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>
