Re: [PATCH 07/11] mm/zsmalloc, zswap: Handle objcg charging and lifetime in zsmalloc

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

On Thu, 12 Mar 2026 17:42:01 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Wed, Mar 11, 2026 at 12:51:44PM -0700, Joshua Hahn wrote:
> > @@ -1244,6 +1297,8 @@ void zs_obj_write(struct zs_pool *pool, unsigned long handle,
> >  	if (objcg) {
> >  		WARN_ON_ONCE(!pool->memcg_aware);
> >  		zspage->objcgs[obj_idx] = objcg;
> > +		obj_cgroup_get(objcg);
> > +		zs_charge_objcg(pool, objcg, class->size);
> >  	}
> >  
> >  	if (!ZsHugePage(zspage))

Hello Johannes, thank you for your review!

> Note that the obj_cgroup_get() reference is for the pointer, not the
> charge. I think it all comes out right in the end, but it's a bit
> confusing to follow and verify through the series.

Thank you for pointing that out. I'll try to make it more explicit via
the placement.

> IOW, it's better move that obj_cgroup_get() when you add and store
> zspage->objcgs[]. If zswap stil has a reference at that point in the
> series, then it's fine for there to be two separate obj_cgroup_get()
> as well, with later patches deleting the zswap one when its
> entry->objcg pointer disappears.

Sounds good with me. Maybe for the code block above I just move it one
line up so that it happens before the zspage->objcgs set and
make it more obvious that it's associated with setting the objcg
pointer and not with the charge?

And for the freeing section, putting after we set the pointer to
NULL could be more obvious?

> > @@ -711,10 +711,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
> >  	zswap_lru_del(&zswap_list_lru, entry, objcg);
> >  	zs_free(entry->pool->zs_pool, entry->handle);
> >  	zswap_pool_put(entry->pool);
> > -	if (objcg) {
> > -		obj_cgroup_uncharge_zswap(objcg, entry->length);
> > -		obj_cgroup_put(objcg);
> > -	}
> >  	if (entry->length == PAGE_SIZE)
> >  		atomic_long_dec(&zswap_stored_incompressible_pages);
> >  	zswap_entry_cache_free(entry);
> 
> [ I can see that this was misleading. It was really getting a
>   reference for the entry->objcg = objcg a few lines down, hitching a
>   ride on that existing `if (objcg)`. ]

Thank you for the clarification! I hope you have a great day : -)
Joshua