s390/mm/pgalloc.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  Page table allocation functions
 *
 *    Copyright IBM Corp. 2016
 *    Author(s): Martin Schwidefsky <schwidefsky@de.ibm.com>
 */

#include <linux/sysctl.h>
#include <linux/slab.h>
#include <linux/mm.h>
#include <asm/mmu_context.h>
#include <asm/pgalloc.h>
#include <asm/gmap.h>
#include <asm/tlb.h>
#include <asm/tlbflush.h>

#ifdef CONFIG_PGSTE

int page_table_allocate_pgste = 0;
EXPORT_SYMBOL(page_table_allocate_pgste);

static struct ctl_table page_table_sysctl[] = {
	{
		.procname	= "allocate_pgste",
		.data		= &page_table_allocate_pgste,
		.maxlen		= sizeof(int),
		.mode		= S_IRUGO | S_IWUSR,
		.proc_handler	= proc_dointvec_minmax,
		.extra1		= SYSCTL_ZERO,
		.extra2		= SYSCTL_ONE,
	},
	{ }
};

static int __init page_table_register_sysctl(void)
{
	return register_sysctl("vm", page_table_sysctl) ? 0 : -ENOMEM;
}
__initcall(page_table_register_sysctl);

#endif /* CONFIG_PGSTE */

unsigned long *crst_table_alloc(struct mm_struct *mm)
{
	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL, CRST_ALLOC_ORDER);

	if (!ptdesc)
		return NULL;
	arch_set_page_dat(ptdesc_page(ptdesc), CRST_ALLOC_ORDER);
	return (unsigned long *) ptdesc_to_virt(ptdesc);
}

void crst_table_free(struct mm_struct *mm, unsigned long *table)
{
	pagetable_free(virt_to_ptdesc(table));
}

static void __crst_table_upgrade(void *arg)
{
	struct mm_struct *mm = arg;

	/* change all active ASCEs to avoid the creation of new TLBs */
	if (current->active_mm == mm) {
		S390_lowcore.user_asce = mm->context.asce;
		__ctl_load(S390_lowcore.user_asce, 7, 7);
	}
	__tlb_flush_local();
}

int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
{
	unsigned long *pgd = NULL, *p4d = NULL, *__pgd;
	unsigned long asce_limit = mm->context.asce_limit;

	/* upgrade should only happen from 3 to 4, 3 to 5, or 4 to 5 levels */
	VM_BUG_ON(asce_limit < _REGION2_SIZE);

	if (end <= asce_limit)
		return 0;

	if (asce_limit == _REGION2_SIZE) {
		p4d = crst_table_alloc(mm);
		if (unlikely(!p4d))
			goto err_p4d;
		crst_table_init(p4d, _REGION2_ENTRY_EMPTY);
	}
	if (end > _REGION1_SIZE) {
		pgd = crst_table_alloc(mm);
		if (unlikely(!pgd))
			goto err_pgd;
		crst_table_init(pgd, _REGION1_ENTRY_EMPTY);
	}

	spin_lock_bh(&mm->page_table_lock);

	/*
	 * This routine gets called with mmap_lock lock held and there is
	 * no reason to optimize for the case of otherwise. However, if
	 * that would ever change, the below check will let us know.
	 */
	VM_BUG_ON(asce_limit != mm->context.asce_limit);

	if (p4d) {
		__pgd = (unsigned long *) mm->pgd;
		p4d_populate(mm, (p4d_t *) p4d, (pud_t *) __pgd);
		mm->pgd = (pgd_t *) p4d;
		mm->context.asce_limit = _REGION1_SIZE;
		mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
			_ASCE_USER_BITS | _ASCE_TYPE_REGION2;
		mm_inc_nr_puds(mm);
	}
	if (pgd) {
		__pgd = (unsigned long *) mm->pgd;
		pgd_populate(mm, (pgd_t *) pgd, (p4d_t *) __pgd);
		mm->pgd = (pgd_t *) pgd;
		mm->context.asce_limit = TASK_SIZE_MAX;
		mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
			_ASCE_USER_BITS | _ASCE_TYPE_REGION1;
	}

	spin_unlock_bh(&mm->page_table_lock);

	on_each_cpu(__crst_table_upgrade, mm, 0);

	return 0;

err_pgd:
	crst_table_free(mm, p4d);
err_p4d:
	return -ENOMEM;
}

static inline unsigned int atomic_xor_bits(atomic_t *v, unsigned int bits)
{
	return atomic_fetch_xor(bits, v) ^ bits;
}

#ifdef CONFIG_PGSTE

struct page *page_table_alloc_pgste(struct mm_struct *mm)
{
	struct ptdesc *ptdesc;
	u64 *table;

	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
	if (ptdesc) {
		table = (u64 *)ptdesc_to_virt(ptdesc);
		memset64(table, _PAGE_INVALID, PTRS_PER_PTE);
		memset64(table + PTRS_PER_PTE, 0, PTRS_PER_PTE);
	}
	return ptdesc_page(ptdesc);
}

void page_table_free_pgste(struct page *page)
{
	pagetable_free(page_ptdesc(page));
}

#endif /* CONFIG_PGSTE */

/*
 * 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.
 *
 * pte_free_defer() overrides those rules: it takes the page off pgtable_list,
 * and prevents both 2K fragments from being reused. pte_free_defer() has to
 * guarantee that its pgtable cannot be reused before the RCU grace period
 * has elapsed (which page_table_free_rcu() does not actually guarantee).
 * But for simplicity, because page->rcu_head overlays page->lru, and because
 * the RCU callback might not be called before the mm_context_t has been freed,
 * pte_free_defer() in this implementation prevents both fragments from being
 * reused, and delays making the call to RCU until both fragments are freed.
 */
unsigned long *page_table_alloc(struct mm_struct *mm)
{
	unsigned long *table;
	struct ptdesc *ptdesc;
	unsigned int mask, bit;

	/* Try to get a fragment of a 4K page as a 2K page table */
	if (!mm_alloc_pgste(mm)) {
		table = NULL;
		spin_lock_bh(&mm->context.lock);
		if (!list_empty(&mm->context.pgtable_list)) {
			ptdesc = list_first_entry(&mm->context.pgtable_list,
						struct ptdesc, pt_list);
			mask = atomic_read(&ptdesc->_refcount) >> 24;
			/*
			 * 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 *) ptdesc_to_virt(ptdesc);
				bit = mask & 1;		/* =1 -> second 2K */
				if (bit)
					table += PTRS_PER_PTE;
				atomic_xor_bits(&ptdesc->_refcount,
							0x01U << (bit + 24));
				list_del_init(&ptdesc->pt_list);
			}
		}
		spin_unlock_bh(&mm->context.lock);
		if (table)
			return table;
	}
	/* Allocate a fresh page */
	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
	if (!ptdesc)
		return NULL;
	if (!pagetable_pte_ctor(ptdesc)) {
		pagetable_free(ptdesc);
		return NULL;
	}
	arch_set_page_dat(ptdesc_page(ptdesc), 0);
	/* Initialize page table */
	table = (unsigned long *) ptdesc_to_virt(ptdesc);
	if (mm_alloc_pgste(mm)) {
		/* Return 4K page table with PGSTEs */
		INIT_LIST_HEAD(&ptdesc->pt_list);
		atomic_xor_bits(&ptdesc->_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(&ptdesc->_refcount, 0x01U << 24);
		memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE);
		spin_lock_bh(&mm->context.lock);
		list_add(&ptdesc->pt_list, &mm->context.pgtable_list);
		spin_unlock_bh(&mm->context.lock);
	}
	return table;
}

static void page_table_release_check(struct page *page, void *table,
				     unsigned int half, unsigned int mask)
{
	char msg[128];

	if (!IS_ENABLED(CONFIG_DEBUG_VM))
		return;
	if (!mask && list_empty(&page->lru))
		return;
	snprintf(msg, sizeof(msg),
		 "Invalid pgtable %p release half 0x%02x mask 0x%02x",
		 table, half, mask);
	dump_page(page, msg);
}

static void pte_free_now(struct rcu_head *head)
{
	struct ptdesc *ptdesc;

	ptdesc = container_of(head, struct ptdesc, pt_rcu_head);
	pagetable_pte_dtor(ptdesc);
	pagetable_free(ptdesc);
}

void page_table_free(struct mm_struct *mm, unsigned long *table)
{
	unsigned int mask, bit, half;
	struct ptdesc *ptdesc = virt_to_ptdesc(table);

	if (!mm_alloc_pgste(mm)) {
		/* 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(&ptdesc->_refcount, 0x11U << (bit + 24));
		mask >>= 24;
		if ((mask & 0x03U) && !folio_test_active(ptdesc_folio(ptdesc))) {
			/*
			 * Other half is allocated, and neither half has had
			 * its free deferred: add page to head of list, to make
			 * this freed half available for immediate reuse.
			 */
			list_add(&ptdesc->pt_list, &mm->context.pgtable_list);
		} else {
			/* If page is on list, now remove it. */
			list_del_init(&ptdesc->pt_list);
		}
		spin_unlock_bh(&mm->context.lock);
		mask = atomic_xor_bits(&ptdesc->_refcount, 0x10U << (bit + 24));
		mask >>= 24;
		if (mask != 0x00U)
			return;
		half = 0x01U << bit;
	} else {
		half = 0x03U;
		mask = atomic_xor_bits(&ptdesc->_refcount, 0x03U << 24);
		mask >>= 24;
	}

	page_table_release_check(ptdesc_page(ptdesc), table, half, mask);
	if (folio_test_clear_active(ptdesc_folio(ptdesc)))
		call_rcu(&ptdesc->pt_rcu_head, pte_free_now);
	else
		pte_free_now(&ptdesc->pt_rcu_head);
}

void page_table_free_rcu(struct mmu_gather *tlb, unsigned long *table,
			 unsigned long vmaddr)
{
	struct mm_struct *mm;
	unsigned int bit, mask;
	struct ptdesc *ptdesc = virt_to_ptdesc(table);

	mm = tlb->mm;
	if (mm_alloc_pgste(mm)) {
		gmap_unlink(mm, table, vmaddr);
		table = (unsigned long *) ((unsigned long)table | 0x03U);
		tlb_remove_ptdesc(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(&ptdesc->_refcount, 0x11U << (bit + 24));
	mask >>= 24;
	if ((mask & 0x03U) && !folio_test_active(ptdesc_folio(ptdesc))) {
		/*
		 * Other half is allocated, and neither half has had
		 * its free deferred: add page to end of list, to make
		 * this freed half available for reuse once its pending
		 * bit has been cleared by __tlb_remove_table().
		 */
		list_add_tail(&ptdesc->pt_list, &mm->context.pgtable_list);
	} else {
		/* If page is on list, now remove it. */
		list_del_init(&ptdesc->pt_list);
	}
	spin_unlock_bh(&mm->context.lock);
	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 & 0x03U, half = mask;
	void *table = (void *)((unsigned long) _table ^ mask);
	struct ptdesc *ptdesc = virt_to_ptdesc(table);

	switch (half) {
	case 0x00U:	/* pmd, pud, or p4d */
		pagetable_free(ptdesc);
		return;
	case 0x01U:	/* lower 2K of a 4K page table */
	case 0x02U:	/* higher 2K of a 4K page table */
		mask = atomic_xor_bits(&ptdesc->_refcount, mask << (4 + 24));
		mask >>= 24;
		if (mask != 0x00U)
			return;
		break;
	case 0x03U:	/* 4K page table with pgstes */
		mask = atomic_xor_bits(&ptdesc->_refcount, 0x03U << 24);
		mask >>= 24;
		break;
	}

	page_table_release_check(ptdesc_page(ptdesc), table, half, mask);
	if (folio_test_clear_active(ptdesc_folio(ptdesc)))
		call_rcu(&ptdesc->pt_rcu_head, pte_free_now);
	else
		pte_free_now(&ptdesc->pt_rcu_head);
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
{
	struct page *page;

	page = virt_to_page(pgtable);
	SetPageActive(page);
	page_table_free(mm, (unsigned long *)pgtable);
	/*
	 * page_table_free() does not do the pgste gmap_unlink() which
	 * page_table_free_rcu() does: warn us if pgste ever reaches here.
	 */
	WARN_ON_ONCE(mm_has_pgste(mm));
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

/*
 * Base infrastructure required to generate basic asces, region, segment,
 * and page tables that do not make use of enhanced features like EDAT1.
 */

static struct kmem_cache *base_pgt_cache;

static unsigned long *base_pgt_alloc(void)
{
	unsigned long *table;

	table = kmem_cache_alloc(base_pgt_cache, GFP_KERNEL);
	if (table)
		memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE);
	return table;
}

static void base_pgt_free(unsigned long *table)
{
	kmem_cache_free(base_pgt_cache, table);
}

static unsigned long *base_crst_alloc(unsigned long val)
{
	unsigned long *table;
	struct ptdesc *ptdesc;

	ptdesc = pagetable_alloc(GFP_KERNEL & ~__GFP_HIGHMEM, CRST_ALLOC_ORDER);
	if (!ptdesc)
		return NULL;
	table = ptdesc_address(ptdesc);

	crst_table_init(table, val);
	return table;
}

static void base_crst_free(unsigned long *table)
{
	pagetable_free(virt_to_ptdesc(table));
}

#define BASE_ADDR_END_FUNC(NAME, SIZE)					\
static inline unsigned long base_##NAME##_addr_end(unsigned long addr,	\
						   unsigned long end)	\
{									\
	unsigned long next = (addr + (SIZE)) & ~((SIZE) - 1);		\
									\
	return (next - 1) < (end - 1) ? next : end;			\
}

BASE_ADDR_END_FUNC(page,    _PAGE_SIZE)
BASE_ADDR_END_FUNC(segment, _SEGMENT_SIZE)
BASE_ADDR_END_FUNC(region3, _REGION3_SIZE)
BASE_ADDR_END_FUNC(region2, _REGION2_SIZE)
BASE_ADDR_END_FUNC(region1, _REGION1_SIZE)

static inline unsigned long base_lra(unsigned long address)
{
	unsigned long real;

	asm volatile(
		"	lra	%0,0(%1)\n"
		: "=d" (real) : "a" (address) : "cc");
	return real;
}

static int base_page_walk(unsigned long *origin, unsigned long addr,
			  unsigned long end, int alloc)
{
	unsigned long *pte, next;

	if (!alloc)
		return 0;
	pte = origin;
	pte += (addr & _PAGE_INDEX) >> _PAGE_SHIFT;
	do {
		next = base_page_addr_end(addr, end);
		*pte = base_lra(addr);
	} while (pte++, addr = next, addr < end);
	return 0;
}

static int base_segment_walk(unsigned long *origin, unsigned long addr,
			     unsigned long end, int alloc)
{
	unsigned long *ste, next, *table;
	int rc;

	ste = origin;
	ste += (addr & _SEGMENT_INDEX) >> _SEGMENT_SHIFT;
	do {
		next = base_segment_addr_end(addr, end);
		if (*ste & _SEGMENT_ENTRY_INVALID) {
			if (!alloc)
				continue;
			table = base_pgt_alloc();
			if (!table)
				return -ENOMEM;
			*ste = __pa(table) | _SEGMENT_ENTRY;
		}
		table = __va(*ste & _SEGMENT_ENTRY_ORIGIN);
		rc = base_page_walk(table, addr, next, alloc);
		if (rc)
			return rc;
		if (!alloc)
			base_pgt_free(table);
		cond_resched();
	} while (ste++, addr = next, addr < end);
	return 0;
}

static int base_region3_walk(unsigned long *origin, unsigned long addr,
			     unsigned long end, int alloc)
{
	unsigned long *rtte, next, *table;
	int rc;

	rtte = origin;
	rtte += (addr & _REGION3_INDEX) >> _REGION3_SHIFT;
	do {
		next = base_region3_addr_end(addr, end);
		if (*rtte & _REGION_ENTRY_INVALID) {
			if (!alloc)
				continue;
			table = base_crst_alloc(_SEGMENT_ENTRY_EMPTY);
			if (!table)
				return -ENOMEM;
			*rtte = __pa(table) | _REGION3_ENTRY;
		}
		table = __va(*rtte & _REGION_ENTRY_ORIGIN);
		rc = base_segment_walk(table, addr, next, alloc);
		if (rc)
			return rc;
		if (!alloc)
			base_crst_free(table);
	} while (rtte++, addr = next, addr < end);
	return 0;
}

static int base_region2_walk(unsigned long *origin, unsigned long addr,
			     unsigned long end, int alloc)
{
	unsigned long *rste, next, *table;
	int rc;

	rste = origin;
	rste += (addr & _REGION2_INDEX) >> _REGION2_SHIFT;
	do {
		next = base_region2_addr_end(addr, end);
		if (*rste & _REGION_ENTRY_INVALID) {
			if (!alloc)
				continue;
			table = base_crst_alloc(_REGION3_ENTRY_EMPTY);
			if (!table)
				return -ENOMEM;
			*rste = __pa(table) | _REGION2_ENTRY;
		}
		table = __va(*rste & _REGION_ENTRY_ORIGIN);
		rc = base_region3_walk(table, addr, next, alloc);
		if (rc)
			return rc;
		if (!alloc)
			base_crst_free(table);
	} while (rste++, addr = next, addr < end);
	return 0;
}

static int base_region1_walk(unsigned long *origin, unsigned long addr,
			     unsigned long end, int alloc)
{
	unsigned long *rfte, next, *table;
	int rc;

	rfte = origin;
	rfte += (addr & _REGION1_INDEX) >> _REGION1_SHIFT;
	do {
		next = base_region1_addr_end(addr, end);
		if (*rfte & _REGION_ENTRY_INVALID) {
			if (!alloc)
				continue;
			table = base_crst_alloc(_REGION2_ENTRY_EMPTY);
			if (!table)
				return -ENOMEM;
			*rfte = __pa(table) | _REGION1_ENTRY;
		}
		table = __va(*rfte & _REGION_ENTRY_ORIGIN);
		rc = base_region2_walk(table, addr, next, alloc);
		if (rc)
			return rc;
		if (!alloc)
			base_crst_free(table);
	} while (rfte++, addr = next, addr < end);
	return 0;
}

/**
 * base_asce_free - free asce and tables returned from base_asce_alloc()
 * @asce: asce to be freed
 *
 * Frees all region, segment, and page tables that were allocated with a
 * corresponding base_asce_alloc() call.
 */
void base_asce_free(unsigned long asce)
{
	unsigned long *table = __va(asce & _ASCE_ORIGIN);

	if (!asce)
		return;
	switch (asce & _ASCE_TYPE_MASK) {
	case _ASCE_TYPE_SEGMENT:
		base_segment_walk(table, 0, _REGION3_SIZE, 0);
		break;
	case _ASCE_TYPE_REGION3:
		base_region3_walk(table, 0, _REGION2_SIZE, 0);
		break;
	case _ASCE_TYPE_REGION2:
		base_region2_walk(table, 0, _REGION1_SIZE, 0);
		break;
	case _ASCE_TYPE_REGION1:
		base_region1_walk(table, 0, TASK_SIZE_MAX, 0);
		break;
	}
	base_crst_free(table);
}

static int base_pgt_cache_init(void)
{
	static DEFINE_MUTEX(base_pgt_cache_mutex);
	unsigned long sz = _PAGE_TABLE_SIZE;

	if (base_pgt_cache)
		return 0;
	mutex_lock(&base_pgt_cache_mutex);
	if (!base_pgt_cache)
		base_pgt_cache = kmem_cache_create("base_pgt", sz, sz, 0, NULL);
	mutex_unlock(&base_pgt_cache_mutex);
	return base_pgt_cache ? 0 : -ENOMEM;
}

/**
 * base_asce_alloc - create kernel mapping without enhanced DAT features
 * @addr: virtual start address of kernel mapping
 * @num_pages: number of consecutive pages
 *
 * Generate an asce, including all required region, segment and page tables,
 * that can be used to access the virtual kernel mapping. The difference is
 * that the returned asce does not make use of any enhanced DAT features like
 * e.g. large pages. This is required for some I/O functions that pass an
 * asce, like e.g. some service call requests.
 *
 * Note: the returned asce may NEVER be attached to any cpu. It may only be
 *	 used for I/O requests. tlb entries that might result because the
 *	 asce was attached to a cpu won't be cleared.
 */
unsigned long base_asce_alloc(unsigned long addr, unsigned long num_pages)
{
	unsigned long asce, *table, end;
	int rc;

	if (base_pgt_cache_init())
		return 0;
	end = addr + num_pages * PAGE_SIZE;
	if (end <= _REGION3_SIZE) {
		table = base_crst_alloc(_SEGMENT_ENTRY_EMPTY);
		if (!table)
			return 0;
		rc = base_segment_walk(table, addr, end, 1);
		asce = __pa(table) | _ASCE_TYPE_SEGMENT | _ASCE_TABLE_LENGTH;
	} else if (end <= _REGION2_SIZE) {
		table = base_crst_alloc(_REGION3_ENTRY_EMPTY);
		if (!table)
			return 0;
		rc = base_region3_walk(table, addr, end, 1);
		asce = __pa(table) | _ASCE_TYPE_REGION3 | _ASCE_TABLE_LENGTH;
	} else if (end <= _REGION1_SIZE) {
		table = base_crst_alloc(_REGION2_ENTRY_EMPTY);
		if (!table)
			return 0;
		rc = base_region2_walk(table, addr, end, 1);
		asce = __pa(table) | _ASCE_TYPE_REGION2 | _ASCE_TABLE_LENGTH;
	} else {
		table = base_crst_alloc(_REGION1_ENTRY_EMPTY);
		if (!table)
			return 0;
		rc = base_region1_walk(table, addr, end, 1);
		asce = __pa(table) | _ASCE_TYPE_REGION1 | _ASCE_TABLE_LENGTH;
	}
	if (rc) {
		base_asce_free(asce);
		asce = 0;
	}
	return asce;
}