Re: [PATCH memcg v3 3/3] memcg: prohibit unconditional exceeding the limit of dying tasks

[Michal Hocko]

On Sat 23-10-21 16:20:51, Vasily Averin wrote:
> Memory cgroup charging allows killed or exiting tasks to exceed the hard
> limit. It is assumed that the amount of the memory charged by those
> tasks is bound and most of the memory will get released while the task
> is exiting. This is resembling a heuristic for the global OOM situation
> when tasks get access to memory reserves. There is no global memory
> shortage at the memcg level so the memcg heuristic is more relieved.
> 
> The above assumption is overly optimistic though. E.g. vmalloc can scale
> to really large requests and the heuristic would allow that. We used to
> have an early break in the vmalloc allocator for killed tasks but this
> has been reverted by commit b8c8a338f75e ("Revert "vmalloc: back off when
> the current task is killed""). There are likely other similar code paths
> which do not check for fatal signals in an allocation&charge loop.
> Also there are some kernel objects charged to a memcg which are not
> bound to a process life time.
> 
> It has been observed that it is not really hard to trigger these
> bypasses and cause global OOM situation.
> 
> One potential way to address these runaways would be to limit the amount
> of excess (similar to the global OOM with limited oom reserves). This is
> certainly possible but it is not really clear how much of an excess is
> desirable and still protects from global OOMs as that would have to
> consider the overall memcg configuration.
> 
> This patch is addressing the problem by removing the heuristic
> altogether. Bypass is only allowed for requests which either cannot fail
> or where the failure is not desirable while excess should be still
> limited (e.g. atomic requests). Implementation wise a killed or dying
> task fails to charge if it has passed the OOM killer stage. That should
> give all forms of reclaim chance to restore the limit before the
> failure (ENOMEM) and tell the caller to back off.
> 
> In addition, this patch renames should_force_charge() helper
> to task_is_dying() because now its use is not associated witch forced
> charging.
> 
> This patch depends on pagefault_out_of_memory() to not trigger
> out_of_memory(), because then a memcg failure can unwind to VM_FAULT_OOM
> and cause a global OOM killer.
> 
> Cc: stable@xxxxxxxxxxxxxxx

My view on stable backport is similar to the previous patch. If we want
to have it there then let's wait for some time to see whether there are
any fallouts as this patch depends on the PF_OOM change.

> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/memcontrol.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6da5020a8656..87e41c3cac10 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -239,7 +239,7 @@ enum res_type {
>  	     iter != NULL;				\
>  	     iter = mem_cgroup_iter(NULL, iter, NULL))
>  
> -static inline bool should_force_charge(void)
> +static inline bool task_is_dying(void)
>  {
>  	return tsk_is_oom_victim(current) || fatal_signal_pending(current) ||
>  		(current->flags & PF_EXITING);
> @@ -1575,7 +1575,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 * A few threads which were not waiting at mutex_lock_killable() can
>  	 * fail to bail out. Therefore, check again after holding oom_lock.
>  	 */
> -	ret = should_force_charge() || out_of_memory(&oc);
> +	ret = task_is_dying() || out_of_memory(&oc);
>  
>  unlock:
>  	mutex_unlock(&oom_lock);
> @@ -2530,6 +2530,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	struct page_counter *counter;
>  	enum oom_status oom_status;
>  	unsigned long nr_reclaimed;
> +	bool passed_oom = false;
>  	bool may_swap = true;
>  	bool drained = false;
>  	unsigned long pflags;
> @@ -2564,15 +2565,6 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (gfp_mask & __GFP_ATOMIC)
>  		goto force;
>  
> -	/*
> -	 * Unlike in global OOM situations, memcg is not in a physical
> -	 * memory shortage.  Allow dying and OOM-killed tasks to
> -	 * bypass the last charges so that they can exit quickly and
> -	 * free their memory.
> -	 */
> -	if (unlikely(should_force_charge()))
> -		goto force;
> -
>  	/*
>  	 * Prevent unbounded recursion when reclaim operations need to
>  	 * allocate memory. This might exceed the limits temporarily,
> @@ -2630,8 +2622,9 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	if (gfp_mask & __GFP_RETRY_MAYFAIL)
>  		goto nomem;
>  
> -	if (fatal_signal_pending(current))
> -		goto force;
> +	/* Avoid endless loop for tasks bypassed by the oom killer */
> +	if (passed_oom && task_is_dying())
> +		goto nomem;
>  
>  	/*
>  	 * keep retrying as long as the memcg oom killer is able to make
> @@ -2640,14 +2633,10 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 */
>  	oom_status = mem_cgroup_oom(mem_over_limit, gfp_mask,
>  		       get_order(nr_pages * PAGE_SIZE));
> -	switch (oom_status) {
> -	case OOM_SUCCESS:
> +	if (oom_status == OOM_SUCCESS) {
> +		passed_oom = true;
>  		nr_retries = MAX_RECLAIM_RETRIES;
>  		goto retry;
> -	case OOM_FAILED:
> -		goto force;
> -	default:
> -		goto nomem;
>  	}
>  nomem:
>  	if (!(gfp_mask & __GFP_NOFAIL))
> -- 
> 2.32.0

-- 
Michal Hocko
SUSE Labs