On Wed, 2012-06-27 at 23:15 +0200, Peter Zijlstra wrote:

S390 doesn't need a TLB flush after ptep_get_and_clear_full() and
before __tlb_remove_page() because its ptep_get_and_clear*() family
already does a full TLB invalidate. Therefore force it to use
tlb_fast_mode. 

On that.. ptep_get_and_clear() says:

/*                                                                                             
 * This is hard to understand. ptep_get_and_clear and ptep_clear_flush                         
 * both clear the TLB for the unmapped pte. The reason is that                                 
 * ptep_get_and_clear is used in common code (e.g. change_pte_range)                           
 * to modify an active pte. The sequence is                                                    
 *   1) ptep_get_and_clear                                                                     
 *   2) set_pte_at                                                                             
 *   3) flush_tlb_range                                                                        
 * On s390 the tlb needs to get flushed with the modification of the pte                       
 * if the pte is active. The only way how this can be implemented is to                        
 * have ptep_get_and_clear do the tlb flush. In exchange flush_tlb_range                       
 * is a nop.                                                                                   
 */ 

I think there is another way, arch_{enter,leave}_lazy_mmu_mode() seems
to wrap these sites so you can do as SPARC64 and PPC do and batch
through there.

That should save a number of TLB invalidates..


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