Date: Thu, 27 Jun 2024 13:45:34 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Eric Biederman <ebiederm@...ssion.com>, Kees Cook <kees@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [RFC PATCH 3/7] mm: unexport vma_expand() / vma_shrink()

* Lorenzo Stoakes <lstoakes@...il.com> [240627 06:39]:
> The vma_expand() and vma_shrink() functions are core VMA manipulaion
> functions which ultimately invoke VMA split/merge. In order to make these
> testable, it is convenient to place all such core functions in a header
> internal to mm/.
> 

The sole user doesn't cause a split or merge, it relocates a vma by
'sliding' the window of the vma by expand/shrink with the moving of page
tables in the middle of the slide.

It slides to relocate the vma start/end and keep the vma pointer
constant.

> In addition, it is safer to abstract direct access to such functionality so
> we can better control how other parts of the kernel use them, which
> provides us the freedom to change how this functionality behaves as needed
> without having to worry about how this functionality is used elsewhere.
> 
> In order to service both these requirements, we provide abstractions for
> the sole external user of these functions, shift_arg_pages() in fs/exec.c.
> 
> We provide vma_expand_bottom() and vma_shrink_top() functions which better
> match the semantics of what shift_arg_pages() is trying to accomplish by
> explicitly wrapping the safe expansion of the bottom of a VMA and the
> shrinking of the top of a VMA.
> 
> As a result, we place the vma_shrink() and vma_expand() functions into
> mm/internal.h to unexport them from use by any other part of the kernel.

There is no point to have vma_shrink() have a wrapper since this is the
only place it's ever used.  So we're wrapping a function that's only
called once.

I'd rather a vma_relocate() do everything in this function than wrap
them.  The only other think it does is the page table moving and freeing
- which we have to do in the vma code.  We;d expose something we want no
one to use - but we already have two of those here..

> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  fs/exec.c          | 26 +++++--------------
>  include/linux/mm.h |  9 +++----
>  mm/internal.h      |  6 +++++
>  mm/mmap.c          | 65 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 82 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 40073142288f..1cb3bf323e0f 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -700,25 +700,14 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
>  	unsigned long length = old_end - old_start;
>  	unsigned long new_start = old_start - shift;
>  	unsigned long new_end = old_end - shift;
> -	VMA_ITERATOR(vmi, mm, new_start);
> +	VMA_ITERATOR(vmi, mm, 0);
>  	struct vm_area_struct *next;
>  	struct mmu_gather tlb;
> +	int ret;
>  
> -	BUG_ON(new_start > new_end);
> -
> -	/*
> -	 * ensure there are no vmas between where we want to go
> -	 * and where we are
> -	 */
> -	if (vma != vma_next(&vmi))
> -		return -EFAULT;
> -
> -	vma_iter_prev_range(&vmi);
> -	/*
> -	 * cover the whole range: [new_start, old_end)
> -	 */
> -	if (vma_expand(&vmi, vma, new_start, old_end, vma->vm_pgoff, NULL))
> -		return -ENOMEM;
> +	ret = vma_expand_bottom(&vmi, vma, shift, &next);
> +	if (ret)
> +		return ret;
>  
>  	/*
>  	 * move the page tables downwards, on failure we rely on
> @@ -730,7 +719,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
>  
>  	lru_add_drain();
>  	tlb_gather_mmu(&tlb, mm);
> -	next = vma_next(&vmi);
> +
>  	if (new_end > old_start) {
>  		/*
>  		 * when the old and new regions overlap clear from new_end.
> @@ -749,9 +738,8 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
>  	}
>  	tlb_finish_mmu(&tlb);
>  
> -	vma_prev(&vmi);
>  	/* Shrink the vma to just the new range */
> -	return vma_shrink(&vmi, vma, new_start, new_end, vma->vm_pgoff);
> +	return vma_shrink_top(&vmi, vma, shift);
>  }
>  
>  /*
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4d2b5538925b..e3220439cf75 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3273,11 +3273,10 @@ void anon_vma_interval_tree_verify(struct anon_vma_chain *node);
>  
>  /* mmap.c */
>  extern int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin);
> -extern int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> -		      unsigned long start, unsigned long end, pgoff_t pgoff,
> -		      struct vm_area_struct *next);
> -extern int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> -		       unsigned long start, unsigned long end, pgoff_t pgoff);
> +extern int vma_expand_bottom(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +			     unsigned long shift, struct vm_area_struct **next);
> +extern int vma_shrink_top(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +			  unsigned long shift);
>  extern struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *);
>  extern int insert_vm_struct(struct mm_struct *, struct vm_area_struct *);
>  extern void unlink_file_vma(struct vm_area_struct *);
> diff --git a/mm/internal.h b/mm/internal.h
> index c8177200c943..f7779727bb78 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1305,6 +1305,12 @@ static inline struct vm_area_struct
>  			  vma_policy(vma), new_ctx, anon_vma_name(vma));
>  }
>  
> +int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +	       unsigned long start, unsigned long end, pgoff_t pgoff,
> +		      struct vm_area_struct *next);
> +int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +	       unsigned long start, unsigned long end, pgoff_t pgoff);
> +
>  enum {
>  	/* mark page accessed */
>  	FOLL_TOUCH = 1 << 16,
> diff --git a/mm/mmap.c b/mm/mmap.c
> index e42d89f98071..574e69a04ebe 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3940,6 +3940,71 @@ void mm_drop_all_locks(struct mm_struct *mm)
>  	mutex_unlock(&mm_all_locks_mutex);
>  }
>  
> +/*
> + * vma_expand_bottom() - Expands the bottom of a VMA downwards. An error will
> + *                       arise if there is another VMA in the expanded range, or
> + *                       if the expansion fails. This function leaves the VMA
> + *                       iterator, vmi, positioned at the newly expanded VMA.
> + * @vmi: The VMA iterator.
> + * @vma: The VMA to modify.
> + * @shift: The number of bytes by which to expand the bottom of the VMA.
> + * @next: Output parameter, pointing at the VMA immediately succeeding the newly
> + *        expanded VMA.
> + *
> + * Returns: 0 on success, an error code otherwise.
> + */
> +int vma_expand_bottom(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +		      unsigned long shift, struct vm_area_struct **next)
> +{
> +	unsigned long old_start = vma->vm_start;
> +	unsigned long old_end = vma->vm_end;
> +	unsigned long new_start = old_start - shift;
> +	unsigned long new_end = old_end - shift;
> +
> +	BUG_ON(new_start > new_end);
> +
> +	vma_iter_set(vmi, new_start);
> +
> +	/*
> +	 * ensure there are no vmas between where we want to go
> +	 * and where we are
> +	 */
> +	if (vma != vma_next(vmi))
> +		return -EFAULT;
> +
> +	vma_iter_prev_range(vmi);
> +
> +	/*
> +	 * cover the whole range: [new_start, old_end)
> +	 */
> +	if (vma_expand(vmi, vma, new_start, old_end, vma->vm_pgoff, NULL))
> +		return -ENOMEM;
> +
> +	*next = vma_next(vmi);
> +	vma_prev(vmi);
> +
> +	return 0;
> +}
> +
> +/*
> + * vma_shrink_top() - Reduce an existing VMA's memory area by shift bytes from
> + *                    the top of the VMA.
> + * @vmi: The VMA iterator, must be positioned at the VMA.
> + * @vma: The VMA to modify.
> + * @shift: The number of bytes by which to shrink the VMA.
> + *
> + * Returns: 0 on success, an error code otherwise.
> + */
> +int vma_shrink_top(struct vma_iterator *vmi, struct vm_area_struct *vma,
> +		   unsigned long shift)
> +{
> +	if (shift >= vma->vm_end - vma->vm_start)
> +		return -EINVAL;
> +
> +	return vma_shrink(vmi, vma, vma->vm_start, vma->vm_end - shift,
> +			  vma->vm_pgoff);
> +}
> +
>  /*
>   * initialise the percpu counter for VM
>   */
> -- 
> 2.45.1
> 

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