[PATCH] khugepaged: khugepaged_test_exit() check mmget_still_valid()

[Hugh Dickins]

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>
Cc: stable@xxxxxxxxxxxxxxx # v4.8+
---
Note on stable backporting: the coredump fix was backported as far as
v3.16, but this extension only needed where shmem or file THP supported.

 mm/khugepaged.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- 5.8-rc7/mm/khugepaged.c	2020-07-26 16:58:02.189038680 -0700
+++ linux/mm/khugepaged.c	2020-08-02 10:56:11.105617241 -0700
@@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(stru
 
 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);
 }
 
 static bool hugepage_vma_check(struct vm_area_struct *vma,
@@ -1100,9 +1100,6 @@ static void collapse_huge_page(struct mm
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	mmap_write_lock(mm);
-	result = SCAN_ANY_PROCESS;
-	if (!mmget_still_valid(mm))
-		goto out;
 	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result)
 		goto out;