Re: [PATCH v5 7/7] mm/memcontrol: add stock to the memsw page_counter

[Michal Koutný]

Hello Joshua.

On Mon, Aug 31, 2026 at 09:37:51AM -0700, Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:
> Before this series, each memcg had one stock shared by all its
> page_counters (memory + memsw). Now that the memcg stock was folded
> into the page_counter level, give memsw its own page_counter_stock
> so that it can benefit from caching charges as well.
> 
> Note that while the allocation is conditional on do_memsw_account(),
> the freeing is not; the freer will only free non-NULL stocks. This
> matters because do_memsw_account() could have changed in between the
> allocation and the free.

(Just a passerby comment, I have no remarks to the overhaul.)

The change of do_memsw_account() is because of re-attaching the memory
controller between v1 and v2 trees. That's not so common operation (but
not ruled out), pre-condition for that is that there's only a single
online memcg, namely the root memcg. So it needs treatment especially at
memcg offlining (which is what drain_all_stock() (also) does).

I don't know how expensive page_counter_drain_stock_async() is going to
be [1] but the v1 parts here could could be guarded with
!cgroup_on_dfl(memcg->css.cgroup).

Regards,
Michal

[1] Now, I see in 5/7 that there's the counter->stock check which should
    be quick bailout on v2 (maybe quicker than the cgroup_on_dfl()
    I proposed above).

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