From: Matthew Wilcox <willy@infradead.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.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>
Subject: Re: xarray breaks thrashing detection and cgroup isolation
Date: Thu, 23 May 2019 12:00:32 -0700 [thread overview]
Message-ID: <20190523190032.GA7873@bombadil.infradead.org> (raw)
In-Reply-To: <CALvZod4o0sA8CM961ZCCp-Vv+i6awFY0U07oJfXFDiVfFiaZfg@mail.gmail.com>
On Thu, May 23, 2019 at 11:49:41AM -0700, Shakeel Butt wrote:
> On Thu, May 23, 2019 at 11:37 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Thu, May 23, 2019 at 01:43:49PM -0400, Johannes Weiner wrote:
> > > I noticed that recent upstream kernels don't account the xarray nodes
> > > of the page cache to the allocating cgroup, like we used to do for the
> > > radix tree nodes.
> > >
> > > This results in broken isolation for cgrouped apps, allowing them to
> > > escape their containment and harm other cgroups and the system with an
> > > excessive build-up of nonresident information.
> > >
> > > It also breaks thrashing/refault detection because the page cache
> > > lives in a different domain than the xarray nodes, and so the shadow
> > > shrinker can reclaim nonresident information way too early when there
> > > isn't much cache in the root cgroup.
> > >
> > > I'm not quite sure how to fix this, since the xarray code doesn't seem
> > > to have per-tree gfp flags anymore like the radix tree did. We cannot
> > > add SLAB_ACCOUNT to the radix_tree_node_cachep slab cache. And the
> > > xarray api doesn't seem to really support gfp flags, either (xas_nomem
> > > does, but the optimistic internal allocations have fixed gfp flags).
> >
> > Would it be a problem to always add __GFP_ACCOUNT to the fixed flags?
> > I don't really understand cgroups.
>
> Does xarray cache allocated nodes, something like radix tree's:
>
> static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
>
> For the cached one, no __GFP_ACCOUNT flag.
No. That was the point of the XArray conversion; no cached nodes.
> Also some users of xarray may not want __GFP_ACCOUNT. That's the
> reason we had __GFP_ACCOUNT for page cache instead of hard coding it
> in radix tree.
This is what I don't understand -- why would someone not want
__GFP_ACCOUNT? For a shared resource? But the page cache is a shared
resource. So what is a good example of a time when an allocation should
_not_ be accounted to the cgroup?
next prev parent reply other threads:[~2019-05-23 19:00 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-23 17:43 xarray breaks thrashing detection and cgroup isolation Johannes Weiner
2019-05-23 18:37 ` Matthew Wilcox
2019-05-23 18:49 ` Shakeel Butt
2019-05-23 19:00 ` Matthew Wilcox [this message]
2019-05-23 19:21 ` Johannes Weiner
2019-05-23 19:41 ` Matthew Wilcox
2019-05-23 19:59 ` Johannes Weiner
2019-05-24 16:11 ` Matthew Wilcox
2019-05-24 17:06 ` Johannes Weiner
2019-05-24 17:18 ` Shakeel Butt
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