pgtable-frag.c

// SPDX-License-Identifier: GPL-2.0

/*
 *  Handling Page Tables through page fragments
 *
 */

#include <linux/kernel.h>
#include <linux/gfp.h>
#include <linux/mm.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/hugetlb.h>
#include <asm/pgalloc.h>
#include <asm/tlbflush.h>
#include <asm/tlb.h>

void pte_frag_destroy(void *pte_frag)
{
	int count;
	struct ptdesc *ptdesc;

	ptdesc = virt_to_ptdesc(pte_frag);
	/* drop all the pending references */
	count = ((unsigned long)pte_frag & ~PAGE_MASK) >> PTE_FRAG_SIZE_SHIFT;
	/* We allow PTE_FRAG_NR fragments from a PTE page */
	if (atomic_sub_and_test(PTE_FRAG_NR - count, &ptdesc->pt_frag_refcount)) {
		pagetable_dtor(ptdesc);
		pagetable_free(ptdesc);
	}
}

static pte_t *get_pte_from_cache(struct mm_struct *mm)
{
	void *pte_frag, *ret;

	if (PTE_FRAG_NR == 1)
		return NULL;

	spin_lock(&mm->page_table_lock);
	ret = pte_frag_get(&mm->context);
	if (ret) {
		pte_frag = ret + PTE_FRAG_SIZE;
		/*
		 * If we have taken up all the fragments mark PTE page NULL
		 */
		if (((unsigned long)pte_frag & ~PAGE_MASK) == 0)
			pte_frag = NULL;
		pte_frag_set(&mm->context, pte_frag);
	}
	spin_unlock(&mm->page_table_lock);
	return (pte_t *)ret;
}

static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
{
	void *ret = NULL;
	struct ptdesc *ptdesc;
	gfp_t gfp = PGALLOC_GFP;

	if (!kernel)
		gfp |= __GFP_ACCOUNT;

	ptdesc = pagetable_alloc(gfp, 0);
	if (!ptdesc)
		return NULL;
	if (!pagetable_pte_ctor(mm, ptdesc)) {
		pagetable_free(ptdesc);
		return NULL;
	}

	atomic_set(&ptdesc->pt_frag_refcount, 1);

	ret = ptdesc_address(ptdesc);
	/*
	 * if we support only one fragment just return the
	 * allocated page.
	 */
	if (PTE_FRAG_NR == 1)
		return ret;
	spin_lock(&mm->page_table_lock);
	/*
	 * If we find ptdesc_page set, we return
	 * the allocated page with single fragment
	 * count.
	 */
	if (likely(!pte_frag_get(&mm->context))) {
		atomic_set(&ptdesc->pt_frag_refcount, PTE_FRAG_NR);
		pte_frag_set(&mm->context, ret + PTE_FRAG_SIZE);
	}
	spin_unlock(&mm->page_table_lock);

	return (pte_t *)ret;
}

pte_t *pte_fragment_alloc(struct mm_struct *mm, int kernel)
{
	pte_t *pte;

	pte = get_pte_from_cache(mm);
	if (pte)
		return pte;

	return __alloc_for_ptecache(mm, kernel);
}

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

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

void pte_fragment_free(unsigned long *table, int kernel)
{
	struct ptdesc *ptdesc = virt_to_ptdesc(table);

	if (pagetable_is_reserved(ptdesc))
		return free_reserved_ptdesc(ptdesc);

	BUG_ON(atomic_read(&ptdesc->pt_frag_refcount) <= 0);
	if (atomic_dec_and_test(&ptdesc->pt_frag_refcount)) {
		if (kernel || !folio_test_clear_active(ptdesc_folio(ptdesc)))
			pte_free_now(&ptdesc->pt_rcu_head);
		else
			call_rcu(&ptdesc->pt_rcu_head, pte_free_now);
	}
}

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

	folio = virt_to_folio(pgtable);
	folio_set_active(folio);
	pte_fragment_free((unsigned long *)pgtable, 0);
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */