[rfc][patch] mm: dirty page accounting race fix

Nick Piggin
2008-08-14T09:45:37Z

Hi,

Wonder if you would be kind enough to verify this? It solves my issues (well
still running after several hours wheras it previously would lock up in a
few seconds).

Thanks,
Nick

---
There is a race with dirty page accounting where a page may not properly
be accounted for.

clear_page_dirty_for_io() calls page_mkclean; then TestClearPageDirty.

page_mkclean walks the rmaps for that page, and for each one it cleans and
write protects the pte if it was dirty. It uses page_check_address to find the
pte. That function has a shortcut to avoid the ptl if the pte is not
present. Unfortunately, the pte can be switched to not-present then back to
present by other code while holding the page table lock -- this should not
be a signal for page_mkclean to ignore that pte, because it may be dirty.

For example, do_wp_page calls ptep_clear_flush_notify before marking the
pte writable and dirty. do_wp_page does subsequently set the page dirty, but
that can happen before clear_page_dirty_for_io() calls TestClearPageDirty.
There may also be other code which does similar things, and/or architectures
which implement other pte manipulation functions using a clear-set sequence,
I haven't looked thoroughly. But it should be fixed.

The consequence of the bug is loss of data integrity (msync) of dirty page
accounting accuracy. XIP's __xip_unmap is probably also unreliable, which
can lead to data corruption.

Fix this by always taking ptl to check the pte in page_check_address.

It's possible to retain this optimization for page_referenced and
try_to_unmap.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h
+++ linux-2.6/include/linux/rmap.h
@@ -102,7 +102,7 @@ int try_to_unmap(struct page *, int igno
* Called from mm/filemap_xip.c to unmap empty zero page
*/
pte_t *page_check_address(struct page *, struct mm_struct *,
- unsigned long, spinlock_t **);
+ unsigned long, spinlock_t **, int);

/*
* Used by swapoff to help locate where page is expected in vma.
Index: linux-2.6/mm/filemap_xip.c
===================================================================
--- linux-2.6.orig/mm/filemap_xip.c
+++ linux-2.6/mm/filemap_xip.c
@@ -185,7 +185,7 @@ __xip_unmap (struct address_space * mapp
address = vma->vm_start +
((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
BUG_ON(address < vma->vm_start || address >= vma->vm_end);
- pte = page_check_address(page, mm, address, &ptl);
+ pte = page_check_address(page, mm, address, &ptl, 1);
if (pte) {
/* Nuke the page table entry. */
flush_cache_page(vma, address, pte_pfn(*pte));
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -224,10 +224,14 @@ unsigned long page_address_in_vma(struct
/*
* Check that @page is mapped at @address into @mm.
*
+ * If @synch is false, page_check_address may perform a racy check to avoid
+ * the page table lock when the pte is not present (helpful when reclaiming
+ * highly shared pages).
+ *
* On success returns with pte mapped and locked.
*/
pte_t *page_check_address(struct page *page, struct mm_struct *mm,
- unsigned long address, spinlock_t **ptlp)
+ unsigned long address, spinlock_t **ptlp, int synch)
{
pgd_t *pgd;
pud_t *pud;
@@ -249,7 +253,7 @@ pte_t *page_check_address(struct page *p

pte = pte_offset_map(pmd, address);
/* Make a quick check before getting the lock */
- if (!pte_present(*pte)) {
+ if (!synch && !pte_present(*pte)) {
pte_unmap(pte);
return NULL;
}
@@ -281,7 +285,7 @@ static int page_referenced_one(struct pa
if (address == -EFAULT)
goto out;

- pte = page_check_address(page, mm, address, &ptl);
+ pte = page_check_address(page, mm, address, &ptl, 0);
if (!pte)
goto out;

@@ -450,7 +454,7 @@ static int page_mkclean_one(struct page
if (address == -EFAULT)
goto out;

- pte = page_check_address(page, mm, address, &ptl);
+ pte = page_check_address(page, mm, address, &ptl, 1);
if (!pte)
goto out;

@@ -697,7 +701,7 @@ static int try_to_unmap_one(struct page
if (address == -EFAULT)
goto out;

- pte = page_check_address(page, mm, address, &ptl);
+ pte = page_check_address(page, mm, address, &ptl, 0);
if (!pte)
goto out;