Re: [PATCH v2] mm: memcg: reset zswap settings in css_reset

[Muchun Song]

> On Jul 2, 2026, at 10:48, Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
> 
> From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>

Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.