Re: [PATCH 2/7] mm/mremap: refactor mremap() system call implementation

[Lorenzo Stoakes]

Thanks for taking a look! :) I know this one is a bit painful...

On Mon, Mar 03, 2025 at 12:12:07PM -0500, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [250303 06:08]:
> > Place checks into a separate function so the mremap() system call is less
> > egregiously long, remove unnecessary mremap_to() offset_in_page() check and
> > just check that earlier so we keep all such basic checks together.
> >
> > Separate out the VMA in-place expansion, hugetlb and expand/move logic into
> > separate, readable functions.
> >
> > De-duplicate code where possible, add comments and ensure that all error
> > handling explicitly specifies the error at the point of it occurring rather
> > than setting a prefixed error value and implicitly setting (which is bug
> > prone).
> >
> > This lays the groundwork for subsequent patches further simplifying and
> > extending the mremap() implementation.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
> >  mm/mremap.c | 405 ++++++++++++++++++++++++++++++++--------------------
> >  1 file changed, 251 insertions(+), 154 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index c3e4c86d0b8d..c4abda8dfc57 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -942,33 +942,14 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> >  	unsigned long ret;
> >  	unsigned long map_flags = 0;
> >
> > -	if (offset_in_page(new_addr))
> > -		return -EINVAL;
> > -
> > +	/* Is the new length or address silly? */
> >  	if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
> >  		return -EINVAL;
> >
> > -	/* Ensure the old/new locations do not overlap */
> > +	/* Ensure the old/new locations do not overlap. */
> >  	if (addr + old_len > new_addr && new_addr + new_len > addr)
> >  		return -EINVAL;
> >
> > -	/*
> > -	 * 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 unmapping 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) {
> >  		/*
> >  		 * In mremap_to().
> > @@ -1035,6 +1016,218 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
> >  	return 1;
> >  }
> >
> > +/* Do the mremap() flags require that the new_addr parameter be specified? */
> > +static bool implies_new_addr(unsigned long flags)
> > +{
> > +	return flags & (MREMAP_FIXED | MREMAP_DONTUNMAP);
> > +}
> > +
> > +/*
> > + * Are the parameters passed to mremap() valid? If so return 0, otherwise return
> > + * error.
> > + */
> > +static unsigned long check_mremap_params(unsigned long addr,
> > +					 unsigned long flags,
> > +					 unsigned long old_len,
> > +					 unsigned long new_len,
> > +					 unsigned long new_addr)
>
> If you use two tabs for the arguments this will be more compact and more
> readable.

Sure, but a later commits eliminates this :)

>
> > +{
> > +	/* Ensure no unexpected flag values. */
> > +	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
> > +		return -EINVAL;
> > +
> > +	/* Start address must be page-aligned. */
> > +	if (offset_in_page(addr))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * 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 (!PAGE_ALIGN(new_len))
> > +		return -EINVAL;
> > +
> > +	/* Remainder of checks are for cases with specific new_addr. */
> > +	if (!implies_new_addr(flags))
> > +		return 0;
> > +
> > +	/* The new address must be page-aligned. */
> > +	if (offset_in_page(new_addr))
> > +		return -EINVAL;
> > +
> > +	/* A fixed address implies a move. */
> > +	if (!(flags & MREMAP_MAYMOVE))
> > +		return -EINVAL;
> > +
> > +	/* MREMAP_DONTUNMAP does not allow resizing in the process. */
> > +	if (flags & MREMAP_DONTUNMAP && old_len != new_len)
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * 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 unmapping 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 ((current->mm->map_count + 2) >= sysctl_max_map_count - 3)
> > +		return -ENOMEM;
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + * We know we can expand the VMA in-place by delta pages, so do so.
> > + *
> > + * If we discover the VMA is locked, update mm_struct statistics accordingly and
> > + * indicate so to the caller.
> > + */
> > +static unsigned long expand_vma_inplace(struct vm_area_struct *vma,
> > +					unsigned long delta, bool *locked)
> > +{
> > +	struct mm_struct *mm = current->mm;
> > +	long pages = delta >> PAGE_SHIFT;
> > +	VMA_ITERATOR(vmi, mm, vma->vm_end);
> > +	long charged = 0;
> > +
> > +	if (vma->vm_flags & VM_ACCOUNT) {
> > +		if (security_vm_enough_memory_mm(mm, pages))
> > +			return -ENOMEM;
> > +
> > +		charged = pages;
> > +	}
> > +
> > +	/*
> > +	 * Function vma_merge_extend() is called on the
> > +	 * extension we are adding to the already existing vma,
> > +	 * vma_merge_extend() will merge this extension with the
> > +	 * already existing vma (expand operation itself) and
> > +	 * possibly also with the next vma if it becomes
> > +	 * adjacent to the expanded vma and otherwise
> > +	 * compatible.
> > +	 */
> > +	vma = vma_merge_extend(&vmi, vma, delta);
> > +	if (!vma) {
> > +		vm_unacct_memory(charged);
> > +		return -ENOMEM;
> > +	}
> > +
> > +	vm_stat_account(mm, vma->vm_flags, pages);
> > +	if (vma->vm_flags & VM_LOCKED) {
> > +		mm->locked_vm += pages;
> > +		*locked = true;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static bool align_hugetlb(struct vm_area_struct *vma,
> > +			  unsigned long addr,
> > +			  unsigned long new_addr,
> > +			  unsigned long *old_len_ptr,
> > +			  unsigned long *new_len_ptr,
> > +			  unsigned long *delta_ptr)
> > +{
> > +	unsigned long old_len = *old_len_ptr;
> > +	unsigned long new_len = *new_len_ptr;
> > +	struct hstate *h __maybe_unused = hstate_vma(vma);
> > +
> > +	old_len = ALIGN(old_len, huge_page_size(h));
> > +	new_len = ALIGN(new_len, huge_page_size(h));
> > +
> > +	/* addrs must be huge page aligned */
> > +	if (addr & ~huge_page_mask(h))
> > +		return false;
> > +	if (new_addr & ~huge_page_mask(h))
> > +		return false;
> > +
> > +	/*
> > +	 * Don't allow remap expansion, because the underlying hugetlb
> > +	 * reservation is not yet capable to handle split reservation.
> > +	 */
> > +	if (new_len > old_len)
> > +		return false;
> > +
> > +	*old_len_ptr = old_len;
> > +	*new_len_ptr = new_len;
> > +	*delta_ptr = abs_diff(old_len, new_len);
> > +	return true;
> > +}
> > +
> > +/*
> > + * We are mremap()'ing without specifying a fixed address to move to, but are
> > + * requesting that the VMA's size be increased.
> > + *
> > + * Try to do so in-place, if this fails, then move the VMA to a new location to
> > + * action the change.
> > + */
> > +static unsigned long expand_vma(struct vm_area_struct *vma,
> > +				unsigned long addr, unsigned long old_len,
> > +				unsigned long new_len, unsigned long flags,
> > +				bool *locked_ptr, unsigned long *new_addr_ptr,
> > +				struct vm_userfaultfd_ctx *uf_ptr,
> > +				struct list_head *uf_unmap_ptr)
> > +{
> > +	unsigned long err;
> > +	unsigned long map_flags;
> > +	unsigned long new_addr; /* We ignore any user-supplied one. */
> > +	pgoff_t pgoff;
> > +
> > +	err = resize_is_valid(vma, addr, old_len, new_len, flags);
> > +	if (err)
> > +		return err;
> > +
> > +	/*
> > +	 * [addr, old_len) spans precisely to the end of the VMA, so try to
> > +	 * expand it in-place.
> > +	 */
> > +	if (old_len == vma->vm_end - addr &&
> > +	    vma_expandable(vma, new_len - old_len)) {
> > +		err = expand_vma_inplace(vma, new_len - old_len, locked_ptr);
>
> You use delta twice here (new_len - old_len).  I assume this is
> different in the next patches.
>
> > +		if (IS_ERR_VALUE(err))
> > +			return err;
>
> Doesn't expand_vma_inplace() return 0 on success, error otherwise?

Yeah, this is copying some already dubious logic from the original (trying to
_somewhat_ minimise the delta here).

At any rate, a later commit addresses this!

>
> > +
> > +		/*
> > +		 * We want to populate the newly expanded portion of the VMA to
> > +		 * satisfy the expectation that mlock()'ing a VMA maintains all
> > +		 * of its pages in memory.
> > +		 */
> > +		if (*locked_ptr)
> > +			*new_addr_ptr = addr;
> > +
> > +		/* OK we're done! */
> > +		return addr;
> > +	}
> > +
> > +	/*
> > +	 * We weren't able to just expand or shrink the area,
> > +	 * we need to create a new one and move it.
> > +	 */
> > +
>
> So it's more of an expand_or_move_vma()?

I think that's misleading, because it would be
expand_or_move_and_expand_vma() or expand_in_place_or_move_and_expand()...

Unavoidably we have to abbreviate, I tried to differentiate between the two
cases with the 'in place' stuff.

So we _try_ to do it in place, if not we have to expand elsewhere.

>
> > +	/* We're not allowed to move the VMA, so error out. */
> > +	if (!(flags & MREMAP_MAYMOVE))
> > +		return -ENOMEM;
>
> and by flags we mean... the flags from the syscall.  This gets confusing
> with map_flags.  At least there's only two and not six flags.

3 flags (MREMAP_FIXED, MREMAP_MAYMOVE, MREMAP_DONTUNMAP) :>)

This becomes clearer later, I'm not sure saying mremap_flags really adds
anything but noise though.

>
> > +
> > +	/* Find a new location to move the VMA to. */
> > +	map_flags = (vma->vm_flags & VM_MAYSHARE) ? MAP_SHARED : 0;
> > +	pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
> > +	new_addr = get_unmapped_area(vma->vm_file, 0, new_len, pgoff, map_flags);
> > +	if (IS_ERR_VALUE(new_addr))
> > +		return new_addr;
> > +	*new_addr_ptr = new_addr;
> > +
> > +	return move_vma(vma, addr, old_len, new_len, new_addr,
> > +			locked_ptr, flags, uf_ptr, uf_unmap_ptr);
> > +}
> > +
> >  /*
> >   * Expand (or shrink) an existing mapping, potentially moving it at the
> >   * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
> > @@ -1048,7 +1241,8 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> >  {
> >  	struct mm_struct *mm = current->mm;
> >  	struct vm_area_struct *vma;
> > -	unsigned long ret = -EINVAL;
> > +	unsigned long ret;
> > +	unsigned long delta;
> >  	bool locked = false;
> >  	struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
> >  	LIST_HEAD(uf_unmap_early);
> > @@ -1067,70 +1261,38 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> >  	 */
> >  	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))
> > +	ret = check_mremap_params(addr, flags, old_len, new_len, new_addr);
> > +	if (ret)
> >  		return ret;
> >
> >  	old_len = PAGE_ALIGN(old_len);
> >  	new_len = PAGE_ALIGN(new_len);
> > +	delta = abs_diff(old_len, 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 (mmap_write_lock_killable(current->mm))
> > +	if (mmap_write_lock_killable(mm))
> >  		return -EINTR;
> > +
> >  	vma = vma_lookup(mm, addr);
> >  	if (!vma) {
> >  		ret = -EFAULT;
> >  		goto out;
> >  	}
> >
> > -	/* Don't allow remapping vmas when they have already been sealed */
> > +	/* If mseal()'d, mremap() is prohibited. */
> >  	if (!can_modify_vma(vma)) {
> >  		ret = -EPERM;
> >  		goto out;
> >  	}
>
> This could be delayed to the munmap() call, which will also check to
> ensure this would fail.
>
> It doesn't really cost anything though so I don't think it's worth it
> here.  Maybe something we can keep in mind for the future...

Happy to address but I think would be better in a later commit, as this one
is more like 'keep things the same but restructure', later commits do
deeper changes.

>
> >
> > -	if (is_vm_hugetlb_page(vma)) {
> > -		struct hstate *h __maybe_unused = hstate_vma(vma);
> > -
> > -		old_len = ALIGN(old_len, huge_page_size(h));
> > -		new_len = ALIGN(new_len, huge_page_size(h));
> > -
> > -		/* addrs must be huge page aligned */
> > -		if (addr & ~huge_page_mask(h))
> > -			goto out;
> > -		if (new_addr & ~huge_page_mask(h))
> > -			goto out;
> > -
> > -		/*
> > -		 * Don't allow remap expansion, because the underlying hugetlb
> > -		 * reservation is not yet capable to handle split reservation.
> > -		 */
> > -		if (new_len > old_len)
> > -			goto out;
> > +	/* Align to hugetlb page size, if required. */
> > +	if (is_vm_hugetlb_page(vma) &&
> > +	    !align_hugetlb(vma, addr, new_addr, &old_len, &new_len, &delta)) {
> > +		ret = -EINVAL;
> > +		goto out;
> >  	}
> >
> > -	if (flags & (MREMAP_FIXED | MREMAP_DONTUNMAP)) {
> > +	/* Are we RELOCATING the VMA to a SPECIFIC address? */
> > +	if (implies_new_addr(flags)) {
> >  		ret = mremap_to(addr, old_len, new_addr, new_len,
> >  				&locked, flags, &uf, &uf_unmap_early,
> >  				&uf_unmap);
> > @@ -1138,109 +1300,44 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> >  	}
> >
> >  	/*
> > -	 * Always allow a shrinking remap: that just unmaps
> > -	 * the unnecessary pages..
> > -	 * do_vmi_munmap does all the needed commit accounting, and
> > -	 * unlocks the mmap_lock if so directed.
> > +	 * From here on in we are only RESIZING the VMA, attempting to do so
> > +	 * in-place, moving the VMA if we cannot.
> >  	 */
> > -	if (old_len >= new_len) {
> > -		VMA_ITERATOR(vmi, mm, addr + new_len);
> >
> > -		if (old_len == new_len) {
> > -			ret = addr;
> > -			goto out;
> > -		}
> > +	/* NO-OP CASE - resizing to the same size. */
> > +	if (new_len == old_len) {
> > +		ret = addr;
> > +		goto out;
> > +	}
> > +
> > +	/* SHRINK CASE. Can always be done in-place. */
> > +	if (new_len < old_len) {
> > +		VMA_ITERATOR(vmi, mm, addr + new_len);
> >
> > -		ret = do_vmi_munmap(&vmi, mm, addr + new_len, old_len - new_len,
> > +		/*
> > +		 * Simply unmap the shrunken portion of the VMA. This does all
> > +		 * the needed commit accounting, unlocking the mmap lock.
> > +		 */
> > +		ret = do_vmi_munmap(&vmi, mm, addr + new_len, delta,
> >  				    &uf_unmap, true);
> >  		if (ret)
> >  			goto out;
> >
> > +		/* We succeeded, mmap lock released for us. */
> >  		ret = addr;
> >  		goto out_unlocked;
> >  	}
> >
> > -	/*
> > -	 * Ok, we need to grow..
> > -	 */
> > -	ret = resize_is_valid(vma, addr, old_len, new_len, flags);
> > -	if (ret)
> > -		goto out;
> > -
> > -	/* old_len exactly to the end of the area..
> > -	 */
> > -	if (old_len == vma->vm_end - addr) {
> > -		unsigned long delta = new_len - old_len;
> > -
> > -		/* can we just expand the current mapping? */
> > -		if (vma_expandable(vma, delta)) {
> > -			long pages = delta >> PAGE_SHIFT;
> > -			VMA_ITERATOR(vmi, mm, vma->vm_end);
> > -			long charged = 0;
> > -
> > -			if (vma->vm_flags & VM_ACCOUNT) {
> > -				if (security_vm_enough_memory_mm(mm, pages)) {
> > -					ret = -ENOMEM;
> > -					goto out;
> > -				}
> > -				charged = pages;
> > -			}
> > -
> > -			/*
> > -			 * Function vma_merge_extend() is called on the
> > -			 * extension we are adding to the already existing vma,
> > -			 * vma_merge_extend() will merge this extension with the
> > -			 * already existing vma (expand operation itself) and
> > -			 * possibly also with the next vma if it becomes
> > -			 * adjacent to the expanded vma and otherwise
> > -			 * compatible.
> > -			 */
> > -			vma = vma_merge_extend(&vmi, vma, delta);
> > -			if (!vma) {
> > -				vm_unacct_memory(charged);
> > -				ret = -ENOMEM;
> > -				goto out;
> > -			}
> > +	/* EXPAND case. We try to do in-place, if we can't, then we move it. */
> > +	ret = expand_vma(vma, addr, old_len, new_len, flags, &locked, &new_addr,
> > +			 &uf, &uf_unmap);
> >
> > -			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))
> >  		locked = false;
> > -	mmap_write_unlock(current->mm);
> > +	mmap_write_unlock(mm);
> >  	if (locked && new_len > old_len)
> > -		mm_populate(new_addr + old_len, new_len - old_len);
> > +		mm_populate(new_addr + old_len, delta);
> >  out_unlocked:
> >  	userfaultfd_unmap_complete(mm, &uf_unmap_early);
> >  	mremap_userfaultfd_complete(&uf, addr, ret, old_len);
> > --
> > 2.48.1
> >