[PATCH] mm: optimize the new mprotect() code a bit

mprotect() resets the page protections, which could result in extra write
faults for those pages whose dirty state we track using write faults and are
dirty already.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 24 insertions, 10 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 367b7f6c0637..955f9d0e38aa 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -27,7 +27,8 @@
 #include <asm/tlbflush.h>
 
 static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
-                unsigned long addr, unsigned long end, pgprot_t newprot)
+                unsigned long addr, unsigned long end, pgprot_t newprot,
+                int dirty_accountable)
 {
         pte_t *pte, oldpte;
         spinlock_t *ptl;
@@ -42,7 +43,14 @@ static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
                          * bits by wiping the pte and then setting the new pte
                          * into place.
                          */
-                        ptent = pte_modify(ptep_get_and_clear(mm, addr, pte), newprot);
+                        ptent = ptep_get_and_clear(mm, addr, pte);
+                        ptent = pte_modify(ptent, newprot);
+                        /*
+                         * Avoid taking write faults for pages we know to be
+                         * dirty.
+                         */
+                        if (dirty_accountable && pte_dirty(ptent))
+                                ptent = pte_mkwrite(ptent);
                         set_pte_at(mm, addr, pte, ptent);
                         lazy_mmu_prot_update(ptent);
 #ifdef CONFIG_MIGRATION
@@ -66,7 +74,8 @@ static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
 }
 
 static inline void change_pmd_range(struct mm_struct *mm, pud_t *pud,
-                unsigned long addr, unsigned long end, pgprot_t newprot)
+                unsigned long addr, unsigned long end, pgprot_t newprot,
+                int dirty_accountable)
 {
         pmd_t *pmd;
         unsigned long next;
@@ -76,12 +85,13 @@ static inline void change_pmd_range(struct mm_struct *mm, pud_t *pud,
                 next = pmd_addr_end(addr, end);
                 if (pmd_none_or_clear_bad(pmd))
                         continue;
-                change_pte_range(mm, pmd, addr, next, newprot);
+                change_pte_range(mm, pmd, addr, next, newprot, dirty_accountable);
         } while (pmd++, addr = next, addr != end);
 }
 
 static inline void change_pud_range(struct mm_struct *mm, pgd_t *pgd,
-                unsigned long addr, unsigned long end, pgprot_t newprot)
+                unsigned long addr, unsigned long end, pgprot_t newprot,
+                int dirty_accountable)
 {
         pud_t *pud;
         unsigned long next;
@@ -91,12 +101,13 @@ static inline void change_pud_range(struct mm_struct *mm, pgd_t *pgd,
                 next = pud_addr_end(addr, end);
                 if (pud_none_or_clear_bad(pud))
                         continue;
-                change_pmd_range(mm, pud, addr, next, newprot);
+                change_pmd_range(mm, pud, addr, next, newprot, dirty_accountable);
         } while (pud++, addr = next, addr != end);
 }
 
 static void change_protection(struct vm_area_struct *vma,
-                unsigned long addr, unsigned long end, pgprot_t newprot)
+                unsigned long addr, unsigned long end, pgprot_t newprot,
+                int dirty_accountable)
 {
         struct mm_struct *mm = vma->vm_mm;
         pgd_t *pgd;
@@ -110,7 +121,7 @@ static void change_protection(struct vm_area_struct *vma,
                 next = pgd_addr_end(addr, end);
                 if (pgd_none_or_clear_bad(pgd))
                         continue;
-                change_pud_range(mm, pgd, addr, next, newprot);
+                change_pud_range(mm, pgd, addr, next, newprot, dirty_accountable);
         } while (pgd++, addr = next, addr != end);
         flush_tlb_range(vma, start, end);
 }
@@ -125,6 +136,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
         unsigned long charged = 0;
         pgoff_t pgoff;
         int error;
+        int dirty_accountable = 0;
 
         if (newflags == oldflags) {
                 *pprev = vma;
@@ -181,14 +193,16 @@ success:
         vma->vm_flags = newflags;
         vma->vm_page_prot = protection_map[newflags &
                 (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
-        if (vma_wants_writenotify(vma))
+        if (vma_wants_writenotify(vma)) {
                 vma->vm_page_prot = protection_map[newflags &
                         (VM_READ|VM_WRITE|VM_EXEC)];
+                dirty_accountable = 1;
+        }
 
         if (is_vm_hugetlb_page(vma))
                 hugetlb_change_protection(vma, start, end, vma->vm_page_prot);
         else
-                change_protection(vma, start, end, vma->vm_page_prot);
+                change_protection(vma, start, end, vma->vm_page_prot, dirty_accountable);
         vm_stat_account(mm, oldflags, vma->vm_file, -nrpages);
         vm_stat_account(mm, newflags, vma->vm_file, nrpages);
         return 0;