[PATCH 4.9 07/39] khugepaged: khugepaged_test_exit() check mmget_still_valid()

[Greg Kroah-Hartman]

From: Hugh Dickins <hughd@xxxxxxxxxx>

[ Upstream commit bbe98f9cadff58cdd6a4acaeba0efa8565dabe65 ]

Move collapse_huge_page()'s mmget_still_valid() check into
khugepaged_test_exit() itself.  collapse_huge_page() is used for anon THP
only, and earned its mmget_still_valid() check because it inserts a huge
pmd entry in place of the page table's pmd entry; whereas
collapse_file()'s retract_page_tables() or collapse_pte_mapped_thp()
merely clears the page table's pmd entry.  But core dumping without mmap
lock must have been as open to mistaking a racily cleared pmd entry for a
page table at physical page 0, as exit_mmap() was.  And we certainly have
no interest in mapping as a THP once dumping core.

Fixes: 59ea6d06cfa9 ("coredump: fix race condition between collapse_huge_page() and core dumping")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.8+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021217020.27773@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/khugepaged.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3080c6415493c..f4ed056d9f863 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -391,7 +391,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
 
 static inline int khugepaged_test_exit(struct mm_struct *mm)
 {
-	return atomic_read(&mm->mm_users) == 0;
+	return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
 }
 
 int __khugepaged_enter(struct mm_struct *mm)
@@ -1004,9 +1004,6 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	down_write(&mm->mmap_sem);
-	result = SCAN_ANY_PROCESS;
-	if (!mmget_still_valid(mm))
-		goto out;
 	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result)
 		goto out;
-- 
2.25.1