Re: [PATCH v2] docs: cgroup: document empty-write behavior of memory limit knobs

[Michal Hocko]

On Wed 26-08-26 10:17:53, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> A maintenance script on a cluster wrote an unset variable into
> memory.max of a workload cgroup; the variable expanded to an empty
> string, the write succeeded, and the workload in the cgroup was
> OOM-killed.  Nothing pointed back at the write, so it took quite
> some time to trace the OOM kills to that script.
> 
> The memory controller documentation does not say what an empty
> write does; the cpuset controller documents its empty-value
> semantics.  The actual behavior is that the empty string is
> accepted as 0.  Reproduced on a k8s cluster (v1.29, cgroup v2,
> two-container pod, 384M limit):
> 
>   # LIMIT=
>   # echo "$LIMIT" > $CG/memory.max
>   # echo $?
>   0
> 
>   m6demo   0/2     OOMKilled   0
> 
>   Memory cgroup out of memory: Killed process 339529 (sleep) ... anon-rss:32kB
> 
> State it where the interface files are introduced, alongside the
> existing notes on units and page rounding.
> 
> Link: https://lore.kernel.org/all/aoVUlFdZYLFn_gvJ@tiehlicka/
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!

> 
> ---
> 
> Changes since v1: document the empty-write behavior instead of
> rejecting it, the outcome of the review discussion at the Link:
> below.
> 
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 86a2a0099178..8d2603751c51 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1321,6 +1321,10 @@ All memory amounts are in bytes.  If a value which is not aligned to
>  PAGE_SIZE is written, the value may be rounded up to the closest
>  PAGE_SIZE multiple when read back.
>  
> +For the limit files described below, an empty or all-whitespace
> +write is accepted and sets the limit to 0.  To disable a limit,
> +write "max"; to set it to zero explicitly, write "0".
> +
>    memory.current
>  	A read-only single value file which exists on non-root
>  	cgroups.
> -- 
> 2.43.0

-- 
Michal Hocko
SUSE Labs