[PATCH V2] mm/powerpc: Fix _PAGE_PTE breaking swapoff

[Michael Ellerman]

On Mon, 2016-01-11 at 21:19 +0530, Aneesh Kumar K.V wrote:

> Core kernel expect swp_entry_t to be consisting of
> only swap type and swap offset. We should not leak pte bits to
> swp_entry_t. This breaks swapoff which use the swap type and offset
> to build a swp_entry_t and later compare that to the swp_entry_t
> obtained from linux page table pte. Leaking pte bits to swp_entry_t
> breaks that comparison and results in us looping in try_to_unuse.
> 
> The stack trace can be anywhere below try_to_unuse() in mm/swapfile.c,
> since swapoff is circling around and around that function, reading from
> each used swap block into a page, then trying to find where that page
> belongs, looking at every non-file pte of every mm that ever swapped.
> 
> Reported-by: Hugh Dickins <hughd at google.com>
> Suggested-by: Hugh Dickins <hughd at google.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>

Thanks. I slightly edited the wording in the change log and added:

Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit")

cheers