Re: [PATCH] x86/mm: do not BUG_ON() on stall pgd entries

[Andy Lutomirski <luto@kernel.org>]

On Wed, May 31, 2017 at 8:03 AM, Jérôme Glisse <jglisse@redhat.com> wrote:
> Since af2cf278ef4f ("Don't remove PGD entries in remove_pagetable()")
> we no longer cleanup stall pgd entries and thus the BUG_ON() inside
> sync_global_pgds() is wrong.
>
> This patch remove the BUG_ON() and unconditionaly update stall pgd
> entries.
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/x86/mm/init_64.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index ff95fe8..36b9020 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -123,12 +123,7 @@ void sync_global_pgds(unsigned long start, unsigned long end)
>                         pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
>                         spin_lock(pgt_lock);
>
> -                       if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
> -                               BUG_ON(p4d_page_vaddr(*p4d)
> -                                      != p4d_page_vaddr(*p4d_ref));
> -
> -                       if (p4d_none(*p4d))
> -                               set_p4d(p4d, *p4d_ref);
> +                       set_p4d(p4d, *p4d_ref);

If we have a mismatch in the vmalloc range, vmalloc_fault is going to
screw up and we'll end up using incorrect page tables.

What's causing the mismatch?  If you're hitting this BUG in practice,
I suspect we have a bug elsewhere.

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