linux-kernel - Re: [PATCH v1] mm: zswap: add per-memcg stat for incompressible pages

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Message-ID: <aYUKpZ8nCB6MTQGY@linux.dev>
Date: Thu, 5 Feb 2026 13:33:50 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: linux-mm@...ck.org, Jiayuan Chen <jiayuan.chen@...pee.com>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, Nhat Pham <nphamcs@...il.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Nick Terrell <terrelln@...com>, David Sterba <dsterba@...e.com>, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm: zswap: add per-memcg stat for incompressible pages

On Thu, Feb 05, 2026 at 01:30:12PM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@...pee.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.
> 
> Add a new memcg stat 'zswpraw' 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 zswpraw stat provides direct
> visibility into the latter case.
> 
> Changes
> -------
> 
> 1. Add zswap_is_raw() helper (include/linux/zswap.h)
>    - Abstract the PAGE_SIZE comparison logic for identifying raw entries
>    - Keep the incompressible check in one place for maintainability
> 
> 2. Add MEMCG_ZSWAP_RAW stat definition (include/linux/memcontrol.h,
>    mm/memcontrol.c)
>    - Add MEMCG_ZSWAP_RAW to memcg_stat_item enum
>    - Register in memcg_stat_items[] and memory_stats[] arrays
>    - Export as "zswpraw" in memory.stat
> 
> 3. Update statistics accounting (mm/memcontrol.c, mm/zswap.c)
>    - Track MEMCG_ZSWAP_RAW in obj_cgroup_charge/uncharge_zswap()
>    - Use zswap_is_raw() helper in zswap.c for consistency
> 
> Test
> ----
> 
> I wrote a simple test program[1] that allocates memory and compresses it
> with zstd, so kernel zswap cannot compress further.
> 
>   $ cgcreate -g memory:test
>   $ cgexec -g memory:test ./test_zswpraw &
>   $ cat /sys/fs/cgroup/test/memory.stat | grep zswp
>   zswpraw 0
>   zswpin 0
>   zswpout 0
>   zswpwb 0
> 
>   $ echo "100M" > /sys/fs/cgroup/test/memory.reclaim
>   $ cat /sys/fs/cgroup/test/memory.stat | grep zswp
>   zswpraw 104800256
>   zswpin 0
>   zswpout 51222
>   zswpwb 0
> 
>   $ pkill test_zswpraw
>   $ cat /sys/fs/cgroup/test/memory.stat | grep zswp
>   zswpraw 0
>   zswpin 1
>   zswpout 51222
>   zswpwb 0
> 
> [1] https://gist.github.com/mrpre/00432c6154250326994fbeaf62e0e6f1
> 
> Signed-off-by: Jiayuan Chen <jiayuan.chen@...pee.com>

Overall looks good but as Nhat suggested please update v2
documentation.


> ---
>  include/linux/memcontrol.h | 1 +
>  include/linux/zswap.h      | 9 +++++++++
>  mm/memcontrol.c            | 6 ++++++
>  mm/zswap.c                 | 6 +++---
>  4 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index b6c82c8f73e1..83d1328f81d1 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_RAW,

Hmm I don't like the name though. How about INCOMPRESSIBLE or INCOMP for
short?