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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 24 May 2019 10:18:04 -0700
Message-ID: <CALvZod5=N_hwGLFzCZY=DG0RfwzSt2sjJDcPZtCRy-NcBsLL+w@mail.gmail.com>
Subject: Re: xarray breaks thrashing detection and cgroup isolation
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>
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On Fri, May 24, 2019 at 10:06 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, May 24, 2019 at 09:11:46AM -0700, Matthew Wilcox wrote:
> > On Thu, May 23, 2019 at 03:59:33PM -0400, Johannes Weiner wrote:
> > > My point is that we cannot have random drivers' internal data
> > > structures charge to and pin cgroups indefinitely just because they
> > > happen to do the modprobing or otherwise interact with the driver.
> > >
> > > It makes no sense in terms of performance or cgroup semantics.
> >
> > But according to Roman, you already have that problem with the page
> > cache.
> > https://lore.kernel.org/linux-mm/20190522222254.GA5700@castle/T/
> >
> > So this argument doesn't make sense to me.
>
> You haven't addressed the rest of the argument though: why every user
> of the xarray, and data structures based on it, should incur the
> performance cost of charging memory to a cgroup, even when we have no
> interest in tracking those allocations on behalf of a cgroup.
>
> Which brings me to repeating the semantics argument: it doesn't make
> sense to charge e.g. driver memory, which is arguably a shared system
> resource, to whoever cgroup happens to do the modprobe / ioctl etc.
>
> Anyway, this seems like a fairly serious regression, and it would make
> sense to find a self-contained, backportable fix instead of something
> that has subtle implications for every user of the xarray / ida code.

Adding to Johannes point, one concrete example of xarray we don't want
to charge is swapper_spaces. Swap is a system level resource. It does
not make any sense to charge the swap overhead to a job and also it
will have negative consequences like pinning zombies.

Shakeel