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

[Hugh Dickins]

On Fri, 5 Mar 2021, Shakeel Butt wrote:
> On Fri, Mar 5, 2021 at 1:26 PM Shakeel Butt <shakeelb@xxxxxxxxxx> 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.

Lgtm
Hugh