Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Andrew Morton]

On Thu,  6 Jul 2023 21:32:10 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:

> With recent changes necessitating mmap_lock to be held for write while
> expanding a stack, per-VMA locks should follow the same rules and be
> write-locked to prevent page faults into the VMA being expanded. Add
> the necessary locking.

What are the possible runtime effects of this change?

> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> @@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> -- 
> 2.41.0.255.g8b1d071c50-goog