This is a note to let you know that I've just added the patch titled khugepaged: retract_page_tables() remember to test exit to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: khugepaged-retract_page_tables-remember-to-test-exit.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 18e77600f7a1ed69f8ce46c9e11cad0985712dfa Mon Sep 17 00:00:00 2001 From: Hugh Dickins <hughd@xxxxxxxxxx> Date: Thu, 6 Aug 2020 23:26:22 -0700 Subject: khugepaged: retract_page_tables() remember to test exit 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); Patches currently in stable-queue which might be from hughd@xxxxxxxxxx are queue-5.4/khugepaged-collapse_pte_mapped_thp-flush-the-right-range.patch queue-5.4/khugepaged-retract_page_tables-remember-to-test-exit.patch queue-5.4/khugepaged-collapse_pte_mapped_thp-protect-the-pmd-lock.patch