Date:   Fri, 5 Mar 2021 11:00:40 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] memcg: charge before adding to swapcache on swapin

On Fri, Mar 5, 2021 at 8:25 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Fri, Mar 05, 2021 at 12:06:31AM -0800, Hugh Dickins wrote:
> > On Wed, 3 Mar 2021, 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@...gle.com>
> >
> > Quite apart from helping with the stat you want, what you've ended
> > up with here is a nice cleanup in several different ways (and I'm
> > glad Johannes talked you out of __GFP_NOFAIL: much better like this).
> > I'll say
> >
> > Acked-by: Hugh Dickins <hughd@...gle.com>
> >
> > but I am quite unhappy with the name mem_cgroup_finish_swapin_page():
> > it doesn't finish the swapin, it doesn't finish the page, and I'm
> > not persuaded by your paragraph above that there's any "transaction"
> > here (if there were, I'd suggest "commit" instead of "finish"'; and
> > I'd get worried by the css_put before it's called - but no, that's
> > fine, it's independent).
> >
> > How about complementing mem_cgroup_charge_swapin_page() with
> > mem_cgroup_uncharge_swapin_swap()?  I think that describes well
> > what it does, at least in the do_memsw_account() case, and I hope
> > we can overlook that it does nothing at all in the other case.
>
> Yes, that's better. The sequence is still somewhat mysterious for
> people not overly familiar with memcg swap internals, but it's much
> clearer for people who are.
>
> I mildly prefer swapping the swapin bit:
>
> mem_cgroup_swapin_charge_page()
> mem_cgroup_swapin_uncharge_swap()
>
> But either way works for me.
>

I will do a coin toss to select one.

> > And it really doesn't need a page argument: both places it's called
> > have just allocated an order-0 page, there's no chance of a THP here;
> > but you might have some idea of future expansion, or matching
> > put_swap_page() - I won't object if you prefer to pass in the page.
>
> Agreed.

Will remove the page parameter.

BTW I will keep mem_cgroup_disabled() check in the uncharge swap
function as I am thinking of removing "swapaccount=0" functionality.

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