mremap.c

// SPDX-License-Identifier: GPL-2.0
/*
 *	mm/mremap.c
 *
 *	(C) Copyright 1996 Linus Torvalds
 *
 *	Address space accounting code	<alan@lxorguk.ukuu.org.uk>
 *	(C) Copyright 2002 Red Hat Inc, All Rights Reserved
 */

#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/shm.h>
#include <linux/ksm.h>
#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/capability.h>
#include <linux/fs.h>
#include <linux/swapops.h>
#include <linux/highmem.h>
#include <linux/security.h>
#include <linux/syscalls.h>
#include <linux/mmu_notifier.h>
#include <linux/uaccess.h>
#include <linux/mm-arch-hooks.h>
#include <linux/userfaultfd_k.h>

#include <asm/cacheflush.h>
#include <asm/tlbflush.h>

#include "internal.h"

static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
{
	pgd_t *pgd;
	p4d_t *p4d;
	pud_t *pud;
	pmd_t *pmd;

	pgd = pgd_offset(mm, addr);
	if (pgd_none_or_clear_bad(pgd))
		return NULL;

	p4d = p4d_offset(pgd, addr);
	if (p4d_none_or_clear_bad(p4d))
		return NULL;

	pud = pud_offset(p4d, addr);
	if (pud_none_or_clear_bad(pud))
		return NULL;

	pmd = pmd_offset(pud, addr);
	if (pmd_none(*pmd))
		return NULL;

	return pmd;
}

static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
			    unsigned long addr)
{
	pgd_t *pgd;
	p4d_t *p4d;
	pud_t *pud;
	pmd_t *pmd;

	pgd = pgd_offset(mm, addr);
	p4d = p4d_alloc(mm, pgd, addr);
	if (!p4d)
		return NULL;
	pud = pud_alloc(mm, p4d, addr);
	if (!pud)
		return NULL;

	pmd = pmd_alloc(mm, pud, addr);
	if (!pmd)
		return NULL;

	VM_BUG_ON(pmd_trans_huge(*pmd));

	return pmd;
}

static void take_rmap_locks(struct vm_area_struct *vma)
{
	if (vma->vm_file)
		i_mmap_lock_write(vma->vm_file->f_mapping);
	if (vma->anon_vma)
		anon_vma_lock_write(vma->anon_vma);
}

static void drop_rmap_locks(struct vm_area_struct *vma)
{
	if (vma->anon_vma)
		anon_vma_unlock_write(vma->anon_vma);
	if (vma->vm_file)
		i_mmap_unlock_write(vma->vm_file->f_mapping);
}

static pte_t move_soft_dirty_pte(pte_t pte)
{
	/*
	 * Set soft dirty bit so we can notice
	 * in userspace the ptes were moved.
	 */
#ifdef CONFIG_MEM_SOFT_DIRTY
	if (pte_present(pte))
		pte = pte_mksoft_dirty(pte);
	else if (is_swap_pte(pte))
		pte = pte_swp_mksoft_dirty(pte);
#endif
	return pte;
}

static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
		unsigned long old_addr, unsigned long old_end,
		struct vm_area_struct *new_vma, pmd_t *new_pmd,
		unsigned long new_addr, bool need_rmap_locks)
{
	struct mm_struct *mm = vma->vm_mm;
	pte_t *old_pte, *new_pte, pte;
	spinlock_t *old_ptl, *new_ptl;
	bool force_flush = false;
	unsigned long len = old_end - old_addr;

	/*
	 * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
	 * locks to ensure that rmap will always observe either the old or the
	 * new ptes. This is the easiest way to avoid races with
	 * truncate_pagecache(), page migration, etc...
	 *
	 * When need_rmap_locks is false, we use other ways to avoid
	 * such races:
	 *
	 * - During exec() shift_arg_pages(), we use a specially tagged vma
	 *   which rmap call sites look for using vma_is_temporary_stack().
	 *
	 * - During mremap(), new_vma is often known to be placed after vma
	 *   in rmap traversal order. This ensures rmap will always observe
	 *   either the old pte, or the new pte, or both (the page table locks
	 *   serialize access to individual ptes, but only rmap traversal
	 *   order guarantees that we won't miss both the old and new ptes).
	 */
	if (need_rmap_locks)
		take_rmap_locks(vma);

	/*
	 * We don't have to worry about the ordering of src and dst
	 * pte locks because exclusive mmap_sem prevents deadlock.
	 */
	old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
	new_pte = pte_offset_map(new_pmd, new_addr);
	new_ptl = pte_lockptr(mm, new_pmd);
	if (new_ptl != old_ptl)
		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
	flush_tlb_batched_pending(vma->vm_mm);
	arch_enter_lazy_mmu_mode();

	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
				   new_pte++, new_addr += PAGE_SIZE) {
		if (pte_none(*old_pte))
			continue;

		pte = ptep_get_and_clear(mm, old_addr, old_pte);
		/*
		 * If we are remapping a valid PTE, make sure
		 * to flush TLB before we drop the PTL for the
		 * PTE.
		 *
		 * NOTE! Both old and new PTL matter: the old one
		 * for racing with page_mkclean(), the new one to
		 * make sure the physical page stays valid until
		 * the TLB entry for the old mapping has been
		 * flushed.
		 */
		if (pte_present(pte))
			force_flush = true;
		pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
		pte = move_soft_dirty_pte(pte);
		set_pte_at(mm, new_addr, new_pte, pte);
	}

	arch_leave_lazy_mmu_mode();
	if (force_flush)
		flush_tlb_range(vma, old_end - len, old_end);
	if (new_ptl != old_ptl)
		spin_unlock(new_ptl);
	pte_unmap(new_pte - 1);
	pte_unmap_unlock(old_pte - 1, old_ptl);
	if (need_rmap_locks)
		drop_rmap_locks(vma);
}

#ifdef CONFIG_HAVE_MOVE_PMD
static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
		  unsigned long new_addr, unsigned long old_end,
		  pmd_t *old_pmd, pmd_t *new_pmd)
{
	spinlock_t *old_ptl, *new_ptl;
	struct mm_struct *mm = vma->vm_mm;
	pmd_t pmd;

	if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK)
	    || old_end - old_addr < PMD_SIZE)
		return false;

	/*
	 * The destination pmd shouldn't be established, free_pgtables()
	 * should have release it.
	 */
	if (WARN_ON(!pmd_none(*new_pmd)))
		return false;

	/*
	 * We don't have to worry about the ordering of src and dst
	 * ptlocks because exclusive mmap_sem prevents deadlock.
	 */
	old_ptl = pmd_lock(vma->vm_mm, old_pmd);
	new_ptl = pmd_lockptr(mm, new_pmd);
	if (new_ptl != old_ptl)
		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);

	/* Clear the pmd */
	pmd = *old_pmd;
	pmd_clear(old_pmd);

	VM_BUG_ON(!pmd_none(*new_pmd));

	/* Set the new pmd */
	set_pmd_at(mm, new_addr, new_pmd, pmd);
	flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
	if (new_ptl != old_ptl)
		spin_unlock(new_ptl);
	spin_unlock(old_ptl);

	return true;
}
#endif

unsigned long move_page_tables(struct vm_area_struct *vma,
		unsigned long old_addr, struct vm_area_struct *new_vma,
		unsigned long new_addr, unsigned long len,
		bool need_rmap_locks)
{
	unsigned long extent, next, old_end;
	struct mmu_notifier_range range;
	pmd_t *old_pmd, *new_pmd;

	old_end = old_addr + len;
	flush_cache_range(vma, old_addr, old_end);

	mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
				old_addr, old_end);
	mmu_notifier_invalidate_range_start(&range);

	for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
		cond_resched();
		next = (old_addr + PMD_SIZE) & PMD_MASK;
		/* even if next overflowed, extent below will be ok */
		extent = next - old_addr;
		if (extent > old_end - old_addr)
			extent = old_end - old_addr;
		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
		if (!old_pmd)
			continue;
		new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
		if (!new_pmd)
			break;
		if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd)) {
			if (extent == HPAGE_PMD_SIZE) {
				bool moved;
				/* See comment in move_ptes() */
				if (need_rmap_locks)
					take_rmap_locks(vma);
				moved = move_huge_pmd(vma, old_addr, new_addr,
						    old_end, old_pmd, new_pmd);
				if (need_rmap_locks)
					drop_rmap_locks(vma);
				if (moved)
					continue;
			}
			split_huge_pmd(vma, old_pmd, old_addr);
			if (pmd_trans_unstable(old_pmd))
				continue;
		} else if (extent == PMD_SIZE) {
#ifdef CONFIG_HAVE_MOVE_PMD
			/*
			 * If the extent is PMD-sized, try to speed the move by
			 * moving at the PMD level if possible.
			 */
			bool moved;

			if (need_rmap_locks)
				take_rmap_locks(vma);
			moved = move_normal_pmd(vma, old_addr, new_addr,
					old_end, old_pmd, new_pmd);
			if (need_rmap_locks)
				drop_rmap_locks(vma);
			if (moved)
				continue;
#endif
		}

		if (pte_alloc(new_vma->vm_mm, new_pmd))
			break;
		next = (new_addr + PMD_SIZE) & PMD_MASK;
		if (extent > next - new_addr)
			extent = next - new_addr;
		move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
			  new_pmd, new_addr, need_rmap_locks);
	}

	mmu_notifier_invalidate_range_end(&range);

	return len + old_addr - old_end;	/* how much done */
}

static unsigned long move_vma(struct vm_area_struct *vma,
		unsigned long old_addr, unsigned long old_len,
		unsigned long new_len, unsigned long new_addr,
		bool *locked, unsigned long flags,
		struct vm_userfaultfd_ctx *uf, struct list_head *uf_unmap)
{
	struct mm_struct *mm = vma->vm_mm;
	struct vm_area_struct *new_vma;
	unsigned long vm_flags = vma->vm_flags;
	unsigned long new_pgoff;
	unsigned long moved_len;
	unsigned long excess = 0;
	unsigned long hiwater_vm;
	int split = 0;
	int err;
	bool need_rmap_locks;

	/*
	 * We'd prefer to avoid failure later on in do_munmap:
	 * which may split one vma into three before unmapping.
	 */
	if (mm->map_count >= sysctl_max_map_count - 3)
		return -ENOMEM;

	/*
	 * Advise KSM to break any KSM pages in the area to be moved:
	 * it would be confusing if they were to turn up at the new
	 * location, where they happen to coincide with different KSM
	 * pages recently unmapped.  But leave vma->vm_flags as it was,
	 * so KSM can come around to merge on vma and new_vma afterwards.
	 */
	err = ksm_madvise(vma, old_addr, old_addr + old_len,
						MADV_UNMERGEABLE, &vm_flags);
	if (err)
		return err;

	new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
	new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
			   &need_rmap_locks);
	if (!new_vma)
		return -ENOMEM;

	moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
				     need_rmap_locks);
	if (moved_len < old_len) {
		err = -ENOMEM;
	} else if (vma->vm_ops && vma->vm_ops->mremap) {
		err = vma->vm_ops->mremap(new_vma);
	}

	if (unlikely(err)) {
		/*
		 * On error, move entries back from new area to old,
		 * which will succeed since page tables still there,
		 * and then proceed to unmap new area instead of old.
		 */
		move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
				 true);
		vma = new_vma;
		old_len = new_len;
		old_addr = new_addr;
		new_addr = err;
	} else {
		mremap_userfaultfd_prep(new_vma, uf);
		arch_remap(mm, old_addr, old_addr + old_len,
			   new_addr, new_addr + new_len);
	}

	/* Conceal VM_ACCOUNT so old reservation is not undone */
	if (vm_flags & VM_ACCOUNT) {
		vma->vm_flags &= ~VM_ACCOUNT;
		excess = vma->vm_end - vma->vm_start - old_len;
		if (old_addr > vma->vm_start &&
		    old_addr + old_len < vma->vm_end)
			split = 1;
	}

	/*
	 * If we failed to move page tables we still do total_vm increment
	 * since do_munmap() will decrement it by old_len == new_len.
	 *
	 * Since total_vm is about to be raised artificially high for a
	 * moment, we need to restore high watermark afterwards: if stats
	 * are taken meanwhile, total_vm and hiwater_vm appear too high.
	 * If this were a serious issue, we'd add a flag to do_munmap().
	 */
	hiwater_vm = mm->hiwater_vm;
	vm_stat_account(mm, vma->vm_flags, new_len >> PAGE_SHIFT);

	/* Tell pfnmap has moved from this vma */
	if (unlikely(vma->vm_flags & VM_PFNMAP))
		untrack_pfn_moved(vma);

	if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {
		if (vm_flags & VM_ACCOUNT) {
			/* Always put back VM_ACCOUNT since we won't unmap */
			vma->vm_flags |= VM_ACCOUNT;

			vm_acct_memory(new_len >> PAGE_SHIFT);
		}

		/*
		 * VMAs can actually be merged back together in copy_vma
		 * calling merge_vma. This can happen with anonymous vmas
		 * which have not yet been faulted, so if we were to consider
		 * this VMA split we'll end up adding VM_ACCOUNT on the
		 * next VMA, which is completely unrelated if this VMA
		 * was re-merged.
		 */
		if (split && new_vma == vma)
			split = 0;

		/* We always clear VM_LOCKED[ONFAULT] on the old vma */
		vma->vm_flags &= VM_LOCKED_CLEAR_MASK;

		/* Because we won't unmap we don't need to touch locked_vm */
		goto out;
	}

	if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
		/* OOM: unable to split vma, just get accounts right */
		vm_unacct_memory(excess >> PAGE_SHIFT);
		excess = 0;
	}

	if (vm_flags & VM_LOCKED) {
		mm->locked_vm += new_len >> PAGE_SHIFT;
		*locked = true;
	}
out:
	mm->hiwater_vm = hiwater_vm;

	/* Restore VM_ACCOUNT if one or two pieces of vma left */
	if (excess) {
		vma->vm_flags |= VM_ACCOUNT;
		if (split)
			vma->vm_next->vm_flags |= VM_ACCOUNT;
	}

	return new_addr;
}

static struct vm_area_struct *vma_to_resize(unsigned long addr,
	unsigned long old_len, unsigned long new_len, unsigned long flags,
	unsigned long *p)
{
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma = find_vma(mm, addr);
	unsigned long pgoff;

	if (!vma || vma->vm_start > addr)
		return ERR_PTR(-EFAULT);

	/*
	 * !old_len is a special case where an attempt is made to 'duplicate'
	 * a mapping.  This makes no sense for private mappings as it will
	 * instead create a fresh/new mapping unrelated to the original.  This
	 * is contrary to the basic idea of mremap which creates new mappings
	 * based on the original.  There are no known use cases for this
	 * behavior.  As a result, fail such attempts.
	 */
	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
		pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
		return ERR_PTR(-EINVAL);
	}

	if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
			vma->vm_flags & VM_SHARED))
		return ERR_PTR(-EINVAL);

	if (is_vm_hugetlb_page(vma))
		return ERR_PTR(-EINVAL);

	/* We can't remap across vm area boundaries */
	if (old_len > vma->vm_end - addr)
		return ERR_PTR(-EFAULT);

	if (new_len == old_len)
		return vma;

	/* Need to be careful about a growing mapping */
	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
	pgoff += vma->vm_pgoff;
	if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
		return ERR_PTR(-EINVAL);

	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
		return ERR_PTR(-EFAULT);

	if (vma->vm_flags & VM_LOCKED) {
		unsigned long locked, lock_limit;
		locked = mm->locked_vm << PAGE_SHIFT;
		lock_limit = rlimit(RLIMIT_MEMLOCK);
		locked += new_len - old_len;
		if (locked > lock_limit && !capable(CAP_IPC_LOCK))
			return ERR_PTR(-EAGAIN);
	}

	if (!may_expand_vm(mm, vma->vm_flags,
				(new_len - old_len) >> PAGE_SHIFT))
		return ERR_PTR(-ENOMEM);

	if (vma->vm_flags & VM_ACCOUNT) {
		unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
		if (security_vm_enough_memory_mm(mm, charged))
			return ERR_PTR(-ENOMEM);
		*p = charged;
	}

	return vma;
}

static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
		unsigned long new_addr, unsigned long new_len, bool *locked,
		unsigned long flags, struct vm_userfaultfd_ctx *uf,
		struct list_head *uf_unmap_early,
		struct list_head *uf_unmap)
{
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma;
	unsigned long ret = -EINVAL;
	unsigned long charged = 0;
	unsigned long map_flags = 0;

	if (offset_in_page(new_addr))
		goto out;

	if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
		goto out;

	/* Ensure the old/new locations do not overlap */
	if (addr + old_len > new_addr && new_addr + new_len > addr)
		goto out;

	/*
	 * move_vma() need us to stay 4 maps below the threshold, otherwise
	 * it will bail out at the very beginning.
	 * That is a problem if we have already unmaped the regions here
	 * (new_addr, and old_addr), because userspace will not know the
	 * state of the vma's after it gets -ENOMEM.
	 * So, to avoid such scenario we can pre-compute if the whole
	 * operation has high chances to success map-wise.
	 * Worst-scenario case is when both vma's (new_addr and old_addr) get
	 * split in 3 before unmaping it.
	 * That means 2 more maps (1 for each) to the ones we already hold.
	 * Check whether current map count plus 2 still leads us to 4 maps below
	 * the threshold, otherwise return -ENOMEM here to be more safe.
	 */
	if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
		return -ENOMEM;

	if (flags & MREMAP_FIXED) {
		ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
		if (ret)
			goto out;
	}

	if (old_len >= new_len) {
		ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
		if (ret && old_len != new_len)
			goto out;
		old_len = new_len;
	}

	vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
	if (IS_ERR(vma)) {
		ret = PTR_ERR(vma);
		goto out;
	}

	/* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
	if (flags & MREMAP_DONTUNMAP &&
		!may_expand_vm(mm, vma->vm_flags, old_len >> PAGE_SHIFT)) {
		ret = -ENOMEM;
		goto out;
	}

	if (flags & MREMAP_FIXED)
		map_flags |= MAP_FIXED;

	if (vma->vm_flags & VM_MAYSHARE)
		map_flags |= MAP_SHARED;

	ret = get_unmapped_area(vma->vm_file, new_addr, new_len, vma->vm_pgoff +
				((addr - vma->vm_start) >> PAGE_SHIFT),
				map_flags);
	if (IS_ERR_VALUE(ret))
		goto out1;

	/* We got a new mapping */
	if (!(flags & MREMAP_FIXED))
		new_addr = ret;

	ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, flags, uf,
		       uf_unmap);

	if (!(offset_in_page(ret)))
		goto out;

out1:
	vm_unacct_memory(charged);

out:
	return ret;
}

static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
{
	unsigned long end = vma->vm_end + delta;
	if (end < vma->vm_end) /* overflow */
		return 0;
	if (vma->vm_next && vma->vm_next->vm_start < end) /* intersection */
		return 0;
	if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
			      0, MAP_FIXED) & ~PAGE_MASK)
		return 0;
	return 1;
}

/*
 * Expand (or shrink) an existing mapping, potentially moving it at the
 * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
 *
 * MREMAP_FIXED option added 5-Dec-1999 by Benjamin LaHaise
 * This option implies MREMAP_MAYMOVE.
 */
SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
		unsigned long, new_len, unsigned long, flags,
		unsigned long, new_addr)
{
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma;
	unsigned long ret = -EINVAL;
	unsigned long charged = 0;
	bool locked = false;
	bool downgraded = false;
	struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
	LIST_HEAD(uf_unmap_early);
	LIST_HEAD(uf_unmap);

	/*
	 * There is a deliberate asymmetry here: we strip the pointer tag
	 * from the old address but leave the new address alone. This is
	 * for consistency with mmap(), where we prevent the creation of
	 * aliasing mappings in userspace by leaving the tag bits of the
	 * mapping address intact. A non-zero tag will cause the subsequent
	 * range checks to reject the address as invalid.
	 *
	 * See Documentation/arm64/tagged-address-abi.rst for more information.
	 */
	addr = untagged_addr(addr);

	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
		return ret;

	if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
		return ret;

	/*
	 * MREMAP_DONTUNMAP is always a move and it does not allow resizing
	 * in the process.
	 */
	if (flags & MREMAP_DONTUNMAP &&
			(!(flags & MREMAP_MAYMOVE) || old_len != new_len))
		return ret;


	if (offset_in_page(addr))
		return ret;

	old_len = PAGE_ALIGN(old_len);
	new_len = PAGE_ALIGN(new_len);

	/*
	 * We allow a zero old-len as a special case
	 * for DOS-emu "duplicate shm area" thing. But
	 * a zero new-len is nonsensical.
	 */
	if (!new_len)
		return ret;

	if (down_write_killable(&current->mm->mmap_sem))
		return -EINTR;

	if (flags & (MREMAP_FIXED | MREMAP_DONTUNMAP)) {
		ret = mremap_to(addr, old_len, new_addr, new_len,
				&locked, flags, &uf, &uf_unmap_early,
				&uf_unmap);
		goto out;
	}

	/*
	 * Always allow a shrinking remap: that just unmaps
	 * the unnecessary pages..
	 * __do_munmap does all the needed commit accounting, and
	 * downgrades mmap_sem to read if so directed.
	 */
	if (old_len >= new_len) {
		int retval;

		retval = __do_munmap(mm, addr+new_len, old_len - new_len,
				  &uf_unmap, true);
		if (retval < 0 && old_len != new_len) {
			ret = retval;
			goto out;
		/* Returning 1 indicates mmap_sem is downgraded to read. */
		} else if (retval == 1)
			downgraded = true;
		ret = addr;
		goto out;
	}

	/*
	 * Ok, we need to grow..
	 */
	vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
	if (IS_ERR(vma)) {
		ret = PTR_ERR(vma);
		goto out;
	}

	/* old_len exactly to the end of the area..
	 */
	if (old_len == vma->vm_end - addr) {
		/* can we just expand the current mapping? */
		if (vma_expandable(vma, new_len - old_len)) {
			int pages = (new_len - old_len) >> PAGE_SHIFT;

			if (vma_adjust(vma, vma->vm_start, addr + new_len,
				       vma->vm_pgoff, NULL)) {
				ret = -ENOMEM;
				goto out;
			}

			vm_stat_account(mm, vma->vm_flags, pages);
			if (vma->vm_flags & VM_LOCKED) {
				mm->locked_vm += pages;
				locked = true;
				new_addr = addr;
			}
			ret = addr;
			goto out;
		}
	}

	/*
	 * We weren't able to just expand or shrink the area,
	 * we need to create a new one and move it..
	 */
	ret = -ENOMEM;
	if (flags & MREMAP_MAYMOVE) {
		unsigned long map_flags = 0;
		if (vma->vm_flags & VM_MAYSHARE)
			map_flags |= MAP_SHARED;

		new_addr = get_unmapped_area(vma->vm_file, 0, new_len,
					vma->vm_pgoff +
					((addr - vma->vm_start) >> PAGE_SHIFT),
					map_flags);
		if (IS_ERR_VALUE(new_addr)) {
			ret = new_addr;
			goto out;
		}

		ret = move_vma(vma, addr, old_len, new_len, new_addr,
			       &locked, flags, &uf, &uf_unmap);
	}
out:
	if (offset_in_page(ret)) {
		vm_unacct_memory(charged);
		locked = 0;
	}
	if (downgraded)
		up_read(&current->mm->mmap_sem);
	else
		up_write(&current->mm->mmap_sem);
	if (locked && new_len > old_len)
		mm_populate(new_addr + old_len, new_len - old_len);
	userfaultfd_unmap_complete(mm, &uf_unmap_early);
	mremap_userfaultfd_complete(&uf, addr, ret, old_len);
	userfaultfd_unmap_complete(mm, &uf_unmap);
	return ret;
}