, pfn,
>   				   vm_get_page_prot(vma->vm_flags));