Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <1585892447-32059-1-git-send-email-iamjoonsoo.kim@lge.com>
 <1585892447-32059-6-git-send-email-iamjoonsoo.kim@lge.com> <CAHbLzkqdupWUv7vPpqDpOARuYkBiTxmQxNi-zaw_TWVB1FsNjQ@mail.gmail.com>
In-Reply-To: <CAHbLzkqdupWUv7vPpqDpOARuYkBiTxmQxNi-zaw_TWVB1FsNjQ@mail.gmail.com>
From:   Joonsoo Kim <js1304@gmail.com>
Date:   Mon, 6 Apr 2020 10:03:31 +0900
Message-ID: <CAAmzW4Mr7NtOpPKpskhnMABJeVoe-CvOuC6TEfJabS7HdcwLbQ@mail.gmail.com>
Subject: Re: [PATCH v5 05/10] mm/swap: charge the page when adding to the swap cache
To:     Yang Shi <shy828301@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>, kernel-team@lge.com,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

2020=EB=85=84 4=EC=9B=94 4=EC=9D=BC (=ED=86=A0) =EC=98=A4=EC=A0=84 3:29, Ya=
ng Shi <shy828301@gmail.com>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On Thu, Apr 2, 2020 at 10:41 PM <js1304@gmail.com> wrote:
> >
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > Currently, some swapped-in pages are not charged to the memcg until
> > actual access to the page happens. I checked the code and found that
> > it could cause a problem. In this implementation, even if the memcg
> > is enabled, one can consume a lot of memory in the system by exploiting
> > this hole. For example, one can make all the pages swapped out and
> > then call madvise_willneed() to load the all swapped-out pages without
> > pressing the memcg. Although actual access requires charging, it's real=
ly
> > big benefit to load the swapped-out pages to the memory without pressin=
g
> > the memcg.
> >
> > And, for workingset detection which is implemented on the following pat=
ch,
> > a memcg should be committed before the workingset detection is executed=
.
> > For this purpose, the best solution, I think, is charging the page when
> > adding to the swap cache. Charging there is not that hard. Caller of
> > adding the page to the swap cache has enough information about the char=
ged
> > memcg. So, what we need to do is just passing this information to
> > the right place.
> >
> > With this patch, specific memcg could be pressured more since readahead
> > pages are also charged to it now. This would result in performance
> > degradation to that user but it would be fair since that readahead is f=
or
> > that user.
>
> If I read the code correctly, the readahead pages may be *not* charged
> to it at all but other memcgs since mem_cgroup_try_charge() would
> retrieve the target memcg id from the swap entry then charge to it
> (generally it is the memcg from who the page is swapped out). So, it
> may open a backdoor to let one memcg stress other memcgs?

It looks like you talk about the call path on CONFIG_MEMCG_SWAP.

The owner (task) for a anonymous page cannot be changed. It means that
the previous owner written on the swap entry will be the next user. So,
I think that using the target memcg id from the swap entry for readahead pa=
ges
is valid way.

As you concerned, if someone can control swap-readahead to readahead
other's swap entry, one memcg could stress other memcg by using the fact ab=
ove.
However, as far as I know, there is no explicit way to readahead other's sw=
ap
entry so no problem.

Thanks.