Re: [PATCH] mm: memcg: reset zswap settings in css_reset

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 6/30/26 6:08 PM, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>
> mem_cgroup_css_reset() is called when the memory controller is disabled
> on a cgroup but the memcg cannot be destroyed because it is pinned by a
> subsystem dependency -- for example, the io controller declares
> .depends_on = 1 << memory_cgrp_id, so memory remains in the cgroup_ss_mask
> and the css is hidden rather than killed.
>
> The purpose of css_reset is to revert the memcg to its vanilla state so
> that no policies are applied and the css can be safely made visible again
> later.  Currently, all page counters (memory.max, swap.max, kmem.max,
> tcpmem.max) and other limits (soft_limit, memory.high, swap.high) are
> reset to their defaults, but zswap_max and zswap_writeback are not.
>
> These fields are initialized in css_alloc (zswap_max = PAGE_COUNTER_MAX,
> zswap_writeback inherited from parent) but were missing from css_reset.
> As a result, stale zswap policies remain in effect after css_reset: the
> zswap charge path (obj_cgroup_may_zswap) continues to enforce the old
> zswap_max limit, and the writeback path continues to honor the old
> zswap_writeback setting, even though the memory controller has been
> "disabled" on this cgroup.
>
> Reset zswap_max to PAGE_COUNTER_MAX and zswap_writeback to true, matching
> their defaults in css_alloc.
>
> Test:
> 	echo "+memory +io" > /sys/fs/cgroup/cgroup.subtree_control
>
> 	mkdir /sys/fs/cgroup/test
> 	mkdir /sys/fs/cgroup/test/child
>
> 	echo "+memory +io" > /sys/fs/cgroup/test/cgroup.subtree_control
> 	echo 10000 > /sys/fs/cgroup/test/child/memory.zswap.max
>
> 	# child/memory.swap.max and child/memory.zswam.max disappear
> 	echo "-memory" > /sys/fs/cgroup/test/cgroup.subtree_control
>
> 	# re-enable memory control
> 	echo "+memory" > /sys/fs/cgroup/test/cgroup.subtree_control
>
> 	# before this patch
> 	cat /sys/fs/cgroup/test/child/memory.zswap.max
> 	    8192
>
> 	# after this patch, same as memory.swap.max
> 	cat /sys/fs/cgroup/test/child/memory.zswap.max
> 	    max
>
> Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>   mm/memcontrol.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d20ffc827306..eeeb22a5e8cc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4362,6 +4362,10 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
>   
>   	page_counter_set_max(&memcg->memory, PAGE_COUNTER_MAX);
>   	page_counter_set_max(&memcg->swap, PAGE_COUNTER_MAX);
> +#ifdef CONFIG_ZSWAP
> +	memcg->zswap_max = PAGE_COUNTER_MAX;
> +	WRITE_ONCE(memcg->zswap_writeback, true);
> +#endif
>   #ifdef CONFIG_MEMCG_V1
>   	page_counter_set_max(&memcg->kmem, PAGE_COUNTER_MAX);
>   	page_counter_set_max(&memcg->tcpmem, PAGE_COUNTER_MAX);


https://sashiko.dev/#/patchset/20260630100832.107062-1-jiayuan.chen%40linux.dev

Ai is right...


Also I'm thinking that should we add a helper instead of using open code 
in css_alloc and  css_reset ?


static void memcg_zswap_reset(struct mem_cgroup *memcg)
{
#ifdef CONFIG_ZSWAP
     WRITE_ONCE(memcg->zswap_max, PAGE_COUNTER_MAX);
     WRITE_ONCE(memcg->zswap_writeback, true);
#endif
}

mem_cgroup_css_reset()
{
...
memcg_zswap_reset()
...
}

mem_cgroup_css_alloc()
{
...
memcg_zswap_reset()
...

}