s390/mm: optimize locking without huge pages in gmap_pmd_op_walk()

Right now we temporarily take the page table lock in
gmap_pmd_op_walk() even though we know we won't need it (if we can
never have 1mb pages mapped into the gmap).

Let's make this a special case, so gmap_protect_range() and
gmap_sync_dirty_log_pmd() will not take the lock when huge pages are
not allowed.

gmap_protect_range() is called quite frequently for managing shadow
page tables in vSIE environments.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20180806155407.15252-1-david@redhat.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index bb44990..d4fa0a4 100644 (file)
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -905,10 +905,16 @@ static inline pmd_t *gmap_pmd_op_walk(struct gmap *gmap, unsigned long gaddr)
        pmd_t *pmdp;
 
        BUG_ON(gmap_is_shadow(gmap));
-       spin_lock(&gmap->guest_table_lock);
        pmdp = (pmd_t *) gmap_table_walk(gmap, gaddr, 1);
+       if (!pmdp)
+               return NULL;
 
-       if (!pmdp || pmd_none(*pmdp)) {
+       /* without huge pages, there is no need to take the table lock */
+       if (!gmap->mm->context.allow_gmap_hpage_1m)
+               return pmd_none(*pmdp) ? NULL : pmdp;
+
+       spin_lock(&gmap->guest_table_lock);
+       if (pmd_none(*pmdp)) {
                spin_unlock(&gmap->guest_table_lock);
                return NULL;
        }