From: Hugh Dickins <hughd@xxxxxxxxxx>
commit 18e77600f7a1ed69f8ce46c9e11cad0985712dfa upstream.
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@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.8+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021215400.27773@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1414,6 +1414,7 @@ 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;
@@ -1442,7 +1443,8 @@ static void retract_page_tables(struct a
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;
/*
@@ -1452,17 +1454,19 @@ static void retract_page_tables(struct a
* mmap_sem while holding page lock. Fault path does it in
* reverse order. Trylock is a way to avoid deadlock.
*/
- if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
- spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
- /* assume page table is clear */
- _pmd = pmdp_collapse_flush(vma, addr, pmd);
- spin_unlock(ptl);
- up_write(&vma->vm_mm->mmap_sem);
- mm_dec_nr_ptes(vma->vm_mm);
- pte_free(vma->vm_mm, pmd_pgtable(_pmd));
+ if (down_write_trylock(&mm->mmap_sem)) {
+ 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));
+ }
+ up_write(&mm->mmap_sem);
} 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);
