Re: [PATCH v2 1/2] mm: zswap: add per-memcg stat for incompressible pages

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Fri, Feb 06, 2026 at 03:22:15PM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> The global zswap_stored_incompressible_pages counter was added in commit
> dca4437a5861 ("mm/zswap: store <PAGE_SIZE compression failed page as-is")
> to track how many pages are stored in raw (uncompressed) form in zswap.
> However, in containerized environments, knowing which cgroup is
> contributing incompressible pages is essential for effective resource
> management [1].
> 
> Add a new memcg stat 'zswap_incomp' to track incompressible pages per
> cgroup. This helps administrators and orchestrators to:
> 
> 1. Identify workloads that produce incompressible data (e.g., encrypted
>    data, already-compressed media, random data) and may not benefit from
>    zswap.
> 
> 2. Make informed decisions about workload placement - moving
>    incompressible workloads to nodes with larger swap backing devices
>    rather than relying on zswap.
> 
> 3. Debug zswap efficiency issues at the cgroup level without needing to
>    correlate global stats with individual cgroups.
> 
> While the compression ratio can be estimated from existing stats
> (zswap / zswapped * PAGE_SIZE), this doesn't distinguish between
> "uniformly poor compression" and "a few completely incompressible pages
> mixed with highly compressible ones". The zswap_incomp stat provides
> direct visibility into the latter case.
> 
> [1]: https://lore.kernel.org/linux-mm/CAF8kJuONDFj4NAksaR4j_WyDbNwNGYLmTe-o76rqU17La=nkOw@mail.gmail.com/
> Acked-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 5 +++++
>  include/linux/memcontrol.h              | 1 +
>  mm/memcontrol.c                         | 8 ++++++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 7f5b59d95fce..78a329414615 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1737,6 +1737,11 @@ The following nested keys are defined.
>  	  zswpwb
>  		Number of pages written from zswap to swap.
>  
> +	  zswap_incomp
> +		Number of incompressible pages currently stored in zswap
> +		without compression. These pages could not be compressed to
> +		a size smaller than PAGE_SIZE, so they are stored as-is.
> +
>  	  thp_fault_alloc (npn)
>  		Number of transparent hugepages which were allocated to satisfy
>  		a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index b6c82c8f73e1..d8ec05dd5d43 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -39,6 +39,7 @@ enum memcg_stat_item {
>  	MEMCG_KMEM,
>  	MEMCG_ZSWAP_B,
>  	MEMCG_ZSWAPPED,
> +	MEMCG_ZSWAP_INCOMP,
>  	MEMCG_NR_STAT,
>  };
>  
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 007413a53b45..a6b6cf5f1aeb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -341,6 +341,7 @@ static const unsigned int memcg_stat_items[] = {
>  	MEMCG_KMEM,
>  	MEMCG_ZSWAP_B,
>  	MEMCG_ZSWAPPED,
> +	MEMCG_ZSWAP_INCOMP,
>  };
>  
>  #define NR_MEMCG_NODE_STAT_ITEMS ARRAY_SIZE(memcg_node_stat_items)
> @@ -1346,6 +1347,7 @@ static const struct memory_stat memory_stats[] = {
>  #ifdef CONFIG_ZSWAP
>  	{ "zswap",			MEMCG_ZSWAP_B			},
>  	{ "zswapped",			MEMCG_ZSWAPPED			},
> +	{ "zswap_incomp",		MEMCG_ZSWAP_INCOMP		},
>  #endif
>  	{ "file_mapped",		NR_FILE_MAPPED			},
>  	{ "file_dirty",			NR_FILE_DIRTY			},
> @@ -5458,6 +5460,9 @@ void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size)
>  	memcg = obj_cgroup_memcg(objcg);
>  	mod_memcg_state(memcg, MEMCG_ZSWAP_B, size);
>  	mod_memcg_state(memcg, MEMCG_ZSWAPPED, 1);
> +	/* size == PAGE_SIZE means compression failed, page is incompressible */

I think the comment is not very useful, but maybe not worth sending a
new version. Otherwise LGTM:

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>

> +	if (size == PAGE_SIZE)
> +		mod_memcg_state(memcg, MEMCG_ZSWAP_INCOMP, 1);
>  	rcu_read_unlock();
>  }
>  
> @@ -5481,6 +5486,9 @@ void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size)
>  	memcg = obj_cgroup_memcg(objcg);
>  	mod_memcg_state(memcg, MEMCG_ZSWAP_B, -size);
>  	mod_memcg_state(memcg, MEMCG_ZSWAPPED, -1);
> +	/* size == PAGE_SIZE means compression failed, page is incompressible */
> +	if (size == PAGE_SIZE)
> +		mod_memcg_state(memcg, MEMCG_ZSWAP_INCOMP, -1);
>  	rcu_read_unlock();
>  }
>  
> -- 
> 2.43.0
>