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 ? 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. Cheers, Laurent. >> Signed-off-by: Laurent Dufour <lduf...@linux.vnet.ibm.com> >> --- >> mm/memcontrol.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 1fd6affcdde7..213f96b2f601 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -2556,7 +2556,7 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t >> *pgdat, int order, >> * is empty. Do it lockless to prevent lock bouncing. Races >> * are acceptable as soft limit is best effort anyway. >> */ >> - if (RB_EMPTY_ROOT(&mctz->rb_root)) >> + if (!mctz || RB_EMPTY_ROOT(&mctz->rb_root)) >> return 0; >> >> /* >> -- >> 2.7.4 >
