s390/mm: fix CMMA vs KSM vs others

commit a8f60d1fadf7b8b54449fcc9d6b15248917478ba upstream.

On heavy paging with KSM I see guest data corruption. Turns out that
KSM will add pages to its tree, where the mapping return true for
pte_unused (or might become as such later).  KSM will unmap such pages
and reinstantiate with different attributes (e.g. write protected or
special, e.g. in replace_page or write_protect_page)). This uncovered
a bug in our pagetable handling: We must remove the unused flag as
soon as an entry becomes present again.

Signed-of-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index 8904e12825623ed418d7a0cdee240d993e6c85d5..589f9c65416ae3e14bae0039b25cf283374b1316 100644 (file)
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -868,6 +868,8 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 {
        pgste_t pgste;
 
+       if (pte_present(entry))
+               pte_val(entry) &= ~_PAGE_UNUSED;
        if (mm_has_pgste(mm)) {
                pgste = pgste_get_lock(ptep);
                pgste_val(pgste) &= ~_PGSTE_GPS_ZERO;