Re: [PATCH] Prevent OOM casualties by enforcing memcg limits

[Chris Down <chris@chrisdown.name>]

Hi Alexander,

Alexander Sosna writes:
>Before this commit memory cgroup limits were not enforced during
>allocation.  If a process within a cgroup tries to allocates more
>memory than allowed, the kernel will not prevent the allocation even if
>OVERCOMMIT_NEVER is set.  Than the OOM killer is activated to kill
>processes in the corresponding cgroup.

Unresolvable cgroup overages are indifferent to vm.overcommit_memory, since 
exceeding memory.max is not overcommitment, it's just a natural consequence of 
the fact that allocation and reclaim are not atomic processes. Overcommitment, 
on the other hand, is about the bounds of available memory at the global 
resource level.

>This behavior is not to be expected
>when setting OVERCOMMIT_NEVER (vm.overcommit_memory = 2) and it is a huge
>problem for applications assuming that the kernel will deny an allocation
>if not enough memory is available, like PostgreSQL.  To prevent this a
>check is implemented to not allow a process to allocate more memory than
>limited by it's cgroup.  This means a process will not be killed while
>accessing pages but will receive errors on memory allocation as
>appropriate.  This gives programs a chance to handle memory allocation
>failures gracefully instead of being reaped.

We don't guarantee that vm.overcommit_memory 2 means "no OOM killer". It can 
still happen for a bunch of reasons, so I really hope PostgreSQL isn't relying 
on that.

Could you please be more clear about the "huge problem" being solved here? I'm 
not seeing it.

>Signed-off-by: Alexander Sosna <alexander@sosna.de>
>
>diff --git a/mm/util.c b/mm/util.c
>index a8bf17f18a81..c84b83c532c6 100644
>--- a/mm/util.c
>+++ b/mm/util.c
>@@ -853,6 +853,7 @@ EXPORT_SYMBOL_GPL(vm_memory_committed);
>  *
>  * Strict overcommit modes added 2002 Feb 26 by Alan Cox.
>  * Additional code 2002 Jul 20 by Robert Love.
>+ * Code to enforce memory cgroup limits added 2021 by Alexander Sosna.
>  *
>  * cap_sys_admin is 1 if the process has admin privileges, 0 otherwise.
>  *
>@@ -891,6 +892,34 @@ int __vm_enough_memory(struct mm_struct *mm, long
>pages, int cap_sys_admin)
> 		long reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
>
> 		allowed -= min_t(long, mm->total_vm / 32, reserve);
>+
>+#ifdef CONFIG_MEMCG
>+		/*
>+		 * If we are in a memory cgroup we also evaluate if the cgroup
>+		 * has enough memory to allocate a new virtual mapping.

This comment confuses me further, I'm afraid. You're talking about virtual 
mappings, but then checking memory.max, which is about allocated pages.

>+		 * This is how we can keep processes from exceeding their
>+		 * limits and also prevent that the OOM killer must be
>+		 * awakened.  This gives programs a chance to handle memory
>+		 * allocation failures gracefully and not being reaped.
>+		 * In the current version mem_cgroup_get_max() is used which
>+		 * allows the processes to exceeded their memory limits if
>+		 * enough SWAP is available.  If this is not intended we could
>+		 * use READ_ONCE(memcg->memory.max) instead.
>+		 *
>+		 * This code is only reached if sysctl_overcommit_memory equals
>+		 * OVERCOMMIT_NEVER, both other options are handled above.
>+		 */
>+		{
>+			struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
>+
>+			if (memcg) {
>+				long available = mem_cgroup_get_max(memcg)
>+						- mem_cgroup_size(memcg);
>+
>+				allowed = min_t(long, available, allowed);
>+			}
>+		}
>+#endif
> 	}
>
> 	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
>