[PATCH v3 4/9] drm/i915: Use kmap_local_page() in gem/selftests/huge_pages.c

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

kmap_local_page()[1], and this patch converts the call from
kmap_atomic() to kmap_local_page().

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/selftests/huge_pages.c, function __cpu_check_shmem()
mainly uses mapping to flush cache and check the value. There're
2 reasons why __cpu_check_shmem() doesn't need to disable pagefaults
and preemption for mapping:

1. The flush operation is safe. Function __cpu_check_shmem() calls
drm_clflush_virt_range() to use CLFLUSHOPT or WBINVD to flush. Since
CLFLUSHOPT is global on x86 and WBINVD is called on each cpu in
drm_clflush_virt_range(), the flush operation is global.

2. Any context switch caused by preemption or page faults (page fault
may cause sleep) doesn't affect the validity of local mapping.

Therefore, __cpu_check_shmem() is a function where the use of
kmap_local_page() in place of kmap_atomic() is correctly suited.

Convert the calls of kmap_atomic() / kunmap_atomic() to
kmap_local_page() / kunmap_local().

[1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com

Suggested-by: Dave Hansen <dave.hansen@intel.com>
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:
Dave: Referred to his explanation about cache flush.
Ira: Referred to his task document, review comments and explanation
about cache flush.
Fabio: Referred to his boiler plate commit message and his description
about why kmap_local_page() should be preferred.
---
drivers/gpu/drm/i915/gem/selftests/huge_pages.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
index 6b9f6cf50bf6..c9e6d77abab0 100644
--- a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
+++ b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
@@ -1082,7 +1082,7 @@ __cpu_check_shmem(struct drm_i915_gem_object *obj, u32 dword, u32 val)
goto err_unlock;

for (n = 0; n < obj->base.size >> PAGE_SHIFT; ++n) {
- u32 *ptr = kmap_atomic(i915_gem_object_get_page(obj, n));
+ u32 *ptr = kmap_local_page(i915_gem_object_get_page(obj, n));

if (needs_flush & CLFLUSH_BEFORE)
drm_clflush_virt_range(ptr, PAGE_SIZE);
@@ -1090,12 +1090,12 @@ __cpu_check_shmem(struct drm_i915_gem_object *obj, u32 dword, u32 val)
if (ptr[dword] != val) {
pr_err("n=%lu ptr[%u]=%u, val=%u\n",
n, dword, ptr[dword], val);
- kunmap_atomic(ptr);
+ kunmap_local(ptr);
err = -EINVAL;
break;
}

- kunmap_atomic(ptr);
+ kunmap_local(ptr);
}

i915_gem_object_finish_access(obj);
--
2.34.1