s390/mm: use full 4KB page for 2KB PTE

Cease using 4KB pages to host two 2KB PTEs. That greatly
simplifies the memory management code at the expense of
page tables memory footprint.

Instead of two PTEs per 4KB page use only upper half of
the parent page for a single PTE. With that the list of
half-used pages pgtable_list becomes unneeded.

Further, the upper byte of the parent page _refcount
counter does not need to be used for fragments tracking
and could be left alone.

Commit 8211dad6 ("s390: add pte_free_defer() for
pgtables sharing page") introduced the use of PageActive
flag to coordinate a deferred free with 2KB page table
fragments tracking. Since there is no tracking anymore,
there is no need for using PageActive flag.
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/mmu.h

@@ -11,7 +11,6 @@ typedef struct {
 	cpumask_t cpu_attach_mask;
 	atomic_t flush_count;
 	unsigned int flush_mm;
-	struct list_head pgtable_list;
 	struct list_head gmap_list;
 	unsigned long gmap_asce;
 	unsigned long asce;
@@ -39,7 +38,6 @@ typedef struct {
 
 #define INIT_MM_CONTEXT(name)						   \
 	.context.lock =	__SPIN_LOCK_UNLOCKED(name.context.lock),	   \
-	.context.pgtable_list = LIST_HEAD_INIT(name.context.pgtable_list), \
 	.context.gmap_list = LIST_HEAD_INIT(name.context.gmap_list),
 
 #endif

arch/s390/include/asm/mmu_context.h

@@ -22,7 +22,6 @@ static inline int init_new_context(struct task_struct *tsk,
 	unsigned long asce_type, init_entry;
 
 	spin_lock_init(&mm->context.lock);
-	INIT_LIST_HEAD(&mm->context.pgtable_list);
 	INIT_LIST_HEAD(&mm->context.gmap_list);
 	cpumask_clear(&mm->context.cpu_attach_mask);
 	atomic_set(&mm->context.flush_count, 0);

arch/s390/include/asm/tlb.h

@@ -69,11 +69,6 @@ static inline void pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
 	tlb->mm->context.flush_mm = 1;
 	tlb->freed_tables = 1;
 	tlb->cleared_pmds = 1;
-	/*
-	 * page_table_free_rcu takes care of the allocation bit masks
-	 * of the 2K table fragments in the 4K page table page,
-	 * then calls tlb_remove_table.
-	 */
 	page_table_free_rcu(tlb, (unsigned long *) pte, address);
 }