The patch below does not apply to the 4.19-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 18e77600f7a1ed69f8ce46c9e11cad0985712dfa Mon Sep 17 00:00:00 2001

From: Hugh Dickins hughd@google.com Date: Thu, 6 Aug 2020 23:26:22 -0700 Subject: [PATCH] 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 a9aca9b71d6f..ac04b332a373 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1532,6 +1532,7 @@ static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot) 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);