On Fri, Oct 16, 2020 at 10:53:08AM -0400, Johannes Weiner wrote: > On Fri, Oct 16, 2020 at 11:47:02AM +0200, Michal Koutný wrote: > > Hello. > > > > On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <rpalethorpe@xxxxxxxx> wrote: > > > SLAB objects which outlive their memcg are moved to their parent > > > memcg where they may be uncharged. However if they are moved to the > > > root memcg, uncharging will result in negative page counter values as > > > root has no page counters. > > Why do you think those are reparented objects? If those are originally > > charged in a non-root cgroup, then the charge value should be propagated up the > > hierarchy, including root memcg, so if they're later uncharged in root > > after reparenting, it should still break even. (Or did I miss some stock > > imbalance?) > > Looking a bit closer at this code, it's kind of a mess right now. > > The central try_charge() function charges recursively all the way up > to and including the root. But not if it's called directly on the > root, in which case it bails and does nothing. > > kmem and objcg use try_charge(), so they have the same > behavior. get_obj_cgroup_from_current() does it's own redundant > filtering for root_mem_cgroup, whereas get_mem_cgroup_from_current() > does not, but its callsite __memcg_kmem_charge_page() does. > > We should clean this up one way or another: either charge the root or > don't, but do it consistently. +1 > > Since we export memory.stat at the root now, we should probably just > always charge the root instead of special-casing it all over the place > and risking bugs. Hm, we export memory.stat but not memory.current. Charging the root memcg seems to be an extra atomic operation, which can be avoided. I wonder if we can handle it in page_counter.c, so there will be a single place where we do the check. > > Indeed, it looks like there is at least one bug where the root-level > memory.stat shows non-root slab objects, but not root ones, whereas it > shows all anon and cache pages, root or no root. I'll take a look.
