[PATCH v5 12/12] Documentation/gpu/drm-mm: Add THP paragraph to GEM mapping section

Loïc Molinari
2025-10-21T11:31:09Z

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

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
Documentation/gpu/drm-mm.rst | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
index d55751cad67c..3d6176adc7ca 100644
--- a/Documentation/gpu/drm-mm.rst
+++ b/Documentation/gpu/drm-mm.rst
@@ -283,6 +283,9 @@ made up of several fields, the more interesting ones being:
void (*open)(struct vm_area_struct * area);
void (*close)(struct vm_area_struct * area);
vm_fault_t (*fault)(struct vm_fault *vmf);
+ vm_fault_t (*map_pages)(struct vm_fault *vmf,
+ pgoff_t start_pgoff,
+ pgoff_t end_pgoff);
};

@@ -290,15 +293,27 @@ 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 and map_pages operations are 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 and
+map_pages handlers 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.
--
2.47.3