Re: [PATCH v3 2/2] mm: ignore memory.min of abandoned memory cgroups

From: Andrew Morton
Date: Wed May 09 2018 - 18:38:13 EST

> Memory controller implements the memory.low best-effort memory
> protection mechanism, which works perfectly in many cases and
> allows protecting working sets of important workloads from
> sudden reclaim.
> But its semantics has a significant limitation: it works
> only as long as there is a supply of reclaimable memory.
> This makes it pretty useless against any sort of slow memory
> leaks or memory usage increases. This is especially true
> for swapless systems. If swap is enabled, memory soft protection
> effectively postpones problems, allowing a leaking application
> to fill all swap area, which makes no sense.
> The only effective way to guarantee the memory protection
> in this case is to invoke the OOM killer.
> It's possible to handle this case in userspace by reacting
> on MEMCG_LOW events; but there is still a place for a fail-safe
> in-kernel mechanism to provide stronger guarantees.
> This patch introduces the memory.min interface for cgroup v2
> memory controller. It works very similarly to memory.low
> (sharing the same hierarchical behavior), except that it's
> not disabled if there is no more reclaimable memory in the system.
> If cgroup is not populated, its memory.min is ignored,
> because otherwise even the OOM killer wouldn't be able
> to reclaim the protected memory, and the system can stall.
> ...
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -1002,6 +1002,29 @@ PAGE_SIZE multiple when read back.
> The total amount of memory currently being used by the cgroup
> and its descendants.
> + memory.min
> + A read-write single value file which exists on non-root
> + cgroups. The default is "0".
> +
> + Hard memory protection. If the memory usage of a cgroup
> + is within its effective min boundary, the cgroup's memory
> + won't be reclaimed under any conditions. If there is no
> + unprotected reclaimable memory available, OOM killer
> + is invoked.
> +
> + Effective low boundary is limited by memory.min values of
> + all ancestor cgroups. If there is memory.min overcommitment
> + (child cgroup or cgroups are requiring more protected memory
> + than parent will allow), then each child cgroup will get
> + the part of parent's protection proportional to its
> + actual memory usage below memory.min.
> +
> + Putting more memory than generally available under this
> + protection is discouraged and may lead to constant OOMs.
> +
> + If a memory cgroup is not populated with processes,
> + its memory.min is ignored.

This is a copy-paste-edit of the memory.low description. Could we
please carefully check that it all remains accurate? Should "Effective
low boundary" be "Effective min boundary"? Does overcommit still apply
to .min? etcetera.