Re: [PATCH] mm/cgroup: avoid panic when init with low memory

[Michal Hocko]

On Mon 20-02-17 18:09:43, Laurent Dufour wrote:
> On 20/02/2017 14:01, Michal Hocko wrote:
> > On Wed 15-02-17 11:36:09, Laurent Dufour wrote:
> >> The system may panic when initialisation is done when almost all the
> >> memory is assigned to the huge pages using the kernel command line
> >> parameter hugepage=xxxx. Panic may occur like this:
> > 
> > I am pretty sure the system might blow up in many other ways when you
> > misconfigure it and pull basically all the memory out. Anyway...
> > 
> > [...]
> > 
> >> This is a chicken and egg issue where the kernel try to get free
> >> memory when allocating per node data in mem_cgroup_init(), but in that
> >> path mem_cgroup_soft_limit_reclaim() is called which assumes that
> >> these data are allocated.
> >>
> >> As mem_cgroup_soft_limit_reclaim() is best effort, it should return
> >> when these data are not yet allocated.
> > 
> > ... this makes some sense. Especially when there is no soft limit
> > configured. So this is a good step. I would just like to ask you to go
> > one step further. Can we make the whole soft reclaim thing uninitialized
> > until the soft limit is actually set? Soft limit is not used in cgroup
> > v2 at all and I would strongly discourage it in v1 as well. We will save
> > few bytes as a bonus.
> 
> Hi Michal, and thanks for the review.
> 
> I'm not familiar with that part of the kernel, so to be sure we are on
> the same line, are you suggesting to set soft_limit_tree at the first
> time mem_cgroup_write() is called to set a soft_limit field ?

yes

> Obviously, all callers to soft_limit_tree_node() and
> soft_limit_tree_from_page() will have to check for the return pointer to
> be NULL.

All callers that need to access the tree unconditionally, yes. Which is
the case anyway, right? I haven't checked the check you have added is
sufficient, but we shouldn't have that many of them because some code
paths are called only when the soft limit is enabled.
-- 
Michal Hocko
SUSE Labs