[PATCH 4/5] mm: optimize the new mprotect() code a bit

Peter Zijlstra
2006-06-27T18:29:06Z

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>
---
mm/mprotect.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)

Index: linux-2.6-dirty/mm/mprotect.c
===================================================================
--- linux-2.6-dirty.orig/mm/mprotect.c 2006-06-27 13:26:19.000000000 +0200
+++ linux-2.6-dirty/mm/mprotect.c 2006-06-27 13:28:49.000000000 +0200
@@ -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_s
* 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_s
}

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(stru
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(stru
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_
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);
}
@@ -126,6 +137,7 @@ mprotect_fixup(struct vm_area_struct *vm
unsigned long mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
pgoff_t pgoff;
int error;
+ int dirty_accountable = 0;

if (newflags == oldflags) {
*pprev = vma;
@@ -180,14 +192,16 @@ success:
* held in write mode.
*/
vma->vm_flags = newflags;
- if (vma_wants_writenotify(vma, VM_NOTIFY_NO_PROT))
+ if (vma_wants_writenotify(vma, VM_NOTIFY_NO_PROT)) {
mask &= ~VM_SHARED;
+ dirty_accountable = 1;
+ }
vma->vm_page_prot = protection_map[newflags & mask];

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;