Re: [PATCH v2 1/4] mm: zswap: fix shrinker count for the root memcg

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Nov 09, 2026 at 05:53:18PM -0800, Joanne Koong wrote:
> zswap_shrinker_count() scales the number of freeable entries on a
> memcg's zswap LRU by the memcg's compression ratio. That compression
> ratio is computed from the memcg's MEMCG_ZSWAP_B and MEMCG_ZSWAPPED
> stats.
> 
> However, for the root memcg, the zswap entries owned by the root memcg
> itself are never accounted for in the stats (obj_cgroup_charge_zswap()
> skips the root objcg, and before the root memcg had an objcg, its
> entries weren't charged at all). The stats only reflect the entries
> owned by the root memcg's descendants. When the root memcg's descendants
> have no zswap entries (eg when the whole workload runs in the root
> cgroup), the shrinker reports nothing to reclaim for the root memcg, and
> its zswap entries are never written back by the shrinker. When its
> descendants do have entries, the root memcg's entries are scaled by the
> descendants' compression ratio instead of a ratio that includes them.
> 
> Fix this by using the zswap pool-wide counters for the root memcg
> instead. The zswap pool-wide counters cover every zswap entry, including
> those owned by the root memcg.
> 
> This also gets rid of having to flush the root memcg's stats, which
> flushes every cgroup in the system.
> 
> In a VM running a reclaim-heavy workload in the root cgroup with the
> zswap shrinker enabled, the shrinker wrote back no pages before this
> change (zswpwb stayed at 0) and 1016 pages with it.
> 
> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
> Acked-by: Yosry Ahmed <yosry@kernel.org>
> Reviewed-by: SJ Park <sj@kernel.org>
> Acked-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>