Re: [PATCH v3] memcg: charge before adding to swapcache on swapin

[Shakeel Butt <shakeelb@google.com>]

On Thu, Mar 4, 2021 at 7:48 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Mar 03, 2021 at 05:42:29PM -0800, Shakeel Butt 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().
> >
> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
>
> The patch looks good to me, I have just a minor documentation nit
> below. But with that addressed, please add:
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks.

>
[...]
>
> It's possible somebody later needs to change things around in the
> swapin path and it's not immediately obvious when exactly these two
> functions need to be called in the swapin sequence.
>
> Maybe add here and above that charge_swapin_page needs to be called
> before we try adding the page to the swapcache, and finish_swapin_page
> needs to be called when swapcache insertion has been successful?

I will update the comments and send v4 after a day or so to see if
someone else has any comments.