Re: xarray breaks thrashing detection and cgroup isolation

[Shakeel Butt]

On Thu, May 23, 2019 at 11:37 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

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.

Shakeel