Re: [PATCH v10 10/10] Documentation/gpu/drm-mm: Add THP paragraph to GEM mapping section

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

On Fri, 28 Nov 2025 19:52:52 +0100
Loïc Molinari <loic.molinari@collabora.com> wrote:

> Add a paragraph to the GEM objects mapping section explaining how
> transparent huge pages are handled by GEM.
> 
> v4:
> - fix wording after huge_pages handler removal
> 
> v6:
> - fix wording after map_pages handler removal
> 
> Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

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

> ---
>  Documentation/gpu/drm-mm.rst | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index d55751cad67c..d69eab0b4093 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -290,15 +290,27 @@ The open and close operations must update the GEM object reference
>  count. Drivers can use the drm_gem_vm_open() and drm_gem_vm_close() helper
>  functions directly as open and close handlers.
>  
> -The fault operation handler is responsible for mapping individual pages
> -to userspace when a page fault occurs. Depending on the memory
> -allocation scheme, drivers can allocate pages at fault time, or can
> -decide to allocate memory for the GEM object at the time the object is
> -created.
> +The fault operation handler is responsible for mapping pages to
> +userspace when a page fault occurs. Depending on the memory allocation
> +scheme, drivers can allocate pages at fault time, or can decide to
> +allocate memory for the GEM object at the time the object is created.
>  
>  Drivers that want to map the GEM object upfront instead of handling page
>  faults can implement their own mmap file operation handler.
>  
> +In order to reduce page table overhead, if the internal shmem mountpoint
> +"shm_mnt" is configured to use transparent huge pages (for builds with
> +CONFIG_TRANSPARENT_HUGEPAGE enabled) and if the shmem backing store
> +managed to allocate a huge page for a faulty address, the fault handler
> +will first attempt to insert that huge page into the VMA before falling
> +back to individual page insertion. mmap() user address alignment for GEM
> +objects is handled by providing a custom get_unmapped_area file
> +operation which forwards to the shmem backing store. For most drivers,
> +which don't create a huge mountpoint by default or through a module
> +parameter, transparent huge pages can be enabled by either setting the
> +"transparent_hugepage_shmem" kernel parameter or the
> +"/sys/kernel/mm/transparent_hugepage/shmem_enabled" sysfs knob.
> +
>  For platforms without MMU the GEM core provides a helper method
>  drm_gem_dma_get_unmapped_area(). The mmap() routines will call this to get a
>  proposed address for the mapping.