[PATCH] drm/i915/gem: Clarify seemingly unaccounted obj refcount inc

[Andi Shyti]

Hi Nirmoy,

On Wed, Mar 15, 2023 at 11:54:46AM +0100, Nirmoy Das wrote:
> Add a comment why there is a obj refcount inc before installing
> the vm_ops for the mmap call. Also remove the invalid older comment
> as drm API(drm_gem_prime_mmap()) will hold an obj reference before
> calling this driver mmap callback so we can't have 0-refcnted
> object here.
> 
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Andi Shyti <andi.shyti at linux.intel.com>
> Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>

fine with me!

Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com>

Andi

> ---
>  drivers/gpu/drm/i915/gem/i915_gem_mman.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> index d3c1dee16af2..0bc8c3818443 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> @@ -952,9 +952,10 @@ int i915_gem_mmap(struct file *filp, struct vm_area_struct *vma)
>  						  vma_pages(vma));
>  	if (node && drm_vma_node_is_allowed(node, priv)) {
>  		/*
> -		 * Skip 0-refcnted objects as it is in the process of being
> -		 * destroyed and will be invalid when the vma manager lock
> -		 * is released.
> +		 * When we install vm_ops for mmap we are too late for
> +		 * the vm_ops->open() which increases the ref_count of
> +		 * this obj and then it gets decreased by the vm_ops->close().
> +		 * To balance this increase the obj ref_count here.
>  		 */
>  		if (!node->driver_private) {
>  			mmo = container_of(node, struct i915_mmap_offset, vma_node);
> -- 
> 2.39.0