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

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Message-ID: <20251201093918.4051d21d@fedora>
Date: Mon, 1 Dec 2025 09:39:18 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Loïc Molinari <loic.molinari@...labora.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Jani Nikula
 <jani.nikula@...ux.intel.com>, Joonas Lahtinen
 <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>,
 Tvrtko Ursulin <tursulin@...ulin.net>, Rob Herring <robh@...nel.org>,
 Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Melissa Wen <mwen@...lia.com>, Maíra Canal
 <mcanal@...lia.com>, Hugh Dickins <hughd@...gle.com>, Baolin Wang
 <baolin.wang@...ux.alibaba.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Al Viro <viro@...iv.linux.org.uk>, Mikołaj Wasiak
 <mikolaj.wasiak@...el.com>, Christian Brauner <brauner@...nel.org>, Nitin
 Gote <nitin.r.gote@...el.com>, Andi Shyti <andi.shyti@...ux.intel.com>,
 Jonathan Corbet <corbet@....net>, Christopher Healy <healych@...zon.com>,
 Matthew Wilcox <willy@...radead.org>, Bagas Sanjaya <bagasdotme@...il.com>,
 linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 intel-gfx@...ts.freedesktop.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v10 10/10] Documentation/gpu/drm-mm: Add THP paragraph
 to GEM mapping section

On Fri, 28 Nov 2025 19:52:52 +0100
Loïc Molinari <loic.molinari@...labora.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@...labora.com>
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>

Reviewed-by: Boris Brezillon <boris.brezillon@...labora.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.