Re: [PATCH v2 2/2] mm/mremap: Remove goto from mremap_to()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Oct 18, 2024 at 01:41:14PM -0400, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
>
> mremap_to() has a goto label at the end that doesn't unwind anything.
> Removing the label makes the code cleaner.
>
> This commit also adds documentation on the function.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>

This is a nice cleanup, thanks!

Other than nits below, LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/mremap.c | 46 +++++++++++++++++++++++++++-------------------
>  1 file changed, 27 insertions(+), 19 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e781ec4573ca..4c79ab92eb8f 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -887,6 +887,20 @@ static int resize_is_valid(struct vm_area_struct *vma, unsigned long addr,
>  	return 0;
>  }
>
> +/*
> + * mremap_to() - remap a vma to a new location
> + * @addr: The old address
> + * @old_len: The old size
> + * @new_addr: The target address
> + * @new_len: The new size
> + * @locked: If the returned vma is locked (VM_LOCKED)
> + * @flags: the mremap flags
> + * @uf: The mremap userfaultfd context
> + * @uf_unmap_early: The userfaultfd unmap early context
> + * @uf_unmap: The userfaultfd unmap context

Nit - these are lists of userfaultfd_unmap_ctx rather than contexts. But I
mean, the naming around this is generally not great and not your fault so
not a big deal.

> + *
> + * Returns: The new address of the vma or an error.
> + */
>  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,
> @@ -895,18 +909,18 @@ static unsigned long mremap_to(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 map_flags = 0;
>
>  	if (offset_in_page(new_addr))
> -		goto out;
> +		return -EINVAL;
>
>  	if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
> -		goto out;
> +		return -EINVAL;
>
>  	/* Ensure the old/new locations do not overlap */
>  	if (addr + old_len > new_addr && new_addr + new_len > addr)
> -		goto out;
> +		return -EINVAL;
>
>  	/*
>  	 * move_vma() need us to stay 4 maps below the threshold, otherwise
> @@ -933,31 +947,28 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  		 */
>  		ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
>  		if (ret)
> -			goto out;
> +			return ret;
>  	}
>
>  	if (old_len > new_len) {
>  		ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
>  		if (ret)
> -			goto out;
> +			return ret;
>  		old_len = new_len;
>  	}
>
>  	vma = vma_lookup(mm, addr);
> -	if (!vma) {
> -		ret = -EFAULT;
> -		goto out;
> -	}
> +	if (!vma)
> +		return -EFAULT;
>
>  	ret = resize_is_valid(vma, addr, old_len, new_len, flags);
>  	if (ret)
> -		goto out;
> +		return ret;
>
>  	/* 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;
> +		return -ENOMEM;
>  	}
>
>  	if (flags & MREMAP_FIXED)
> @@ -970,17 +981,14 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  				((addr - vma->vm_start) >> PAGE_SHIFT),
>  				map_flags);

Unrelated to your change, but I'm really confused as to why we're doing
this even if MREMAP_FIXED might be set, then ignore its return value if it
is set?

Would be way nicer to do something like:

	if (!(flags & MREMAP_FIXED)) {
		new_addr = 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(new_addr))
			return new_addr;
	}

Anyway one for another series.

I'd say the 'ret' variable is badly named now, but it was badly named
before so there's zero delta on that.

>  	if (IS_ERR_VALUE(ret))
> -		goto out;
> +		return ret;
>
>  	/* 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);
> -
> -out:
> -	return ret;
> +	return move_vma(vma, addr, old_len, new_len, new_addr, locked, flags,
> +			uf, uf_unmap);
>  }
>
>  static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
> --
> 2.43.0
>