s390/mm: better annotate 2KB pagetable fragments handling

Explicitly encode immediate value of pending remove nibble
(bits 31-28) and tracking nibble (bits 27-24) of the page
refcount whenever these nibbles are tested or changed, for
better readability. Also, add some comments describing how
the fragments are handled.
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/pgalloc.c

@@ -176,7 +176,75 @@ void page_table_free_pgste(struct page *page)
 #endif /* CONFIG_PGSTE */
 
 /*
- * page table entry allocation/free routines.
+ * A 2KB-pgtable is either upper or lower half of a normal page.
+ * The second half of the page may be unused or used as another
+ * 2KB-pgtable.
+ *
+ * Whenever possible the parent page for a new 2KB-pgtable is picked
+ * from the list of partially allocated pages mm_context_t::pgtable_list.
+ * In case the list is empty a new parent page is allocated and added to
+ * the list.
+ *
+ * When a parent page gets fully allocated it contains 2KB-pgtables in both
+ * upper and lower halves and is removed from mm_context_t::pgtable_list.
+ *
+ * When 2KB-pgtable is freed from to fully allocated parent page that
+ * page turns partially allocated and added to mm_context_t::pgtable_list.
+ *
+ * If 2KB-pgtable is freed from the partially allocated parent page that
+ * page turns unused and gets removed from mm_context_t::pgtable_list.
+ * Furthermore, the unused parent page is released.
+ *
+ * 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
+ * for tracking contained 2KB-pgtables and has the following format:
+ *
+ *   PP  AA
+ * 01234567    upper byte (bits 24-31) of struct page::_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
+ *     parent page is still unallocated;
+ *   - removed from mm_context_t::pgtable_list in case both hales of the
+ *     parent page are allocated;
+ * These operations are protected with mm_context_t::lock.
+ *
+ * When 2KB-pgtable is deallocated the corresponding AA bit is set to 0
+ * and the corresponding PP bit is set to 1 in a single atomic operation.
+ * Thus, PP and AA bits corresponding to the same 2KB-pgtable are mutually
+ * exclusive and may never be both set to 1!
+ * The parent page is either:
+ *   - added to mm_context_t::pgtable_list in case the second half of the
+ *     parent page is still allocated;
+ *   - removed from mm_context_t::pgtable_list in case the second half of
+ *     the parent page is unallocated;
+ * These operations are protected with mm_context_t::lock.
+ *
+ * It is important to understand that mm_context_t::lock only protects
+ * mm_context_t::pgtable_list and AA bits, but not the parent page itself
+ * and PP bits.
+ *
+ * Releasing the parent page happens whenever the PP bit turns from 1 to 0,
+ * while both AA bits and the second PP bit are already unset. Then the
+ * parent page does not contain any 2KB-pgtable fragment anymore, and it has
+ * also been removed from mm_context_t::pgtable_list. It is safe to release
+ * the page therefore.
+ *
+ * PGSTE memory spaces use full 4KB-pgtables and do not need most of the
+ * logic described above. Both AA bits are set to 1 to denote a 4KB-pgtable
+ * while the PP bits are never used, nor such a page is added to or removed
+ * from mm_context_t::pgtable_list.
  */
 unsigned long *page_table_alloc(struct mm_struct *mm)
 {
@@ -192,14 +260,23 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
 			page = list_first_entry(&mm->context.pgtable_list,
 						struct page, lru);
 			mask = atomic_read(&page->_refcount) >> 24;
-			mask = (mask | (mask >> 4)) & 3;
-			if (mask != 3) {
+			/*
+			 * 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.
+			 * 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
+			 * a whole bunch of races and corruptions.
+			 */
+			mask = (mask | (mask >> 4)) & 0x03U;
+			if (mask != 0x03U) {
 				table = (unsigned long *) page_to_virt(page);
 				bit = mask & 1;		/* =1 -> second 2K */
 				if (bit)
 					table += PTRS_PER_PTE;
 				atomic_xor_bits(&page->_refcount,
-							1U << (bit + 24));
+							0x01U << (bit + 24));
 				list_del(&page->lru);
 			}
 		}
@@ -220,12 +297,12 @@ 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, 3 << 24);
+		atomic_xor_bits(&page->_refcount, 0x03U << 24);
 		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, 1 << 24);
+		atomic_xor_bits(&page->_refcount, 0x01U << 24);
 		memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE);
 		spin_lock_bh(&mm->context.lock);
 		list_add(&page->lru, &mm->context.pgtable_list);
@@ -244,19 +321,24 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
 		/* Free 2K page table fragment of a 4K page */
 		bit = ((unsigned long) table & ~PAGE_MASK)/(PTRS_PER_PTE*sizeof(pte_t));
 		spin_lock_bh(&mm->context.lock);
+		/*
+		 * Mark the page for delayed release. The actual release
+		 * 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;
-		if (mask & 3)
+		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;
-		if (mask != 0)
+		if (mask != 0x00U)
 			return;
 	} else {
-		atomic_xor_bits(&page->_refcount, 3U << 24);
+		atomic_xor_bits(&page->_refcount, 0x03U << 24);
 	}
 
 	pgtable_pte_page_dtor(page);
@@ -274,43 +356,48 @@ void page_table_free_rcu(struct mmu_gather *tlb, unsigned long *table,
 	page = virt_to_page(table);
 	if (mm_alloc_pgste(mm)) {
 		gmap_unlink(mm, table, vmaddr);
-		table = (unsigned long *) ((unsigned long)table | 3);
+		table = (unsigned long *) ((unsigned long)table | 0x03U);
 		tlb_remove_table(tlb, table);
 		return;
 	}
 	bit = ((unsigned long) table & ~PAGE_MASK) / (PTRS_PER_PTE*sizeof(pte_t));
 	spin_lock_bh(&mm->context.lock);
+	/*
+	 * Mark the page for delayed release. The actual release will happen
+	 * 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;
-	if (mask & 3)
+	if (mask & 0x03U)
 		list_add_tail(&page->lru, &mm->context.pgtable_list);
 	else
 		list_del(&page->lru);
 	spin_unlock_bh(&mm->context.lock);
-	table = (unsigned long *) ((unsigned long) table | (1U << bit));
+	table = (unsigned long *) ((unsigned long) table | (0x01U << bit));
 	tlb_remove_table(tlb, table);
 }
 
 void __tlb_remove_table(void *_table)
 {
-	unsigned int mask = (unsigned long) _table & 3;
+	unsigned int mask = (unsigned long) _table & 0x03U;
 	void *table = (void *)((unsigned long) _table ^ mask);
 	struct page *page = virt_to_page(table);
 
 	switch (mask) {
-	case 0:		/* pmd, pud, or p4d */
+	case 0x00U:	/* pmd, pud, or p4d */
 		free_pages((unsigned long) table, 2);
 		break;
-	case 1:		/* lower 2K of a 4K page table */
-	case 2:		/* higher 2K of a 4K page table */
+	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;
-		if (mask != 0)
+		if (mask != 0x00U)
 			break;
 		fallthrough;
-	case 3:		/* 4K page table with pgstes */
-		if (mask & 3)
-			atomic_xor_bits(&page->_refcount, 3 << 24);
+	case 0x03U:	/* 4K page table with pgstes */
+		if (mask & 0x03U)
+			atomic_xor_bits(&page->_refcount, 0x03U << 24);
 		pgtable_pte_page_dtor(page);
 		__free_page(page);
 		break;