[PATCH v3 3/9] drm/i915: Use kmap_local_page() in gem/i915_gem_shmem.c

Zhao Liu
2023-12-03T13:18:14Z

kmap_local_page()[1].

The main difference between atomic and local mappings is that local
mappings doesn't disable page faults or preemption (the preemption is
disabled for !PREEMPT_RT case, otherwise it only disables migration).

With kmap_local_page(), we can avoid the often unwanted side effect of
unnecessary page faults or preemption disables.

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>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
Suggested by credits:
Ira: Referred to his suggestions about keeping pagefault_disable().
Fabio: Referred to his description about why kmap_local_page() should
be preferred.
---
drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index 73a4a4eb29e0..38b72d86560f 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -485,11 +485,13 @@ 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);
--
2.34.1