Re: [External] Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

[Shakeel Butt]

On Fri, Feb 12, 2021 at 10:48 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Sat, Feb 13, 2021 at 2:57 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >
> > CCing more folks.
> >
> > On Fri, Feb 12, 2021 at 9:14 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > >
> > > The swap charges the actual number of swap entries on cgroup v2.
> > > If a swap cache page is charged successful, and then we uncharge
> > > the swap counter. It is wrong on cgroup v2. Because the swap
> > > entry is not freed.
> > >
> > > Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > What's the user visible impact of this change?
>
> IIUC, I think that we cannot limit the swap to memory.swap.max
> on cgroup v2.
>
>   cd /sys/fs/cgroup/
>   mkdir test
>   cd test
>   echo 8192 > memory.max
>   echo 4096 > memory.swap.max
>
> OK. Now we limit swap to 1 page and memory to 2 pages.
> Firstly, we allocate 1 page from this memory cgroup and
> swap this page to swap disk. We can see:
>
>   memory.current: 0
>   memory.swap.current: 1
>
> Then we touch this page, we will swap in and charge
> the swap cache page to the memory counter and uncharge
> the swap counter.
>
>   memory.current: 1
>   memory.swap.current: 0 (but actually we use a swap entry)
>
> Then we allocate another 1 page from this memory cgroup.
>
>   memory.current: 2
>   memory.swap.current: 0 (but actually we use a swap entry)
>
> If we swap those 2 pages to swap disk. We can charge and swap
> those 2 pages successfully. Right? Maybe I am wrong.
>

I was trying to repro this but couldn't and later remembered that swap
on zram skips the swapcache and thus is not impacted by this issue.
This is reproducible on swap on disk and I see Johannes has already
described in good detail.