Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting

[Johannes Weiner]

On Tue, Aug 14, 2018 at 05:36:19PM -0700, Roman Gushchin wrote:
> @@ -224,9 +224,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		return s->addr;
>  	}
>  
> +	/*
> +	 * Allocated stacks are cached and later reused by new threads,
> +	 * so memcg accounting is performed manually on assigning/releasing
> +	 * stacks to tasks. Drop __GFP_ACCOUNT.
> +	 */
>  	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
>  				     VMALLOC_START, VMALLOC_END,
> -				     THREADINFO_GFP,
> +				     THREADINFO_GFP & ~__GFP_ACCOUNT,
>  				     PAGE_KERNEL,
>  				     0, node, __builtin_return_address(0));
>  
> @@ -246,12 +251,41 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  #endif
>  }
>  
> +static void memcg_charge_kernel_stack(struct task_struct *tsk)
> +{
> +#ifdef CONFIG_VMAP_STACK
> +	struct vm_struct *vm = task_stack_vm_area(tsk);
> +
> +	if (vm) {
> +		int i;
> +
> +		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> +			memcg_kmem_charge(vm->pages[i], __GFP_NOFAIL,
> +					  compound_order(vm->pages[i]));
> +
> +		/* All stack pages belong to the same memcg. */
> +		mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
> +				     THREAD_SIZE / 1024);
> +	}
> +#endif
> +}

Before this change, the memory limit can fail the fork, but afterwards
fork() can grow memory consumption unimpeded by the cgroup settings.

Can we continue to use try_charge() here and fail the fork?