Re: [PATCH] khugepaged: retract_page_tables() remember to test exit

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Sun, Aug 02, 2020 at 12:16:53PM -0700, Hugh Dickins wrote:
> 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).

Hm. I'm not sure I follow. vma->vm_mm has to be valid as long as we hold
i_mmap lock, no? Unlinking a VMA requires it.

-- 
 Kirill A. Shutemov