On Mon, Feb 29, 2016 at 08:16:33PM +0300, Vladimir Davydov wrote:

When a cgroup directory is removed, the memory cgroup subsys state does
not disappear immediately. Instead, it's left hanging around until the
last reference to it is gone, which implies reclaiming all pages from
its lruvec.

In the unified hierarchy, there's the memory.low knob, which can be used
to set a best-effort protection for a memory cgroup - the reclaimer
first scans those cgroups whose consumption is above memory.low, and
only if it fails to reclaim enough pages, it gets to the rest.

Currently this protection is not reset when the cgroup directory is
removed. As a result, if a dead memory cgroup has a lot of page cache
charged to it and a high value of memory.low, it will result in higher
pressure exerted on live cgroups, and userspace will have no ways to
detect such consumers and reconfigure memory.low properly.

To fix this, let's reset memory.low on css offline.

We already have mem_cgroup_css_reset() for soft-offlining a css - when
the css is asked to be disabled but another subsystem still uses it.
Can we just call that function during offline as well? The css can be
around for quite a bit after the user deleted it. Eliminating *any*
user-supplied configurations and zapping it back to defaults makes
sense in general, so that we never have to worry about any remnants.

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