Re: xarray breaks thrashing detection and cgroup isolation

[Matthew Wilcox]

On Thu, May 23, 2019 at 03:21:17PM -0400, Johannes Weiner wrote:
> On Thu, May 23, 2019 at 12:00:32PM -0700, Matthew Wilcox wrote:
> > 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@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.
> > 
> > > 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?
> 
> We used to cgroup-account every slab charge to cgroups per default,
> until we changed it to a whitelist behavior:
> 
> commit b2a209ffa605994cbe3c259c8584ba1576d3310c
> Author: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Date:   Thu Jan 14 15:18:05 2016 -0800
> 
>     Revert "kernfs: do not account ino_ida allocations to memcg"
>     
>     Currently, all kmem allocations (namely every kmem_cache_alloc, kmalloc,
>     alloc_kmem_pages call) are accounted to memory cgroup automatically.
>     Callers have to explicitly opt out if they don't want/need accounting
>     for some reason.  Such a design decision leads to several problems:
>     
>      - kmalloc users are highly sensitive to failures, many of them
>        implicitly rely on the fact that kmalloc never fails, while memcg
>        makes failures quite plausible.

Doesn't apply here.  The allocation under spinlock is expected to fail,
and then we'll use xas_nomem() with the caller's specified GFP flags
which may or may not include __GFP_ACCOUNT.

>      - A lot of objects are shared among different containers by design.
>        Accounting such objects to one of containers is just unfair.
>        Moreover, it might lead to pinning a dead memcg along with its kmem
>        caches, which aren't tiny, which might result in noticeable increase
>        in memory consumption for no apparent reason in the long run.

These objects are in the slab of radix_tree_nodes, and we'll already be
accounting page cache nodes to the cgroup, so accounting random XArray
nodes to the cgroups isn't going to make the problem worse.

>      - There are tons of short-lived objects. Accounting them to memcg will
>        only result in slight noise and won't change the overall picture, but
>        we still have to pay accounting overhead.

XArray nodes are generally not short-lived objects.