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Message-ID: <hevovghpl2udhpof66oz26ulrpqcrtuwjxcakyskoeoil2wo6x@osbrncj7ifwz>
Date: Fri, 6 Feb 2026 00:39:53 +0000
From: Yosry Ahmed <yosry.ahmed@...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>, 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
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.
> #endif
> { "file_mapped", NR_FILE_MAPPED },
> { "file_dirty", NR_FILE_DIRTY },
> @@ -5458,6 +5460,8 @@ 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);
> + if (zswap_is_raw(size))
> + mod_memcg_state(memcg, MEMCG_ZSWAP_RAW, 1);
> rcu_read_unlock();
> }
>
> @@ -5481,6 +5485,8 @@ 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);
> + if (zswap_is_raw(size))
> + mod_memcg_state(memcg, MEMCG_ZSWAP_RAW, -1);
> rcu_read_unlock();
> }
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 3d2d59ac3f9c..54ab4d126f64 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -723,7 +723,7 @@ static void zswap_entry_free(struct zswap_entry *entry)
> obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
> obj_cgroup_put(entry->objcg);
> }
> - if (entry->length == PAGE_SIZE)
> + if (zswap_is_raw(entry->length))
> atomic_long_dec(&zswap_stored_incompressible_pages);
> zswap_entry_cache_free(entry);
> atomic_long_dec(&zswap_stored_pages);
> @@ -941,7 +941,7 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> zs_obj_read_sg_begin(pool->zs_pool, entry->handle, input, entry->length);
>
> /* zswap entries of length PAGE_SIZE are not compressed. */
> - if (entry->length == PAGE_SIZE) {
> + if (zswap_is_raw(entry->length)) {
> WARN_ON_ONCE(input->length != PAGE_SIZE);
> memcpy_from_sglist(kmap_local_folio(folio, 0), input, 0, PAGE_SIZE);
> dlen = PAGE_SIZE;
> @@ -1448,7 +1448,7 @@ static bool zswap_store_page(struct page *page,
> obj_cgroup_charge_zswap(objcg, entry->length);
> }
> atomic_long_inc(&zswap_stored_pages);
> - if (entry->length == PAGE_SIZE)
> + if (zswap_is_raw(entry->length))
> atomic_long_inc(&zswap_stored_incompressible_pages);
>
> /*
> --
> 2.43.0
>
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