Re: xarray breaks thrashing detection and cgroup isolation

[Matthew Wilcox]

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.