text: #ifndef _ASM_PGALLOC_H
  #define _ASM_PGALLOC_H
  
  #include <linux/gfp.h>
  #include <linux/mm.h>
  #include <linux/threads.h>
  #include <asm/processor.h>
  #include <asm/fixmap.h>
  
  #include <asm/cache.h>
  
  /* Allocate the top level pgd (page directory)
   *
   * Here (for 64 bit kernels) we implement a Hybrid L2/L3 scheme: we
   * allocate the first pmd adjacent to the pgd.  This means that we can
   * subtract a constant offset to get to it.  The pmd and pgd sizes are
   * arranged so that a single pmd covers 4GB (giving a full 64-bit
   * process access to 8TB) so our lookups are effectively L2 for the
   * first 4GB of the kernel (i.e. for all ILP32 processes and all the
   * kernel for machines with under 4GB of memory) */
  static inline pgd_t *pgd_alloc(struct mm_struct *mm)
  {
  	pgd_t *pgd = (pgd_t *)__get_free_pages(GFP_KERNEL,
  					       PGD_ALLOC_ORDER);
  	pgd_t *actual_pgd = pgd;
  
  	if (likely(pgd != NULL)) {
  		memset(pgd, 0, PAGE_SIZE<<PGD_ALLOC_ORDER);
  #if PT_NLEVELS == 3
  		actual_pgd += PTRS_PER_PGD;
  		/* Populate first pmd with allocated memory.  We mark it
  		 * with PxD_FLAG_ATTACHED as a signal to the system that this
  		 * pmd entry may not be cleared. */
  		__pgd_val_set(*actual_pgd, (PxD_FLAG_PRESENT | 
  				        PxD_FLAG_VALID | 
  					PxD_FLAG_ATTACHED) 
  			+ (__u32)(__pa((unsigned long)pgd) >> PxD_VALUE_SHIFT));
  		/* The first pmd entry also is marked with _PAGE_GATEWAY as
  		 * a signal that this pmd may not be freed */
  		__pgd_val_set(*pgd, PxD_FLAG_ATTACHED);
  #endif
  	}
  	return actual_pgd;
  }
  
  static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)
  {
  #if PT_NLEVELS == 3
  	pgd -= PTRS_PER_PGD;
  #endif
  	free_pages((unsigned long)pgd, PGD_ALLOC_ORDER);
  }
  
  #if PT_NLEVELS == 3
  
  /* Three Level Page Table Support for pmd's */
  
  static inline void pgd_populate(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmd)
  {
  	__pgd_val_set(*pgd, (PxD_FLAG_PRESENT | PxD_FLAG_VALID) +
  		        (__u32)(__pa((unsigned long)pmd) >> PxD_VALUE_SHIFT));
  }
  
  static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
  {
  	pmd_t *pmd = (pmd_t *)__get_free_pages(GFP_KERNEL|__GFP_REPEAT,
  					       PMD_ORDER);
  	if (pmd)
  		memset(pmd, 0, PAGE_SIZE<<PMD_ORDER);
  	return pmd;
  }
  
  static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
  {
  	if(pmd_flag(*pmd) & PxD_FLAG_ATTACHED)
  		/*
  		 * This is the permanent pmd attached to the pgd;
  		 * cannot free it.
  		 * Increment the counter to compensate for the decrement
  		 * done by generic mm code.
  		 */
  		mm_inc_nr_pmds(mm);
  		return;
  	free_pages((unsigned long)pmd, PMD_ORDER);
  }
  
  #else
  
  /* Two Level Page Table Support for pmd's */
  
  /*
   * allocating and freeing a pmd is trivial: the 1-entry pmd is
   * inside the pgd, so has no extra memory associated with it.
   */
  
  #define pmd_alloc_one(mm, addr)		({ BUG(); ((pmd_t *)2); })
  #define pmd_free(mm, x)			do { } while (0)
  #define pgd_populate(mm, pmd, pte)	BUG()
  
  #endif
  
  static inline void
  pmd_populate_kernel(struct mm_struct *mm, pmd_t *pmd, pte_t *pte)
  {
  #if PT_NLEVELS == 3
  	/* preserve the gateway marker if this is the beginning of
  	 * the permanent pmd */
  	if(pmd_flag(*pmd) & PxD_FLAG_ATTACHED)
  		__pmd_val_set(*pmd, (PxD_FLAG_PRESENT |
  				 PxD_FLAG_VALID |
  				 PxD_FLAG_ATTACHED) 
  			+ (__u32)(__pa((unsigned long)pte) >> PxD_VALUE_SHIFT));
  	else
  #endif
  		__pmd_val_set(*pmd, (PxD_FLAG_PRESENT | PxD_FLAG_VALID) 
  			+ (__u32)(__pa((unsigned long)pte) >> PxD_VALUE_SHIFT));
  }
  
  #define pmd_populate(mm, pmd, pte_page) \
  	pmd_populate_kernel(mm, pmd, page_address(pte_page))
  #define pmd_pgtable(pmd) pmd_page(pmd)
  
  static inline pgtable_t
  pte_alloc_one(struct mm_struct *mm, unsigned long address)
  {
  	struct page *page = alloc_page(GFP_KERNEL|__GFP_REPEAT|__GFP_ZERO);
  	if (!page)
  		return NULL;
  	if (!pgtable_page_ctor(page)) {
  		__free_page(page);
  		return NULL;
  	}
  	return page;
  }
  
  static inline pte_t *
  pte_alloc_one_kernel(struct mm_struct *mm, unsigned long addr)
  {
  	pte_t *pte = (pte_t *)__get_free_page(GFP_KERNEL|__GFP_REPEAT|__GFP_ZERO);
  	return pte;
  }
  
  static inline void pte_free_kernel(struct mm_struct *mm, pte_t *pte)
  {
  	free_page((unsigned long)pte);
  }
  
  static inline void pte_free(struct mm_struct *mm, struct page *pte)
  {
  	pgtable_page_dtor(pte);
  	pte_free_kernel(mm, page_address(pte));
  }
  
  #define check_pgt_cache()	do { } while (0)
  
  #endif
  