[PATCH v4 03/34] s390: Use pt_frag_refcount for pagetables

["Vishal Moola (Oracle)" <vishal.moola@xxxxxxxxx>]

s390 currently uses _refcount to identify fragmented page tables.
The page table struct already has a member pt_frag_refcount used by
powerpc, so have s390 use that instead of the _refcount field as well.
This improves the safety for _refcount and the page table tracking.

This also allows us to simplify the tracking since we can once again use
the lower byte of pt_frag_refcount instead of the upper byte of _refcount.

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
---
 arch/s390/mm/pgalloc.c | 38 +++++++++++++++-----------------------
 1 file changed, 15 insertions(+), 23 deletions(-)

diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
index 66ab68db9842..6b99932abc66 100644
--- a/arch/s390/mm/pgalloc.c
+++ b/arch/s390/mm/pgalloc.c
@@ -182,20 +182,17 @@ void page_table_free_pgste(struct page *page)
  * As follows from the above, no unallocated or fully allocated parent
  * pages are contained in mm_context_t::pgtable_list.
  *
- * The upper byte (bits 24-31) of the parent page _refcount is used
+ * The lower byte (bits 0-7) of the parent page pt_frag_refcount is used
  * for tracking contained 2KB-pgtables and has the following format:
  *
  *   PP  AA
- * 01234567    upper byte (bits 24-31) of struct page::_refcount
+ * 01234567    upper byte (bits 0-7) of struct page::pt_frag_refcount
  *   ||  ||
  *   ||  |+--- upper 2KB-pgtable is allocated
  *   ||  +---- lower 2KB-pgtable is allocated
  *   |+------- upper 2KB-pgtable is pending for removal
  *   +-------- lower 2KB-pgtable is pending for removal
  *
- * (See commit 620b4e903179 ("s390: use _refcount for pgtables") on why
- * using _refcount is possible).
- *
  * When 2KB-pgtable is allocated the corresponding AA bit is set to 1.
  * The parent page is either:
  *   - added to mm_context_t::pgtable_list in case the second half of the
@@ -243,11 +240,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
                if (!list_empty(&mm->context.pgtable_list)) {
                        page = list_first_entry(&mm->context.pgtable_list,
                                                struct page, lru);
-                       mask = atomic_read(&page->_refcount) >> 24;
+                       mask = atomic_read(&page->pt_frag_refcount);
                        /*
                         * The pending removal bits must also be checked.
                         * Failure to do so might lead to an impossible
-                        * value of (i.e 0x13 or 0x23) written to _refcount.
+                        * value of (i.e 0x13 or 0x23) written to
+                        * pt_frag_refcount.
                         * Such values violate the assumption that pending and
                         * allocation bits are mutually exclusive, and the rest
                         * of the code unrails as result. That could lead to
@@ -259,8 +257,8 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
                                bit = mask & 1;         /* =1 -> second 2K */
                                if (bit)
                                        table += PTRS_PER_PTE;
-                               atomic_xor_bits(&page->_refcount,
-                                                       0x01U << (bit + 24));
+                               atomic_xor_bits(&page->pt_frag_refcount,
+                                                       0x01U << bit);
                                list_del(&page->lru);
                        }
                }
@@ -281,12 +279,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
        table = (unsigned long *) page_to_virt(page);
        if (mm_alloc_pgste(mm)) {
                /* Return 4K page table with PGSTEs */
-               atomic_xor_bits(&page->_refcount, 0x03U << 24);
+               atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
                memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE);
                memset64((u64 *)table + PTRS_PER_PTE, 0, PTRS_PER_PTE);
        } else {
                /* Return the first 2K fragment of the page */
-               atomic_xor_bits(&page->_refcount, 0x01U << 24);
+               atomic_xor_bits(&page->pt_frag_refcount, 0x01U);
                memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE);
                spin_lock_bh(&mm->context.lock);
                list_add(&page->lru, &mm->context.pgtable_list);
@@ -323,22 +321,19 @@ void page_table_free(struct mm_struct *mm, unsigned long 
*table)
                 * will happen outside of the critical section from this
                 * function or from __tlb_remove_table()
                 */
-               mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
-               mask >>= 24;
+               mask = atomic_xor_bits(&page->pt_frag_refcount, 0x11U << bit);
                if (mask & 0x03U)
                        list_add(&page->lru, &mm->context.pgtable_list);
                else
                        list_del(&page->lru);
                spin_unlock_bh(&mm->context.lock);
-               mask = atomic_xor_bits(&page->_refcount, 0x10U << (bit + 24));
-               mask >>= 24;
+               mask = atomic_xor_bits(&page->pt_frag_refcount, 0x10U << bit);
                if (mask != 0x00U)
                        return;
                half = 0x01U << bit;
        } else {
                half = 0x03U;
-               mask = atomic_xor_bits(&page->_refcount, 0x03U << 24);
-               mask >>= 24;
+               mask = atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
        }
 
        page_table_release_check(page, table, half, mask);
@@ -368,8 +363,7 @@ void page_table_free_rcu(struct mmu_gather *tlb, unsigned 
long *table,
         * outside of the critical section from __tlb_remove_table() or from
         * page_table_free()
         */
-       mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
-       mask >>= 24;
+       mask = atomic_xor_bits(&page->pt_frag_refcount, 0x11U << bit);
        if (mask & 0x03U)
                list_add_tail(&page->lru, &mm->context.pgtable_list);
        else
@@ -391,14 +385,12 @@ void __tlb_remove_table(void *_table)
                return;
        case 0x01U:     /* lower 2K of a 4K page table */
        case 0x02U:     /* higher 2K of a 4K page table */
-               mask = atomic_xor_bits(&page->_refcount, mask << (4 + 24));
-               mask >>= 24;
+               mask = atomic_xor_bits(&page->pt_frag_refcount, mask << 4);
                if (mask != 0x00U)
                        return;
                break;
        case 0x03U:     /* 4K page table with pgstes */
-               mask = atomic_xor_bits(&page->_refcount, 0x03U << 24);
-               mask >>= 24;
+               mask = atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
                break;
        }
 
-- 
2.40.1