Re: [PATCH 3/9] drm/i915: Use kmap_local_page() in gem/i915_gem_shmem.c

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On lunedì 17 ottobre 2022 11:37:19 CET Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@intel.com>
> 
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1].
> 
> The main difference between atomic and local mappings is that local
> mappings doesn't disable page faults or preemption.
> 
> In drm/i915/gem/i915_gem_shmem.c, the function shmem_pwrite() need to
> disable pagefault to eliminate the potential recursion fault[2]. But
> here __copy_from_user_inatomic() doesn't need to disable preemption and
> local mapping is valid for sched in/out.
> So it can use kmap_local_page() / kunmap_local() with
> pagefault_disable() / pagefault_enable() to replace atomic mapping.
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> [2]: https://patchwork.freedesktop.org/patch/295840/
> 
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> ---
> Suggested by credits:
>   Ira: Referred to his suggestions about keeping pagefault_disable().
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)

The commit message explains clearly and the changes to the code look good. The 
necessity to reuse pagefault_disable() / pagefault_enable() from the main 
kmap_atomic() side effect is a nice catch.

Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Thanks!

> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c index f42ca1179f37..e279a3e30c02 
100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -472,11 +472,13 @@ shmem_pwrite(struct drm_i915_gem_object *obj,
>  		if (err < 0)
>  			return err;
> 
> -		vaddr = kmap_atomic(page);
> +		vaddr = kmap_local_page(page);
> +		pagefault_disable();
>  		unwritten = __copy_from_user_inatomic(vaddr + pg,
>  						      user_data,
>  						      len);
> -		kunmap_atomic(vaddr);
> +		pagefault_enable();
> +		kunmap_local(vaddr);
> 
>  		err = aops->write_end(obj->base.filp, mapping, offset, 
len,
>  				      len - unwritten, page, data);