Re: [patch] mm: mlock: document scary-looking stack expansion mlockchain

[Ric Mason]

On 02/01/2013 02:10 PM, Johannes Weiner wrote:
> The fact that mlock calls get_user_pages, and get_user_pages might
> call mlock when expanding a stack looks like a potential recursion.

Why expand stack need call mlock? I can't find it in the codes, could 
you point out to me?

> However, mlock makes sure the requested range is already contained
> within a vma, so no stack expansion will actually happen from mlock.
>
> Should this ever change: the stack expansion mlocks only the newly
> expanded range and so will not result in recursive expansion.
>
> Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>   mm/mlock.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index b1647fb..78c4924 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -185,6 +185,10 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
>   	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
>   		gup_flags |= FOLL_FORCE;
>   
> +	/*
> +	 * We made sure addr is within a VMA, so the following will
> +	 * not result in a stack expansion that recurses back here.
> +	 */
>   	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
>   				NULL, NULL, nonblocking);
>   }

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