[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>

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 367b7f6..955f9d0 100644 (file)
--- 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;