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