On Thu, Jul 02, 2020 at 06:35:31PM +0200, Vlastimil Babka wrote: > On 7/2/20 6:22 PM, Michal Hocko wrote: > > On Wed 01-07-20 11:45:52, Roman Gushchin wrote: > > [...] > >> >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001 > >> From: Roman Gushchin <guro@xxxxxx> > >> Date: Wed, 1 Jul 2020 11:05:32 -0700 > >> Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible > >> > >> Historically the kernel memory accounting was an opt-in feature, which > >> could be enabled for individual cgroups. But now it's not true, and > >> it's on by default both on cgroup v1 and cgroup v2. And as long as a > >> user has at least one non-root memory cgroup, the kernel memory > >> accounting is on. So in most setups it's either always on (if memory > >> cgroups are in use and kmem accounting is not disabled), either always > >> off (otherwise). > >> > >> memcg_kmem_enabled() is used in many places to guard the kernel memory > >> accounting code. If memcg_kmem_enabled() can reverse from returning > >> true to returning false (as now), we can't rely on it on release paths > >> and have to check if it was on before. > >> > >> If we'll make memcg_kmem_enabled() irreversible (always returning true > >> after returning it for the first time), it'll make the general logic > >> more simple and robust. It also will allow to guard some checks which > >> otherwise would stay unguarded. > >> > >> Signed-off-by: Roman Gushchin <guro@xxxxxx> > > Fixes: ? or let Andrew squash it to some patch of your series (it's in mmotm I > think)? Hm, it's actually complicated. One obvious case was added by "mm: memcg/slab: save obj_cgroup for non-root slab objects", which is currently in the mm tree, so no stable hash. But I suspect that there are more cases where we just silently leaking a memcg reference. But because the whole setup (going back and forth between 0 and 1+ memory cgroups) can not be easily found in the real life, nobody cares. So I don't think we really need a stable backport. So IMO the best option is to put it as a standalone patch _before_ my series. Does it sound good to you? > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Thanks! > > But see below: > > >> --- > >> mm/memcontrol.c | 6 ++---- > >> 1 file changed, 2 insertions(+), 4 deletions(-) > >> > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c > >> index 50ae77f3985e..2d018a51c941 100644 > >> --- a/mm/memcontrol.c > >> +++ b/mm/memcontrol.c > >> @@ -3582,7 +3582,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg) > >> objcg->memcg = memcg; > >> rcu_assign_pointer(memcg->objcg, objcg); > >> > >> - static_branch_inc(&memcg_kmem_enabled_key); > >> + if (!memcg_kmem_enabled()) > >> + static_branch_inc(&memcg_kmem_enabled_key); > > > > Wouldn't be static_branch_enable() more readable? > > Yes, and drop the if(). It will just do nothing and return if already enabled. > Maybe slightly less efficient, but this is not a fast path anyway, and it feels > weird to modify the static key in a branch controlled by the static key itself > (CC peterz in case he wants to add something). Ok, will do in v2. Thanks!
