Re: [PATCH v2 6/9] drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_context.c

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 31/03/2023 04:33, Ira Weiny wrote:
> Zhao Liu wrote:
>> From: Zhao Liu <zhao1.liu@intel.com>
>>
>> The use of kmap_atomic() is being deprecated in favor of
>> 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.
>>
>> With kmap_local_page(), we can avoid the often unwanted side effect of
>> unnecessary page faults or preemption disables.
>>
>> In drm/i915/gem/selftests/i915_gem_context.c, functions cpu_fill() and
>> cpu_check() mainly uses mapping to flush cache and check/assign the
>> value.
>>
>> There're 2 reasons why cpu_fill() and cpu_check() don't need to disable
>> pagefaults and preemption for mapping:
>>
>> 1. The flush operation is safe. cpu_fill() and cpu_check() call
>> 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_fill() and cpu_check() are functions 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
>>
>> v2:
>> * Dropped hot plug related description since it has nothing to do with
>>    kmap_local_page().
>> * No code change since v1, and added description of the motivation of
>>    using kmap_local_page().
>>
>> Suggested-by: Dave Hansen <dave.hansen@intel.com>
>> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> 
> First off I think this is fine.
> 
> But as I looked at this final selftests patch I began to wonder how the
> memory being mapped here and in the previous selftests patches are
> allocated.  Does highmem need to be considered at all?  Unfortunately, I
> could not determine where the memory in the SG list of this test gem
> object was allocated.
> 
> AFAICS cpu_fill() is only called in create_test_object().  Digging into
> huge_gem_object() did not reveal where these pages were allocated from.
> 
> I wonder if these kmap_local_page() calls could be removed entirely based
> on knowing that the pages were allocated from low mem?  Removing yet
> another user of highmem altogether would be best if possible.
> 
> Do you know how these test objects are created?  Do the pages come from
> user space somehow?

FWIW

create_test_object
  -> huge_gem_object
      -> i915_gem_object_init(obj, &huge_ops, &lock_class, 0);

Which is:

static const struct drm_i915_gem_object_ops huge_ops = {
	.name = "huge-gem",
	.get_pages = huge_get_pages,
	.put_pages = huge_put_pages,
};

And:

huge_get_pages()
...
#define GFP (GFP_KERNEL | __GFP_NOWARN | __GFP_RETRY_MAYFAIL)
...
		page = alloc_page(GFP | __GFP_HIGHMEM);

> 
> Regardless this is still a step in the right direction so:
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>

Yeah LGTM.

FYI I am yet to read through the rest of the series, but I don't think 
there will be anything problematic and it passed our CI so likely is 
good to pull in.

Regards,

Tvrtko

> 
>> Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
>> Signed-off-by: Zhao Liu <zhao1.liu@intel.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/i915_gem_context.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>> index a81fa6a20f5a..dcbc0b8e3323 100644
>> --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>> +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>> @@ -481,12 +481,12 @@ static int cpu_fill(struct drm_i915_gem_object *obj, u32 value)
>>   	for (n = 0; n < real_page_count(obj); n++) {
>>   		u32 *map;
>>   
>> -		map = kmap_atomic(i915_gem_object_get_page(obj, n));
>> +		map = kmap_local_page(i915_gem_object_get_page(obj, n));
>>   		for (m = 0; m < DW_PER_PAGE; m++)
>>   			map[m] = value;
>>   		if (!has_llc)
>>   			drm_clflush_virt_range(map, PAGE_SIZE);
>> -		kunmap_atomic(map);
>> +		kunmap_local(map);
>>   	}
>>   
>>   	i915_gem_object_finish_access(obj);
>> @@ -512,7 +512,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
>>   	for (n = 0; n < real_page_count(obj); n++) {
>>   		u32 *map, m;
>>   
>> -		map = kmap_atomic(i915_gem_object_get_page(obj, n));
>> +		map = kmap_local_page(i915_gem_object_get_page(obj, n));
>>   		if (needs_flush & CLFLUSH_BEFORE)
>>   			drm_clflush_virt_range(map, PAGE_SIZE);
>>   
>> @@ -538,7 +538,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
>>   		}
>>   
>>   out_unmap:
>> -		kunmap_atomic(map);
>> +		kunmap_local(map);
>>   		if (err)
>>   			break;
>>   	}
>> -- 
>> 2.34.1
>>
> 
>