Re: [PATCH v2 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c

[Ira Weiny]

Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@xxxxxxxxx>
> 
> 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 (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 and preemption disables.
> 
> There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't
> need to disable pagefaults and preemption for mapping:
> 
> 1. The flush operation is safe. In drm/i915/gem/i915_gem_object.c,
> i915_gem_object_read_from_page_kmap() 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, i915_gem_object_read_from_page_kmap() 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().
> 
> And remove the redundant variable that stores the address of the mapped
> page since kunmap_local() can accept any pointer within the page.
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx
> 
> v2:
> * Dropped hot plug related description since it has nothing to do with
>   kmap_local_page().
> * Rebased on f47e630 (drm/i915/gem: Typecheck page lookups) to keep
>   the "idx" variable of type pgoff_t here.
> * Added description of the motivation of using kmap_local_page().
> 
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>

Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>