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
(using an mm variable instead of vma->vm_mm repeatedly).
Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem
pages")
Signed-off-by: Hugh Dickins <[email protected]>
Cc: [email protected] # v4.8+
---
mm/khugepaged.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)
--- 5.8-rc7/mm/khugepaged.c 2020-07-26 16:58:02.189038680 -0700
+++ linux/mm/khugepaged.c 2020-08-02 10:53:37.892660983 -0700
@@ -1538,6 +1538,7 @@ 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;
@@ -1566,7 +1567,8 @@ 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;
/*
@@ -1576,17 +1578,19 @@ static void retract_page_tables(struct a
* 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);
