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Date:   Fri, 24 May 2019 13:06:42 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Shakeel Butt <shakeelb@google.com>,
        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>
Subject: Re: xarray breaks thrashing detection and cgroup isolation
Message-ID: <20190524170642.GA20546@cmpxchg.org>
References: <20190523174349.GA10939@cmpxchg.org>
 <20190523183713.GA14517@bombadil.infradead.org>
 <CALvZod4o0sA8CM961ZCCp-Vv+i6awFY0U07oJfXFDiVfFiaZfg@mail.gmail.com>
 <20190523190032.GA7873@bombadil.infradead.org>
 <20190523192117.GA5723@cmpxchg.org>
 <20190523194130.GA4598@bombadil.infradead.org>
 <20190523195933.GA6404@cmpxchg.org>
 <20190524161146.GC1075@bombadil.infradead.org>
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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.