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)? Acked-by: Vlastimil Babka <vbabka@xxxxxxx> 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). >> /* >> * A memory cgroup is considered kmem-online as soon as it gets >> * kmemcg_id. Setting the id after enabling static branching will >> @@ -3643,9 +3644,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg) >> /* css_alloc() failed, offlining didn't happen */ >> if (unlikely(memcg->kmem_state == KMEM_ONLINE)) >> memcg_offline_kmem(memcg); >> - >> - if (memcg->kmem_state == KMEM_ALLOCATED) >> - static_branch_dec(&memcg_kmem_enabled_key); >> } >> #else >> static int memcg_online_kmem(struct mem_cgroup *memcg) >> -- >> 2.26.2 >
