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mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared 

For VMAs that don't want write notifications, PTEs created for read faults
have their write bit set.  If the read fault happens after VM_SOFTDIRTY is
cleared, then the PTE's softdirty bit will remain clear after subsequent
writes.

Here's a simple code snippet to demonstrate the bug:

  char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
                 MAP_ANONYMOUS | MAP_SHARED, -1, 0);
  system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */
  assert(*m == '\0');     /* new PTE allows write access */
  assert(!soft_dirty(x));
  *m = 'x';               /* should dirty the page */
  assert(soft_dirty(x));  /* fails */

With this patch, write notifications are enabled when VM_SOFTDIRTY is
cleared.  Furthermore, to avoid unnecessary faults, write notifications
are disabled when VM_SOFTDIRTY is set.

As a side effect of enabling and disabling write notifications with
care, this patch fixes a bug in mprotect where vm_page_prot bits set by
drivers were zapped on mprotect.  An analogous bug was fixed in mmap by
commit c9d0bf2414 ("mm: uncached vma support with writenotify").

Signed-off-by: Peter Feiner <pfeiner@google.com>
Reported-by: Peter Feiner <pfeiner@google.com>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Jamie Liu <jamieliu@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Peter Feiner 2014-10-13 15:55:46 -07:00 committed by Linus Torvalds
parent 63a12d9d01
commit 64e455079e
6 changed files with 68 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
fs/proc/task_mmu.c
View File

@ -827,8 +827,21 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
.private = &cp,
};
down_read(&mm->mmap_sem);
if (type == CLEAR_REFS_SOFT_DIRTY)
if (type == CLEAR_REFS_SOFT_DIRTY) {
for (vma = mm->mmap; vma; vma = vma->vm_next) {
if (!(vma->vm_flags & VM_SOFTDIRTY))
continue;
up_read(&mm->mmap_sem);
down_write(&mm->mmap_sem);
for (vma = mm->mmap; vma; vma = vma->vm_next) {
vma->vm_flags &= ~VM_SOFTDIRTY;
vma_set_page_prot(vma);
}
downgrade_write(&mm->mmap_sem);
break;
}
mmu_notifier_invalidate_range_start(mm, 0, -1);
}
for (vma = mm->mmap; vma; vma = vma->vm_next) {
cp.vma = vma;
if (is_vm_hugetlb_page(vma))
@ -848,10 +861,6 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
continue;
if (type == CLEAR_REFS_MAPPED && !vma->vm_file)
continue;
if (type == CLEAR_REFS_SOFT_DIRTY) {
if (vma->vm_flags & VM_SOFTDIRTY)
vma->vm_flags &= ~VM_SOFTDIRTY;
}
walk_page_range(vma->vm_start, vma->vm_end,
&clear_refs_walk);
}

14
include/asm-generic/pgtable.h
View File

@ -253,6 +253,20 @@ static inline int pmd_same(pmd_t pmd_a, pmd_t pmd_b)
#define pgprot_device pgprot_noncached
#endif
#ifndef pgprot_modify
#define pgprot_modify pgprot_modify
static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
{
if (pgprot_val(oldprot) == pgprot_val(pgprot_noncached(oldprot)))
newprot = pgprot_noncached(newprot);
if (pgprot_val(oldprot) == pgprot_val(pgprot_writecombine(oldprot)))
newprot = pgprot_writecombine(newprot);
if (pgprot_val(oldprot) == pgprot_val(pgprot_device(oldprot)))
newprot = pgprot_device(newprot);
return newprot;
}
#endif
/*
* When walking page tables, get the address of the next boundary,
* or the end address of the range if that comes earlier. Although no

5
include/linux/mm.h
View File

@ -1974,11 +1974,16 @@ static inline struct vm_area_struct *find_exact_vma(struct mm_struct *mm,
#ifdef CONFIG_MMU
pgprot_t vm_get_page_prot(unsigned long vm_flags);
void vma_set_page_prot(struct vm_area_struct *vma);
#else
static inline pgprot_t vm_get_page_prot(unsigned long vm_flags)
{
return __pgprot(0);
}
static inline void vma_set_page_prot(struct vm_area_struct *vma)
{
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
}
#endif
#ifdef CONFIG_NUMA_BALANCING

3
mm/memory.c
View File

@ -2053,7 +2053,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
old_page = vm_normal_page(vma, address, orig_pte);
if (!old_page) {
/*
* VM_MIXEDMAP !pfn_valid() case
* VM_MIXEDMAP !pfn_valid() case, or VM_SOFTDIRTY clear on a
* VM_PFNMAP VMA.
*
* We should not cow pages in a shared writeable mapping.
* Just mark the pages writable as we can't do any dirty

45
mm/mmap.c
View File

@ -89,6 +89,25 @@ pgprot_t vm_get_page_prot(unsigned long vm_flags)
}
EXPORT_SYMBOL(vm_get_page_prot);
static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
{
return pgprot_modify(oldprot, vm_get_page_prot(vm_flags));
}
/* Update vma->vm_page_prot to reflect vma->vm_flags. */
void vma_set_page_prot(struct vm_area_struct *vma)
{
unsigned long vm_flags = vma->vm_flags;
vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot, vm_flags);
if (vma_wants_writenotify(vma)) {
vm_flags &= ~VM_SHARED;
vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot,
vm_flags);
}
}
int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS; /* heuristic overcommit */
int sysctl_overcommit_ratio __read_mostly = 50; /* default is 50% */
unsigned long sysctl_overcommit_kbytes __read_mostly;
@ -1475,11 +1494,16 @@ int vma_wants_writenotify(struct vm_area_struct *vma)
if (vma->vm_ops && vma->vm_ops->page_mkwrite)
return 1;
/* The open routine did something to the protections already? */
/* The open routine did something to the protections that pgprot_modify
* won't preserve? */
if (pgprot_val(vma->vm_page_prot) !=
pgprot_val(vm_get_page_prot(vm_flags)))
pgprot_val(vm_pgprot_modify(vma->vm_page_prot, vm_flags)))
return 0;
/* Do we need to track softdirty? */
if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) && !(vm_flags & VM_SOFTDIRTY))
return 1;
/* Specialty mapping? */
if (vm_flags & VM_PFNMAP)
return 0;
@ -1615,21 +1639,6 @@ munmap_back:
goto free_vma;
}
if (vma_wants_writenotify(vma)) {
pgprot_t pprot = vma->vm_page_prot;
/* Can vma->vm_page_prot have changed??
*
* Answer: Yes, drivers may have changed it in their
* f_op->mmap method.
*
* Ensures that vmas marked as uncached stay that way.
*/
vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
if (pgprot_val(pprot) == pgprot_val(pgprot_noncached(pprot)))
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
}
vma_link(mm, vma, prev, rb_link, rb_parent);
/* Once vma denies write, undo our temporary denial count */
if (file) {
@ -1663,6 +1672,8 @@ out:
*/
vma->vm_flags |= VM_SOFTDIRTY;
vma_set_page_prot(vma);
return addr;
unmap_and_free_vma:

20
mm/mprotect.c
View File

@ -29,13 +29,6 @@
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>
#ifndef pgprot_modify
static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
{
return newprot;
}
#endif
/*
* For a prot_numa update we only hold mmap_sem for read so there is a
* potential race with faulting where a pmd was temporarily none. This
@ -93,7 +86,9 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
* Avoid taking write faults for pages we
* know to be dirty.
*/
if (dirty_accountable && pte_dirty(ptent))
if (dirty_accountable && pte_dirty(ptent) &&
(pte_soft_dirty(ptent) ||
!(vma->vm_flags & VM_SOFTDIRTY)))
ptent = pte_mkwrite(ptent);
ptep_modify_prot_commit(mm, addr, pte, ptent);
updated = true;
@ -320,13 +315,8 @@ success:
* held in write mode.
*/
vma->vm_flags = newflags;
vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
vm_get_page_prot(newflags));
if (vma_wants_writenotify(vma)) {
vma->vm_page_prot = vm_get_page_prot(newflags & ~VM_SHARED);
dirty_accountable = 1;
}
dirty_accountable = vma_wants_writenotify(vma);
vma_set_page_prot(vma);
change_protection(vma, start, end, vma->vm_page_prot,
dirty_accountable, 0);