Re: [PATCH 03/11] mm/zsmalloc: Introduce conditional memcg awareness to zs_pool

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Wed, 11 Mar 2026 16:16:34 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Wed, Mar 11, 2026 at 12:51:40PM -0700, Joshua Hahn wrote:
> > Introduce 3 new fields to struct zs_pool to allow individual zpools to
> > be "memcg-aware": memcg_aware, compressed_stat, and uncompressed_stat.
> > 
> > memcg_aware is used in later patches to determine whether memory
> > should be allocated to keep track of per-compresed object objgs.
> > compressed_stat and uncompressed_stat are enum indices that point into
> > memcg (node) stats that zsmalloc will account towards.
> > 
> > In reality, these fields help distinguish between the two users of
> > zsmalloc, zswap and zram. The enum indices compressed_stat and
> > uncompressed_stat are parametrized to minimize zswap-specific hardcoding
> > in zsmalloc.
> > 
> > Suggested-by: Yosry Ahmed <yosry@kernel.org>
> > Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> > ---
> >  drivers/block/zram/zram_drv.c |  3 ++-
> >  include/linux/zsmalloc.h      |  5 ++++-
> >  mm/zsmalloc.c                 | 13 ++++++++++++-
> >  mm/zswap.c                    |  3 ++-
> >  4 files changed, 20 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index bca33403fc8b..d1eae5c20df7 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -1980,7 +1980,8 @@ static bool zram_meta_alloc(struct zram *zram, u64 disksize)
> >  	if (!zram->table)
> >  		return false;
> >  
> > -	zram->mem_pool = zs_create_pool(zram->disk->disk_name);
> > +	/* zram does not support memcg accounting */
> > +	zram->mem_pool = zs_create_pool(zram->disk->disk_name, false, 0, 0);

Hello Johannes,

Thank you for your review! I hope you are doing well : -)

> It's a bit awkward that 0 is valid (MEMCG_SWAP). Plus you store these
> values in every pool, even though they're always the same for all
> zswap pools.

Agreed. Originally I thought of removing memcg_aware and doing a bitwise
OR check of the two stats, but thought this was also a bit strange
(also because 0 is not a valid enum state for memcg_stat_item anyways)

> How about:
> 
> /* zsmalloc.h */
> struct zs_memcg_params {
> 	enum memcg_stat_item compressed;
> 	enum memcg_stat_item uncompressed;
> };
> struct zs_pool *zs_create_pool(const char *name, struct zs_memcg_params *memcg_params);
> 
> /* zswap.c */
> static struct zs_memcg_params zswap_memcg_params = {
> 	.compressed = MEMCG_ZSWAP_B,
> 	.uncompressed = MEMCG_ZSWAPPED,
> };
> 
> then pass &zswap_memcg_params from zswap and NULL from zram.
> 
> > @@ -2071,6 +2079,9 @@ struct zs_pool *zs_create_pool(const char *name)
> >  	rwlock_init(&pool->lock);
> >  	atomic_set(&pool->compaction_in_progress, 0);
> >  
> > +	pool->memcg_aware = memcg_aware;
> > +	pool->compressed_stat = compressed_stat;
> > +	pool->uncompressed_stat = uncompressed_stat;
> 
> 	pool->memcg_params = memcg_params;
> 
> And then use if (pool->memcg_params) to gate in zsmalloc.c.

These definitely look a lot cleaner. Will make these changes in v3!

Thanks again. I hope you have a great day!
Joshua