Re: [PATCH 1/1] fork,memcg: alloc_thread_stack_node needs to set tsk->stack

[Michal Hocko <mhocko@kernel.org>]

On Tue 18-06-19 21:14:50, Andrea Arcangeli wrote:
> Commit 5eed6f1dff87bfb5e545935def3843edf42800f2 corrected two
> instances, but there was a third instance of this bug.

Sigh. I should have noticed when reviewing the above. My bad.

> Without setting tsk->stack, if memcg_charge_kernel_stack fails, it'll
> execute free_thread_stack() on a dangling pointer.
> 
> Enterprise kernels are compiled with VMAP_STACK=y so this isn't
> critical, but custom VMAP_STACK=n builds should have some performance
> advantage, with the drawback of risking to fail fork because
> compaction didn't succeed. So as long as VMAP_STACK=n is a supported
> option it's worth fixing it upstream.
> 
> Fixes: 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>

I have double checked now and it seems we have covered all the cases
finally.

Thanks!
> ---
>  kernel/fork.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d6c324b1b29e..9ee28dfe7c21 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -248,7 +248,11 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
>  					     THREAD_SIZE_ORDER);
>  
> -	return page ? page_address(page) : NULL;
> +	if (likely(page)) {
> +		tsk->stack = page_address(page);
> +		return tsk->stack;
> +	}
> +	return NULL;
>  #endif
>  }
>  

-- 
Michal Hocko
SUSE Labs