khugepaged: retract_page_tables() remember to test exit

Only once have I seen this scenario (and forgot even to notice what forced
the eventual crash): a sequence of "BUG: Bad page map" alerts from
vm_normal_page(), from zap_pte_range() servicing exit_mmap();
pmd:00000000, pte values corresponding to data in physical page 0.

The pte mappings being zapped in this case were supposed to be from a huge
page of ext4 text (but could as well have been shmem): my belief is that
it was racing with collapse_file()'s retract_page_tables(), found *pmd
pointing to a page table, locked it, but *pmd had become 0 by the time
start_pte was decided.

In most cases, that possibility is excluded by holding mmap lock; but
exit_mmap() proceeds without mmap lock.  Most of what's run by khugepaged
checks khugepaged_test_exit() after acquiring mmap lock:
khugepaged_collapse_pte_mapped_thps() and hugepage_vma_revalidate() do so,
for example.  But retract_page_tables() did not: fix that.

The fix is for retract_page_tables() to check khugepaged_test_exit(),
after acquiring mmap lock, before doing anything to the page table.
Getting the mmap lock serializes with __mmput(), which briefly takes and
drops it in __khugepaged_exit(); then the khugepaged_test_exit() check on
mm_users makes sure we don't touch the page table once exit_mmap() might
reach it, since exit_mmap() will be proceeding without mmap lock, not
expecting anyone to be racing with it.

Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: <stable@vger.kernel.org>	[4.8+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021215400.27773@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a9aca9b..ac04b33 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1532,6 +1532,7 @@ out:
 static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 {
        struct vm_area_struct *vma;
+       struct mm_struct *mm;
        unsigned long addr;
        pmd_t *pmd, _pmd;
 
@@ -1560,7 +1561,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
                        continue;
                if (vma->vm_end < addr + HPAGE_PMD_SIZE)
                        continue;
-               pmd = mm_find_pmd(vma->vm_mm, addr);
+               mm = vma->vm_mm;
+               pmd = mm_find_pmd(mm, addr);
                if (!pmd)
                        continue;
                /*
@@ -1570,17 +1572,19 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
                 * mmap_lock while holding page lock. Fault path does it in
                 * reverse order. Trylock is a way to avoid deadlock.
                 */
-               if (mmap_write_trylock(vma->vm_mm)) {
-                       spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
-                       /* assume page table is clear */
-                       _pmd = pmdp_collapse_flush(vma, addr, pmd);
-                       spin_unlock(ptl);
-                       mmap_write_unlock(vma->vm_mm);
-                       mm_dec_nr_ptes(vma->vm_mm);
-                       pte_free(vma->vm_mm, pmd_pgtable(_pmd));
+               if (mmap_write_trylock(mm)) {
+                       if (!khugepaged_test_exit(mm)) {
+                               spinlock_t *ptl = pmd_lock(mm, pmd);
+                               /* assume page table is clear */
+                               _pmd = pmdp_collapse_flush(vma, addr, pmd);
+                               spin_unlock(ptl);
+                               mm_dec_nr_ptes(mm);
+                               pte_free(mm, pmd_pgtable(_pmd));
+                       }
+                       mmap_write_unlock(mm);
                } else {
                        /* Try again later */
-                       khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+                       khugepaged_add_pte_mapped_thp(mm, addr);
                }
        }
        i_mmap_unlock_write(mapping);