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From: "Yosry Ahmed" Message-ID: <75c6f71f794c04e6c6fc90e6ac0a3d4656651362@linux.dev> TLS-Required: No Subject: Re: [PATCH v1] mm: zswap: add per-memcg stat for incompressible pages To: "Jiayuan Chen" , "SeongJae Park" Cc: "SeongJae Park" , linux-mm@kvack.org, "Jiayuan Chen" , "Johannes Weiner" , "Michal Hocko" , "Roman Gushchin" , "Shakeel Butt" , "Muchun Song" , "Nhat Pham" , "Chengming Zhou" , "Andrew Morton" , "Nick Terrell" , "David Sterba" , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: References: <20260206022152.67992-1-sj@kernel.org> X-Migadu-Flow: FLOW_OUT On Thu, 5 Feb 2026 13:30:12 +0800 Jiayuan Chen w= rote: > >=20=20 >=20>=20=20 >=20> From: Jiayuan Chen > >=20=20 >=20> The global zswap_stored_incompressible_pages counter was added in = commit > > dca4437a5861 ("mm/zswap: store > to track how many pages are stored in raw (uncompressed) form in zsw= ap. > > However, in containerized environments, knowing which cgroup is > > contributing incompressible pages is essential for effective resourc= e > > management. > >=20=20 >=20> Add a new memcg stat 'zswpraw' to track incompressible pages per c= group. > > This helps administrators and orchestrators to: > >=20=20 >=20> 1. Identify workloads that produce incompressible data (e.g., encr= ypted > > data, already-compressed media, random data) and may not benefit fro= m > > zswap. > >=20=20 >=20> 2. Make informed decisions about workload placement - moving > > incompressible workloads to nodes with larger swap backing devices > > rather than relying on zswap. > >=20=20 >=20> 3. Debug zswap efficiency issues at the cgroup level without needi= ng to > > correlate global stats with individual cgroups. > >=20=20 >=20> 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 pa= ges > > mixed with highly compressible ones". The zswpraw stat provides dire= ct > > visibility into the latter case. > >=20=20 >=20> Changes > > ------- > >=20=20 >=20> 1. Add zswap_is_raw() helper (include/linux/zswap.h) > > - Abstract the PAGE_SIZE comparison logic for identifying raw entrie= s > > - Keep the incompressible check in one place for maintainability > >=20=20 >=20> 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 > >=20=20 >=20> 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 > >=20=20 >=20> Test > > ---- > >=20=20 >=20> I wrote a simple test program[1] that allocates memory and compres= ses it > > with zstd, so kernel zswap cannot compress further. > >=20=20 >=20> $ 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 > >=20=20 >=20> $ 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 > >=20=20 >=20> $ pkill test_zswpraw > > $ cat /sys/fs/cgroup/test/memory.stat | grep zswp > > zswpraw 0 > > zswpin 1 > > zswpout 51222 > > zswpwb 0 > >=20=20 >=20> [1] https://gist.github.com/mrpre/00432c6154250326994fbeaf62e0e6f1 > >=20=20 >=20> Signed-off-by: Jiayuan Chen > > --- > > 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(-) > >=20=20 >=20> As others also mentioned, the documentation of the new stat would = be needed. > >=20=20 >=20>=20=20 >=20> 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, > > MEMCG_NR_STAT, > > }; > >=20=20 >=20> 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 @@ > >=20=20 >=20> struct lruvec; > >=20=20 >=20> +/* > > + * 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) > > +{ > > + return size =3D=3D PAGE_SIZE; > > +} > > + > >=20=20 >=20> No strong opinion, but I'm not really sure if the helper is needed= , because it > > feels quite simple logic: > >=20=20 >=20> "If an object is compressed and the size is same to the original o= ne, the > > object is incompressible." > >=20=20 >=20> I also feel the function name bit odd, given the type of the param= eter. Based > > on the function name and the comment, I'd expect it to receive a zsw= ap_entry > > object. I understand it is better to receive a size_t, to be called = from > > obj_cgroup_[un]charge_zswap(), though. Even in the case, I think the= name can > > be better (e.g., zswap_compression_failed() or zswap_was_incompressi= ble() ?), > > or at least the coment can be more kindly explain the fact that the = parameter > > is the size of object after the compression attempt. > >=20=20 >=20>=20=20 >=20> extern atomic_long_t zswap_stored_pages; > >=20=20 >=20> #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[] =3D= { > > MEMCG_KMEM, > > MEMCG_ZSWAP_B, > > MEMCG_ZSWAPPED, > > + MEMCG_ZSWAP_RAW, > > }; > >=20=20 >=20> No strong opinion, but I think Shakeel's suggestion of other names= is > > reasonable. > >=20=20 >=20>=20=20 >=20> #define NR_MEMCG_NODE_STAT_ITEMS ARRAY_SIZE(memcg_node_stat_items) > > @@ -1346,6 +1347,7 @@ static const struct memory_stat memory_stats[]= =3D { > > #ifdef CONFIG_ZSWAP > > { "zswap", MEMCG_ZSWAP_B }, > > { "zswapped", MEMCG_ZSWAPPED }, > > + { "zswpraw", MEMCG_ZSWAP_RAW }, > >=20=20 >=20> Ditto. > >=20=20 >=20>=20=20 >=20> #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 =3D 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); > >=20=20 >=20> I understand the helper function is better to receive size_t rathe= r than > > zswap_entry for this. > >=20=20 >=20>=20=20 >=20> rcu_read_unlock(); > > } > >=20=20 >=20> @@ -5481,6 +5485,8 @@ void obj_cgroup_uncharge_zswap(struct obj_cg= roup *objcg, size_t size) > > memcg =3D 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(); > > } > >=20=20 >=20> 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 =3D=3D 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->len= gth); > >=20=20 >=20> /* zswap entries of length PAGE_SIZE are not compressed. */ > > - if (entry->length =3D=3D PAGE_SIZE) { > > + if (zswap_is_raw(entry->length)) { > > WARN_ON_ONCE(input->length !=3D PAGE_SIZE); > > memcpy_from_sglist(kmap_local_folio(folio, 0), input, 0, PAGE_SIZE); > > dlen =3D PAGE_SIZE; > >=20=20 >=20> Below this part, I show 'dlen =3D=3D PAGE_SIZE'. Should it also be= converted to > > use the helper function? > >=20 >=20The dlen variable represents the decompressed (plaintext) size. > Since we compress individual pages, the decompressed output should > always be PAGE_SIZE in normal cases. >=20 >=20This check validates whether decompression produced the expected resu= lt, not whether the entry is incompressible. >=20 >=20Using zswap_is_incomp() here would be semantically incorrect - the he= lper is meant to check if an > entry was stored without compression (i.e., compression failed to reduc= e size), while dlen =3D=3D PAGE_SIZE > verifies the output of decompression is valid. This confusion is exactly why we should drop the helper.