[PATCH] .text page fault SMP scalability optimization

We had a problem on ppc64 where with more than 4 threads a large system
wouldn't scale well while faulting in the .text (most of the time was spent
in the kernel despite it was an userland compute intensive app).  The
reason is the useless overwrite of the same pte from all cpu.

I fixed it this way (verified on an older kernel but the forward port is
almost identical).  This will benefit all archs not just ppc64.
Signed-off-by: NAndrea Arcangeli <andrea@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/memory.c

@@ -1980,9 +1980,10 @@ static inline int handle_pte_fault(struct mm_struct *mm,
 		pte_t *pte, pmd_t *pmd, int write_access)
 {
 	pte_t entry;
+	pte_t old_entry;
 	spinlock_t *ptl;
 
-	entry = *pte;
+	old_entry = entry = *pte;
 	if (!pte_present(entry)) {
 		if (pte_none(entry)) {
 			if (!vma->vm_ops || !vma->vm_ops->nopage)
@@ -2009,9 +2010,20 @@ static inline int handle_pte_fault(struct mm_struct *mm,
 		entry = pte_mkdirty(entry);
 	}
 	entry = pte_mkyoung(entry);
-	ptep_set_access_flags(vma, address, pte, entry, write_access);
-	update_mmu_cache(vma, address, entry);
-	lazy_mmu_prot_update(entry);
+	if (!pte_same(old_entry, entry)) {
+		ptep_set_access_flags(vma, address, pte, entry, write_access);
+		update_mmu_cache(vma, address, entry);
+		lazy_mmu_prot_update(entry);
+	} else {
+		/*
+		 * This is needed only for protection faults but the arch code
+		 * is not yet telling us if this is a protection fault or not.
+		 * This still avoids useless tlb flushes for .text page faults
+		 * with threads.
+		 */
+		if (write_access)
+			flush_tlb_page(vma, address);
+	}
 unlock:
 	pte_unmap_unlock(pte, ptl);
 	return VM_FAULT_MINOR;