Re: [PATCH v3 7/8] mm: zswap: Refactor and optimize zswap_store_pages() batch operations.

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 3 potential
issues to consider:
- [High] mm: zswap: missing !CONFIG_MEMCG stub for obj_cgroup_get_many()
- [Low] mm: zswap: unconditional incompressible page stat updates on
  zero delta
- [Low] mm: memcontrol: unused and duplicated obj_cgroup_charge_zswap()
--

commit 55351c710f10c5fbb79d9201d80496dc658bbc19
Author: Kanchana P. Sridhar <kanchanapsridhar2026@gmail.com>

mm: zswap: Refactor and optimize zswap_store_pages() batch operations.

This commit factors zswap_store_pages() into batch helpers
zswap_init_entries() and zswap_tree_stores(), and introduces
obj_cgroup_charge_zswap_many() to charge and account zswap batches at
once.

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ac6c43ea8c5a6..ab280a448e8c4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6368,6 +6368,42 @@ void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size)
>  	rcu_read_unlock();
>  }
>  
> +/**
> + * obj_cgroup_charge_zswap_many - bulk charge compression backend memory
> + *                                from compressing a batch of pages.
> + * @objcg: the object cgroup
> + * @size: total compressed length of a batch of compressed objects
> + * @nr_pages: # of pages that were compressed to store in zswap
> + * @nr_incomp_pages: # of incompressible pages to store in zswap
> + *
> + * This forces the charge after obj_cgroup_may_zswap() allowed
> + * compression and storage in zswap for this cgroup to go ahead.
> + */
> +void obj_cgroup_charge_zswap_many(struct obj_cgroup *objcg, size_t size,
> +				  u8 nr_pages, u8 nr_incomp_pages)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +		return;
> +
> +	if (obj_cgroup_is_root(objcg))
> +		return;
> +
> +	VM_WARN_ON_ONCE(!(current->flags & PF_MEMALLOC));
> +
> +	/* PF_MEMALLOC context, charging must succeed */
> +	if (obj_cgroup_charge(objcg, GFP_KERNEL, size))
> +		VM_WARN_ON_ONCE(1);
> +
> +	rcu_read_lock();
> +	memcg = obj_cgroup_memcg(objcg);
> +	mod_memcg_state(memcg, MEMCG_ZSWAP_B, size);
> +	mod_memcg_state(memcg, MEMCG_ZSWAPPED, nr_pages);
> +	mod_memcg_state(memcg, MEMCG_ZSWAP_INCOMP, nr_incomp_pages);
> +	rcu_read_unlock();
> +}

[Severity: Low]
Could obj_cgroup_charge_zswap() be removed now that its only caller in
mm/zswap.c has switched to obj_cgroup_charge_zswap_many(), just as
zswap_pool_get() was replaced by zswap_pool_get_many()?  Leaving
obj_cgroup_charge_zswap() in mm/memcontrol.c and include/linux/memcontrol.h
duplicates the charging logic with no remaining callers.

> diff --git a/mm/zswap.c b/mm/zswap.c
> index 8c36344000f51..5ef1ae67e67a9 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c

[ ... ]

> @@ -1711,17 +1702,86 @@ static bool zswap_store_pages(struct zswap_store_context *zc,
>  			zswap_lru_add(entry);
>  	}
>  
> +	/*
> +	 * Each batch entry is successfully compressed and stored in the tree,
> +	 * and further failures will be cleaned up in zswap_store().
> +	 * Next, we do bulk accounting for the batch: grab refs to the pool and
> +	 * objcg, charge zswap memory, increment zswap_stored_pages and
> +	 * zswap_stored_incompressible_pages. The opposite actions will be
> +	 * performed by zswap_entry_free() when the entry is removed from the
> +	 * tree.
> +	 */
> +	zswap_pool_get_many(pool, zc->nr_pages);
> +	if (objcg) {
> +		obj_cgroup_get_many(objcg, zc->nr_pages);

[Severity: High]
Does calling obj_cgroup_get_many() here break the build when CONFIG_ZSWAP=y
and CONFIG_MEMCG=n?

include/linux/memcontrol.h only defines obj_cgroup_get_many() inside the
#ifdef CONFIG_MEMCG block, while the #else section only provides stubs for
obj_cgroup_get() and obj_cgroup_put().

> +		obj_cgroup_charge_zswap_many(objcg, total_batch_comp_length,
> +					     zc->nr_pages, nr_incomp);
> +	}
> +	atomic_long_add(zc->nr_pages, &zswap_stored_pages);
> +	atomic_long_add(nr_incomp, &zswap_stored_incompressible_pages);

[Severity: Low]
Should the incompressible page updates here and in
obj_cgroup_charge_zswap_many() be skipped when nr_incomp is 0?

In the common case where all pages in the batch are compressible,
atomic_long_add(0, &zswap_stored_incompressible_pages) still performs an
atomic read-modify-write on the shared global counter, and
mod_memcg_state(memcg, MEMCG_ZSWAP_INCOMP, 0) still runs get_cpu(), updates
per-CPU stats by 0, and fires trace_mod_memcg_state() with val == 0.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261106072412.7909-1-kanchanapsridhar2026@gmail.com?part=7