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authorNick Piggin <npiggin@suse.de>2008-08-20 14:09:18 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-08-20 15:40:32 -0700
commit479db0bf408e65baa14d2a9821abfcbc0804b847 (patch)
treeacdaaed567afefa36ac2fe27cfe22cfefeb50cd5
parent2d70b68d42b5196a48ccb639e3797f097ef5bea3 (diff)
mm: dirty page tracking race fix
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, powerpc64's set_pte_at will clear a previously present pte before setting it to the desired value. There may also be other code in core mm or in arch which do similar things. The consequence of the bug is loss of data integrity due to msync, and loss of dirty page accounting accuracy. XIP's __xip_unmap could easily also be unreliable (depending on the exact XIP locking scheme), which can lead to data corruption. Fix this by having an option to always take 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> Cc: Jared Hulbert <jaredeh@gmail.com> Cc: Carsten Otte <cotte@freenet.de> Cc: Hugh Dickins <hugh@veritas.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/rmap.h2
-rw-r--r--mm/filemap_xip.c2
-rw-r--r--mm/rmap.c14
3 files changed, 11 insertions, 7 deletions
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 69407f85e10b..fed6f5e0b411 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -102,7 +102,7 @@ int try_to_unmap(struct page *, int ignore_refs);
* 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.
diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index 380ab402d711..8b710ca13247 100644
--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -185,7 +185,7 @@ __xip_unmap (struct address_space * mapping,
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));
diff --git a/mm/rmap.c b/mm/rmap.c
index 059774712c08..0383acfcb068 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -224,10 +224,14 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
/*
* Check that @page is mapped at @address into @mm.
*
+ * If @sync 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 sync)
{
pgd_t *pgd;
pud_t *pud;
@@ -249,7 +253,7 @@ pte_t *page_check_address(struct page *page, struct mm_struct *mm,
pte = pte_offset_map(pmd, address);
/* Make a quick check before getting the lock */
- if (!pte_present(*pte)) {
+ if (!sync && !pte_present(*pte)) {
pte_unmap(pte);
return NULL;
}
@@ -281,7 +285,7 @@ static int page_referenced_one(struct page *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;
@@ -450,7 +454,7 @@ static int page_mkclean_one(struct page *page, struct vm_area_struct *vma)
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;
@@ -704,7 +708,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
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;

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