Fix sys_remap_file_pages BUG at highmem.c:15!

Gurudas Pai reports kernel BUG at arch/i386/mm/highmem.c:15! below
sys_remap_file_pages, while running Oracle database test on x86 in 6GB
RAM: kunmap thinks we're in_interrupt because the preempt count has
wrapped.

That's because __do_fault expected to unmap page_table, but one of its
two callers do_nonlinear_fault already unmapped it: let do_linear_fault
unmap it first too, and then there's no need to pass the page_table arg
down.

Why have we been so slow to notice this? Probably through forgetting
that the mapping_cap_account_dirty test means that sys_remap_file_pages
nowadays only goes the full nonlinear vma route on a few memory-backed
filesystems like ramfs, tmpfs and hugetlbfs.

[ It also depends on CONFIG_HIGHPTE, so it becomes even harder to
  trigger in practice. Many who have need of large memory have probably
  migrated to x86-64..

  Problem introduced by commit d0217ac0
  ("mm: fault feedback #1")                -- Linus ]
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: gurudas pai <gurudas.pai@oracle.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2307,13 +2307,14 @@ oom:
  * do not need to flush old virtual caches or the TLB.
  *
  * We enter with non-exclusive mmap_sem (to exclude vma changes,
- * but allow concurrent faults), and pte mapped but not yet locked.
+ * but allow concurrent faults), and pte neither mapped nor locked.
  * We return with mmap_sem still held, but pte unmapped and unlocked.
  */
 static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
-		unsigned long address, pte_t *page_table, pmd_t *pmd,
+		unsigned long address, pmd_t *pmd,
 		pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
 {
+	pte_t *page_table;
 	spinlock_t *ptl;
 	struct page *page;
 	pte_t entry;
@@ -2327,7 +2328,6 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	vmf.flags = flags;
 	vmf.page = NULL;
 
-	pte_unmap(page_table);
 	BUG_ON(vma->vm_flags & VM_PFNMAP);
 
 	if (likely(vma->vm_ops->fault)) {
@@ -2468,8 +2468,8 @@ static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			- vma->vm_start) >> PAGE_CACHE_SHIFT) + vma->vm_pgoff;
 	unsigned int flags = (write_access ? FAULT_FLAG_WRITE : 0);
 
-	return __do_fault(mm, vma, address, page_table, pmd, pgoff,
-							flags, orig_pte);
+	pte_unmap(page_table);
+	return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
 
 
@@ -2552,9 +2552,7 @@ static int do_nonlinear_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	}
 
 	pgoff = pte_to_pgoff(orig_pte);
-
-	return __do_fault(mm, vma, address, page_table, pmd, pgoff,
-							flags, orig_pte);
+	return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
 
 /*