um: Avoid marking pages with "changed protection"

Changing protection is a very high cost operation in UML
because in addition to an extra syscall it also interrupts
mmap merge sequences generated by the tlb.

While the condition is not particularly common it is worth
avoiding.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/um/include/asm/pgtable.h b/arch/um/include/asm/pgtable.h
index 7485398d0737..9c04562310b3 100644
--- a/arch/um/include/asm/pgtable.h
+++ b/arch/um/include/asm/pgtable.h
@@ -197,12 +197,17 @@ static inline pte_t pte_mkold(pte_t pte)
 
 static inline pte_t pte_wrprotect(pte_t pte)
 { 
-        pte_clear_bits(pte, _PAGE_RW);
+        if (likely(pte_get_bits(pte, _PAGE_RW)))
+                pte_clear_bits(pte, _PAGE_RW);
+        else
+                return pte;
         return(pte_mknewprot(pte)); 
 }
 
 static inline pte_t pte_mkread(pte_t pte)
 { 
+        if (unlikely(pte_get_bits(pte, _PAGE_USER)))
+                return pte;
         pte_set_bits(pte, _PAGE_USER);
         return(pte_mknewprot(pte)); 
 }
@@ -221,6 +226,8 @@ static inline pte_t pte_mkyoung(pte_t pte)
 
 static inline pte_t pte_mkwrite(pte_t pte)      
 {
+        if (unlikely(pte_get_bits(pte,  _PAGE_RW)))
+                return pte;
         pte_set_bits(pte, _PAGE_RW);
         return(pte_mknewprot(pte)); 
 }