linux/arch/s390/mm/gmap_helpers.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  Helper functions for KVM guest address space mapping code
 *
 *    Copyright IBM Corp. 2007, 2025
 */

#include <linux/export.h>
#include <linux/mm_types.h>
#include <linux/mmap_lock.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/swap.h>
#include <linux/leafops.h>
#include <linux/pagewalk.h>
#include <linux/ksm.h>
#include <asm/gmap_helpers.h>

/**
 * try_get_locked_pte() - like get_locked_pte(), but atomic and with trylock
 * @mm: the mm
 * @vmaddr: the userspace virtual address whose pte is to be found
 * @ptl: will be set to the pointer to the lock used to lock the pte in case
 *       of success.
 *
 * This function returns the pointer to the pte corresponding to @addr in @mm,
 * similarly to get_locked_pte(). Unlike get_locked_pte(), no attempt is made
 * to allocate missing page tables. If a missing or large entry is found, the
 * function will return NULL. If the ptl lock is contended, %-EAGAIN is
 * returned.
 *
 * In case of success, *@ptl will point to the locked pte lock for the returned
 * pte, like get_locked_pte() does.
 *
 * Context: mmap_lock or vma lock for read or for write needs to be held.
 * Return:
 * * %NULL if the pte cannot be reached.
 * * %-EAGAIN if the pte can be reached, but cannot be locked.
 * * the pointer to the pte corresponding to @addr in @mm, if it can be reached
 *   and locked.
 */
pte_t *try_get_locked_pte(struct mm_struct *mm, unsigned long vmaddr, spinlock_t **ptl)
{
	pmd_t *pmdp, pmd, pmdval;
	pud_t *pudp, pud;
	p4d_t *p4dp, p4d;
	pgd_t *pgdp, pgd;
	pte_t *ptep;

	pgdp = pgd_offset(mm, vmaddr);
	pgd = pgdp_get(pgdp);
	if (pgd_none(pgd) || !pgd_present(pgd))
		return NULL;
	p4dp = p4d_offset_lockless(pgdp, pgd, vmaddr);
	p4d = p4dp_get(p4dp);
	if (p4d_none(p4d) || !p4d_present(p4d))
		return NULL;
	pudp = pud_offset_lockless(p4dp, p4d, vmaddr);
	pud = pudp_get(pudp);
	if (pud_none(pud) || pud_leaf(pud) || !pud_present(pud))
		return NULL;
	pmdp = pmd_offset_lockless(pudp, pud, vmaddr);
	pmd = pmdp_get_lockless(pmdp);
	if (pmd_none(pmd) || pmd_leaf(pmd) || !pmd_present(pmd))
		return NULL;
	ptep = pte_offset_map_rw_nolock(mm, pmdp, vmaddr, &pmdval, ptl);
	if (!ptep)
		return NULL;

	if (spin_trylock(*ptl)) {
		if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmdp)))) {
			pte_unmap_unlock(ptep, *ptl);
			return ERR_PTR(-EAGAIN);
		}
		return ptep;
	}

	pte_unmap(ptep);
	return ERR_PTR(-EAGAIN);
}
EXPORT_SYMBOL_GPL(try_get_locked_pte);

/**
 * gmap_helper_zap_one_page() - discard a page if it was swapped.
 * @mm: the mm
 * @vmaddr: the userspace virtual address that needs to be discarded
 *
 * If the given address maps to a swap entry, discard it.
 *
 * Context: needs to be called while holding the mmap lock.
 */
void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr)
{
	struct vm_area_struct *vma;
	spinlock_t *ptl;	/* Lock for the host (userspace) page table */
	softleaf_t sl;
	pte_t *ptep;

	mmap_assert_locked(mm);

	/* Find the vm address for the guest address */
	vma = vma_lookup(mm, vmaddr);
	if (!vma || is_vm_hugetlb_page(vma))
		return;

	/* Get pointer to the page table entry */
	ptep = try_get_locked_pte(mm, vmaddr, &ptl);
	if (IS_ERR_OR_NULL(ptep))
		return;
	sl = softleaf_from_pte(*ptep);
	if (pte_swap(*ptep) && softleaf_is_swap(sl)) {
		dec_mm_counter(mm, MM_SWAPENTS);
		swap_put_entries_direct(sl, 1);
		pte_clear(mm, vmaddr, ptep);
	}
	pte_unmap_unlock(ptep, ptl);
}
EXPORT_SYMBOL_GPL(gmap_helper_zap_one_page);

/**
 * gmap_helper_discard() - discard user pages in the given range
 * @mm: the mm
 * @vmaddr: starting userspace address
 * @end: end address (first address outside the range)
 *
 * All userpace pages in the range [@vamddr, @end) are discarded and unmapped.
 *
 * Context: needs to be called while holding the mmap lock.
 */
void gmap_helper_discard(struct mm_struct *mm, unsigned long vmaddr, unsigned long end)
{
	struct vm_area_struct *vma;

	mmap_assert_locked(mm);

	while (vmaddr < end) {
		vma = find_vma_intersection(mm, vmaddr, end);
		if (!vma)
			return;
		if (!is_vm_hugetlb_page(vma))
			zap_vma_range(vma, vmaddr, min(end, vma->vm_end) - vmaddr);
		vmaddr = vma->vm_end;
	}
}
EXPORT_SYMBOL_GPL(gmap_helper_discard);

/**
 * gmap_helper_try_set_pte_unused() - mark a pte entry as unused
 * @mm: the mm
 * @vmaddr: the userspace address whose pte is to be marked
 *
 * Mark the pte corresponding the given address as unused. This will cause
 * core mm code to just drop this page instead of swapping it.
 *
 * This function needs to be called with interrupts disabled (for example
 * while holding a spinlock), or while holding the mmap lock. Normally this
 * function is called as a result of an unmap operation, and thus KVM common
 * code will already hold kvm->mmu_lock in write mode.
 *
 * Context: Needs to be called while holding the mmap lock or with interrupts
 *          disabled.
 */
void gmap_helper_try_set_pte_unused(struct mm_struct *mm, unsigned long vmaddr)
{
	spinlock_t *ptl;	/* Lock for the host (userspace) page table */
	pte_t *ptep;

	/*
	 * Several paths exists that takes the ptl lock and then call the
	 * mmu_notifier, which takes the mmu_lock. The unmap path, instead,
	 * takes the mmu_lock in write mode first, and then potentially
	 * calls this function, which takes the ptl lock. This can lead to a
	 * deadlock.
	 * The unused page mechanism is only an optimization, if the
	 * _PAGE_UNUSED bit is not set, the unused page is swapped as normal
	 * instead of being discarded.
	 * If the lock is contended the bit is not set and the deadlock is
	 * avoided.
	 */
	ptep = try_get_locked_pte(mm, vmaddr, &ptl);
	if (IS_ERR_OR_NULL(ptep))
		return;

	if (pte_present(*ptep))
		__atomic64_or(_PAGE_UNUSED, (long *)ptep);
	pte_unmap_unlock(ptep, ptl);
}
EXPORT_SYMBOL_GPL(gmap_helper_try_set_pte_unused);

static int find_zeropage_pte_entry(pte_t *pte, unsigned long addr,
				   unsigned long end, struct mm_walk *walk)
{
	unsigned long *found_addr = walk->private;

	/* Return 1 of the page is a zeropage. */
	if (is_zero_pfn(pte_pfn(*pte))) {
		/*
		 * Shared zeropage in e.g., a FS DAX mapping? We cannot do the
		 * right thing and likely don't care: FAULT_FLAG_UNSHARE
		 * currently only works in COW mappings, which is also where
		 * mm_forbids_zeropage() is checked.
		 */
		if (!is_cow_mapping(walk->vma->vm_flags))
			return -EFAULT;

		*found_addr = addr;
		return 1;
	}
	return 0;
}

static const struct mm_walk_ops find_zeropage_ops = {
	.pte_entry      = find_zeropage_pte_entry,
	.walk_lock      = PGWALK_WRLOCK,
};

/** __gmap_helper_unshare_zeropages() - unshare all shared zeropages
 * @mm: the mm whose zero pages are to be unshared
 *
 * Unshare all shared zeropages, replacing them by anonymous pages. Note that
 * we cannot simply zap all shared zeropages, because this could later
 * trigger unexpected userfaultfd missing events.
 *
 * This must be called after mm->context.allow_cow_sharing was
 * set to 0, to avoid future mappings of shared zeropages.
 *
 * mm contracts with s390, that even if mm were to remove a page table,
 * and racing with walk_page_range_vma() calling pte_offset_map_lock()
 * would fail, it will never insert a page table containing empty zero
 * pages once mm_forbids_zeropage(mm) i.e.
 * mm->context.allow_cow_sharing is set to 0.
 */
static int __gmap_helper_unshare_zeropages(struct mm_struct *mm)
{
	struct vm_area_struct *vma;
	VMA_ITERATOR(vmi, mm, 0);
	unsigned long addr;
	vm_fault_t fault;
	int rc;

	for_each_vma(vmi, vma) {
		/*
		 * We could only look at COW mappings, but it's more future
		 * proof to catch unexpected zeropages in other mappings and
		 * fail.
		 */
		if ((vma->vm_flags & VM_PFNMAP) || is_vm_hugetlb_page(vma))
			continue;
		addr = vma->vm_start;

retry:
		rc = walk_page_range_vma(vma, addr, vma->vm_end,
					 &find_zeropage_ops, &addr);
		if (rc < 0)
			return rc;
		else if (!rc)
			continue;

		/* addr was updated by find_zeropage_pte_entry() */
		fault = handle_mm_fault(vma, addr,
					FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
					NULL);
		if (fault & VM_FAULT_OOM)
			return -ENOMEM;
		/*
		 * See break_ksm(): even after handle_mm_fault() returned 0, we
		 * must start the lookup from the current address, because
		 * handle_mm_fault() may back out if there's any difficulty.
		 *
		 * VM_FAULT_SIGBUS and VM_FAULT_SIGSEGV are unexpected but
		 * maybe they could trigger in the future on concurrent
		 * truncation. In that case, the shared zeropage would be gone
		 * and we can simply retry and make progress.
		 */
		cond_resched();
		goto retry;
	}

	return 0;
}

/**
 * gmap_helper_disable_cow_sharing() - disable all COW sharing
 *
 * Disable most COW-sharing of memory pages for the whole process:
 * (1) Disable KSM and unmerge/unshare any KSM pages.
 * (2) Disallow shared zeropages and unshare any zerpages that are mapped.
 *
 * Not that we currently don't bother with COW-shared pages that are shared
 * with parent/child processes due to fork().
 */
int gmap_helper_disable_cow_sharing(void)
{
	struct mm_struct *mm = current->mm;
	int rc;

	mmap_assert_write_locked(mm);

	if (!mm->context.allow_cow_sharing)
		return 0;

	mm->context.allow_cow_sharing = 0;

	/* Replace all shared zeropages by anonymous pages. */
	rc = __gmap_helper_unshare_zeropages(mm);
	/*
	 * Make sure to disable KSM (if enabled for the whole process or
	 * individual VMAs). Note that nothing currently hinders user space
	 * from re-enabling it.
	 */
	if (!rc)
		rc = ksm_disable(mm);
	if (rc)
		mm->context.allow_cow_sharing = 1;
	return rc;
}
EXPORT_SYMBOL_GPL(gmap_helper_disable_cow_sharing);