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

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Message-ID: <2928382d5614d5632e3508c81878c8ad720ccd7e@linux.dev>
Date: Fri, 06 Feb 2026 02:05:25 +0000
From: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
To: "Yosry Ahmed" <yosry.ahmed@...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>, "Shakeel Butt"
 <shakeel.butt@...ux.dev>, "Muchun Song" <muchun.song@...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

February 6, 2026 at 08:39, "Yosry Ahmed" <yosry.ahmed@...ux.dev mailto:yosry.ahmed@...ux.dev?to=%22Yosry%20Ahmed%22%20%3Cyosry.ahmed%40linux.dev%3E > wrote:


> 
> 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>
> >  ---
> >  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,
> > 
> Please change the name as Shakeel suggested.
> 
> > 
> > MEMCG_NR_STAT,
> >  };
> >  
> >  diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> >  index 30c193a1207e..94f84b154b71 100644
> >  --- a/include/linux/zswap.h
> >  +++ b/include/linux/zswap.h
> >  @@ -7,6 +7,15 @@
> >  
> >  struct lruvec;
> >  
> >  +/*
> >  + * Check if a zswap entry is stored in raw (uncompressed) form.
> >  + * This happens when compression doesn't reduce the size.
> >  + */
> >  +static inline bool zswap_is_raw(size_t size)
> > 
> Internall as well, please rename this to zswap_is_incompressible() or
> zswap_is_incomp(). Not a big fan of the helper because it doesn't add
> much, but I don't feel strongly either way.
> 
> > 
> > +{
> >  + return size == PAGE_SIZE;
> >  +}
> >  +
> >  extern atomic_long_t zswap_stored_pages;
> >  
> >  #ifdef CONFIG_ZSWAP
> >  diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >  index 007413a53b45..32fb801530a3 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_RAW,
> >  };
> >  
> >  #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 },
> >  + { "zswpraw", MEMCG_ZSWAP_RAW },
> > 
> Here as well: zswap_incompressible or zswap_incomp?
> 
> Other than the renames and doc, LGTM.
> 
> > 


Thanks Yosry!

I'll rename everything to use "incomp" (MEMCG_ZSWAP_INCOMP, zswap_is_incomp(), "zswap_incomp")
and update the documentation in v2.