[PATCH next] powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff

[Cyrill Gorcunov]

On Sat, Jan 09, 2016 at 04:54:59PM -0800, Hugh Dickins wrote:
> Swapoff after swapping hangs on the G5, when CONFIG_CHECKPOINT_RESTORE=y
> but CONFIG_MEM_SOFT_DIRTY is not set.  That's because the non-zero
> _PAGE_SWP_SOFT_DIRTY bit, added by CONFIG_HAVE_ARCH_SOFT_DIRTY=y, is not
> discounted when CONFIG_MEM_SOFT_DIRTY is not set: so swap ptes cannot be
> recognized.
> 
> (I suspect that the peculiar dependence of HAVE_ARCH_SOFT_DIRTY on
> CHECKPOINT_RESTORE in arch/powerpc/Kconfig comes from an incomplete
> attempt to solve this problem.)
> 
> It's true that the relationship between CONFIG_HAVE_ARCH_SOFT_DIRTY and
> and CONFIG_MEM_SOFT_DIRTY is too confusing, and it's true that swapoff
> should be made more robust; but nevertheless, fix up the powerpc ifdefs
> as x86_64 and s390 (which met the same problem) have them, defining the
> bits as 0 if CONFIG_MEM_SOFT_DIRTY is not set.
> 
> Signed-off-by: Hugh Dickins <hughd at google.com>
Reviewed-by: Cyrill Gorcunov <gorcunov at openvz.org>

Thank you, Hugh!