[PATCH] mm: pte_offset_map_lock loops

Convert those common loops using page_table_lock on the outside and
pte_offset_map within to use just pte_offset_map_lock within instead.

These all hold mmap_sem (some exclusively, some not), so at no level can a
page table be whipped away from beneath them.  But whereas pte_alloc loops
tested with the "atomic" pmd_present, these loops are testing with pmd_none,
which on i386 PAE tests both lower and upper halves.

That's now unsafe, so add a cast into pmd_none to test only the vital lower
half: we lose a little sensitivity to a corrupt middle directory, but not
enough to worry about.  It appears that i386 and UML were the only
architectures vulnerable in this way, and pgd and pud no problem.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 6 insertions, 15 deletions

diff --git a/mm/msync.c b/mm/msync.c
index 8603954..0e040e9 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -17,28 +17,22 @@
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
 
-/*
- * Called with mm->page_table_lock held to protect against other
- * threads/the swapper from ripping pte's out from under us.
- */
-
 static void msync_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				unsigned long addr, unsigned long end)
 {
-	struct mm_struct *mm = vma->vm_mm;
 	pte_t *pte;
+	spinlock_t *ptl;
 	int progress = 0;
 
 again:
-	pte = pte_offset_map(pmd, addr);
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
 	do {
 		unsigned long pfn;
 		struct page *page;
 
 		if (progress >= 64) {
 			progress = 0;
-			if (need_resched() ||
-			    need_lockbreak(&mm->page_table_lock))
+			if (need_resched() || need_lockbreak(ptl))
 				break;
 		}
 		progress++;
@@ -58,8 +52,8 @@ again:
 			set_page_dirty(page);
 		progress += 3;
 	} while (pte++, addr += PAGE_SIZE, addr != end);
-	pte_unmap(pte - 1);
-	cond_resched_lock(&mm->page_table_lock);
+	pte_unmap_unlock(pte - 1, ptl);
+	cond_resched();
 	if (addr != end)
 		goto again;
 }
@@ -97,7 +91,6 @@ static inline void msync_pud_range(struct vm_area_struct *vma, pgd_t *pgd,
 static void msync_page_range(struct vm_area_struct *vma,
 				unsigned long addr, unsigned long end)
 {
-	struct mm_struct *mm = vma->vm_mm;
 	pgd_t *pgd;
 	unsigned long next;
 
@@ -110,16 +103,14 @@ static void msync_page_range(struct vm_area_struct *vma,
 		return;
 
 	BUG_ON(addr >= end);
-	pgd = pgd_offset(mm, addr);
+	pgd = pgd_offset(vma->vm_mm, addr);
 	flush_cache_range(vma, addr, end);
-	spin_lock(&mm->page_table_lock);
 	do {
 		next = pgd_addr_end(addr, end);
 		if (pgd_none_or_clear_bad(pgd))
 			continue;
 		msync_pud_range(vma, pgd, addr, next);
 	} while (pgd++, addr = next, addr != end);
-	spin_unlock(&mm->page_table_lock);
 }
 
 /*