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Subject[PATCH 4/6] mm: optimize the new mprotect() code a bit

From: Peter Zijlstra <a.p.zijlstra@chello.nl>

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

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
mm/mprotect.c | 33 +++++++++++++++++++++++----------
1 file changed, 23 insertions(+), 10 deletions(-)

Index: linux-2.6/mm/mprotect.c
===================================================================
--- linux-2.6.orig/mm/mprotect.c 2006-06-12 13:09:55.000000000 +0200
+++ linux-2.6/mm/mprotect.c 2006-06-13 11:43:13.000000000 +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 is_accountable)
{
pte_t *pte;
spinlock_t *ptl;
@@ -41,7 +42,13 @@ 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 (is_accountable && pte_dirty(ptent))
+ ptent = pte_mkwrite(ptent);
set_pte_at(mm, addr, pte, ptent);
lazy_mmu_prot_update(ptent);
}
@@ -50,7 +57,8 @@ 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 is_accountable)
{
pmd_t *pmd;
unsigned long next;
@@ -60,12 +68,13 @@ 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, is_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 is_accountable)
{
pud_t *pud;
unsigned long next;
@@ -75,12 +84,13 @@ 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, is_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 is_accountable)
{
struct mm_struct *mm = vma->vm_mm;
pgd_t *pgd;
@@ -94,7 +104,7 @@ 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, is_accountable);
} while (pgd++, addr = next, addr != end);
flush_tlb_range(vma, start, end);
}
@@ -112,6 +122,7 @@ mprotect_fixup(struct vm_area_struct *vm
pgprot_t newprot;
pgoff_t pgoff;
int error;
+ int is_accountable = 0;

if (newflags == oldflags) {
*pprev = vma;
@@ -165,8 +176,10 @@ success:
mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
if (is_shared_writable(newflags) && vma->vm_file)
mapping = vma->vm_file->f_mapping;
- if (mapping && mapping_cap_account_dirty(mapping))
+ if (mapping && mapping_cap_account_dirty(mapping)) {
mask &= ~VM_SHARED;
+ is_accountable = 1;
+ }

newprot = protection_map[newflags & mask];

@@ -179,7 +192,7 @@ success:
if (is_vm_hugetlb_page(vma))
hugetlb_change_protection(vma, start, end, newprot);
else
- change_protection(vma, start, end, newprot);
+ change_protection(vma, start, end, newprot, is_accountable);
vm_stat_account(mm, oldflags, vma->vm_file, -nrpages);
vm_stat_account(mm, newflags, vma->vm_file, nrpages);
return 0;
-
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