Re: [PATCH v4] memcg: charge before adding to swapcache on swapin

[Shakeel Butt <shakeelb@google.com>]

On Fri, Mar 5, 2021 at 1:26 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> Currently the kernel adds the page, allocated for swapin, to the
> swapcache before charging the page. This is fine but now we want a
> per-memcg swapcache stat which is essential for folks who wants to
> transparently migrate from cgroup v1's memsw to cgroup v2's memory and
> swap counters. In addition charging a page before exposing it to other
> parts of the kernel is a step in the right direction.
>
> To correctly maintain the per-memcg swapcache stat, this patch has
> adopted to charge the page before adding it to swapcache. One
> challenge in this option is the failure case of add_to_swap_cache() on
> which we need to undo the mem_cgroup_charge(). Specifically undoing
> mem_cgroup_uncharge_swap() is not simple.
>
> To resolve the issue, this patch introduces transaction like interface
> to charge a page for swapin. The function mem_cgroup_charge_swapin_page()
> initiates the charging of the page and mem_cgroup_finish_swapin_page()
> completes the charging process. So, the kernel starts the charging
> process of the page for swapin with mem_cgroup_charge_swapin_page(),
> adds the page to the swapcache and on success completes the charging
> process with mem_cgroup_finish_swapin_page().

And of course I forgot to update the commit message.

Andrew, please replace the third paragraph with the following para:

To resolve the issue, this patch decouples the charging for swapin pages from
mem_cgroup_charge(). Two new functions are introduced,
mem_cgroup_swapin_charge_page() for just charging the swapin page and
mem_cgroup_swapin_uncharge_swap() for uncharging the swap slot once the
page has been successfully added to the swapcache.