Re: [PATCH] memcg: drop kmem.limit_in_bytes

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jul 04, 2023 at 01:52:40PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> kmem.limit_in_bytes (v1 way to limit kernel memory usage) has been
> deprecated since 58056f77502f ("memcg, kmem: further deprecate
> kmem.limit_in_bytes") merged in 5.16. We haven't heard about any
> serious users since then but it seems that the mere presence of the file
> is causing more harm thatn good. We (SUSE) have had several bug reports
> from customers where Docker based containers started to fail because a
> write to kmem.limit_in_bytes has failed.
> 
> This was unexpected because runc code only expects ENOENT (kmem
> disabled) or EBUSY (tasks already running within cgroup). So a new error
> code was unexpected and the whole container startup failed. This has
> been later addressed by
> https://github.com/opencontainers/runc/commit/52390d68040637dfc77f9fda6bbe70952423d380
> so current Docker runtimes do not suffer from the problem anymore. There
> are still older version of Docker in use and likely hard to get rid of
> completely.
> 
> Address this by wiping out the file completely and effectively get back
> to pre 4.5 era and CONFIG_MEMCG_KMEM=n configuration.
> 
> I would recommend backporting to stable trees which have picked up
> 58056f77502f ("memcg, kmem: further deprecate kmem.limit_in_bytes").
> 
> Cc: stable
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  Documentation/admin-guide/cgroup-v1/memory.rst |  2 --
>  mm/memcontrol.c                                | 13 -------------
>  2 files changed, 15 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
> index 47d1d7d932a8..b92c71f39172 100644
> --- a/Documentation/admin-guide/cgroup-v1/memory.rst
> +++ b/Documentation/admin-guide/cgroup-v1/memory.rst
> @@ -92,8 +92,6 @@ Brief summary of control files.
>   memory.oom_control		     set/show oom controls.
>   memory.numa_stat		     show the number of memory usage per numa
>  				     node
> - memory.kmem.limit_in_bytes          This knob is deprecated and writing to
> -                                     it will return -ENOTSUPP.
>   memory.kmem.usage_in_bytes          show current kernel memory allocation
>   memory.kmem.failcnt                 show the number of kernel memory usage
>  				     hits limits
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4b27e245a055..a0d3ed8d02e2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3750,9 +3750,6 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
>  	case _MEMSWAP:
>  		counter = &memcg->memsw;
>  		break;
> -	case _KMEM:
> -		counter = &memcg->kmem;
> -		break;
>  	case _TCP:
>  		counter = &memcg->tcpmem;
>  		break;

This case is still needed for the remaining kmem files:

	{
		.name = "kmem.usage_in_bytes",
		.private = MEMFILE_PRIVATE(_KMEM, RES_USAGE),
		.read_u64 = mem_cgroup_read_u64,
	},
	{
		.name = "kmem.failcnt",
		.private = MEMFILE_PRIVATE(_KMEM, RES_FAILCNT),
		.write = mem_cgroup_reset,
		.read_u64 = mem_cgroup_read_u64,
	},
	{
		.name = "kmem.max_usage_in_bytes",
		.private = MEMFILE_PRIVATE(_KMEM, RES_MAX_USAGE),
		.write = mem_cgroup_reset,
		.read_u64 = mem_cgroup_read_u64,
	},

otherwise they BUG() when reading.

Without this hunk, the patch looks good to me.