Subject: Re: [PATCH v4 18/33] mm: write-lock VMAs before removing them from VMA tree

From: Hyeonggon Yoo <[email-masked]>
Date: Wed, 1 Mar 2023 07:43:33 +0000

On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:
> Write-locking VMAs before isolating them ensures that page fault
> handlers don't operate on isolated VMAs.
> 
> Signed-off-by: Suren Baghdasaryan <[email-masked]>
> ---
>  mm/mmap.c  | 1 +
>  mm/nommu.c | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 1f42b9a52b9b..f7ed357056c4 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  static inline int munmap_sidetree(struct vm_area_struct *vma,
>  				   struct ma_state *mas_detach)
>  {
> +	vma_start_write(vma);
>  	mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);

I may be missing something, but have few questions:

	1) Why does a writer need to both write-lock a VMA and mark the VMA detached
	   when unmapping it, isn't it enough to just only write-lock a VMA?

	2) as VMAs that are going to be removed are already locked in vma_prepare(),
	   so I think this hunk could be dropped?

>  	if (mas_store_gfp(mas_detach, vma, GFP_KERNEL))
>  		return -ENOMEM;
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 57ba243c6a37..2ab162d773e2 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -588,6 +588,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
>  		       current->pid);
>  		return -ENOMEM;
>  	}
> +	vma_start_write(vma);
>  	cleanup_vma_from_mm(vma);

	3) I think this hunk could be dropped as Per-VMA lock depends on MMU anyway.

Thanks,
Hyeonggon

>  
>  	/* remove from the MM's tree and list */
> @@ -1519,6 +1520,10 @@ void exit_mmap(struct mm_struct *mm)
>  	 */
>  	mmap_write_lock(mm);
>  	for_each_vma(vmi, vma) {
> +		/*
> +		 * No need to lock VMA because this is the only mm user and no
> +		 * page fault handled can race with it.
> +		 */
>  		cleanup_vma_from_mm(vma);
>  		delete_vma(mm, vma);
>  		cond_resched();
> -- 
> 2.39.2.722.g9855ee24e9-goog
> 
>