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Date: Mon, 14 Dec 2015 20:14:55 +0300
From: Vladimir Davydov <vdavydov@...tuozzo.com>
To: Johannes Weiner <hannes@...xchg.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<kernel-team@...com>
Subject: Re: [PATCH 4/4] mm: memcontrol: clean up alloc, online, offline,
free functions
On Fri, Dec 11, 2015 at 02:54:13PM -0500, Johannes Weiner wrote:
...
> -static int
> -mem_cgroup_css_online(struct cgroup_subsys_state *css)
> +static struct cgroup_subsys_state * __ref
> +mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
> {
> - struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> - struct mem_cgroup *parent = mem_cgroup_from_css(css->parent);
> - int ret;
> -
> - if (css->id > MEM_CGROUP_ID_MAX)
> - return -ENOSPC;
> + struct mem_cgroup *parent = mem_cgroup_from_css(parent_css);
> + struct mem_cgroup *memcg;
> + long error = -ENOMEM;
>
> - if (!parent)
> - return 0;
> + memcg = mem_cgroup_alloc();
> + if (!memcg)
> + return ERR_PTR(error);
>
> mutex_lock(&memcg_create_mutex);
It is pointless to take memcg_create_mutex in ->css_alloc. It won't
prevent setting use_hierarchy for parent after a new child was
allocated, but before it was added to the list of children (see
create_css()). Taking the mutex in ->css_online renders this race
impossible. That is, your cleanup breaks use_hierarchy consistency
check.
Can we drop this use_hierarchy consistency check at all and allow
children of a cgroup with use_hierarchy=1 have use_hierarchy=0? Yeah,
that might result in some strangeness if cgroups are created in parallel
with use_hierarchy flipped, but is it a valid use case? I surmise, one
just sets use_hierarchy for a cgroup once and for good before starting
to create sub-cgroups.
> -
> - memcg->use_hierarchy = parent->use_hierarchy;
> - memcg->oom_kill_disable = parent->oom_kill_disable;
> - memcg->swappiness = mem_cgroup_swappiness(parent);
> -
> - if (parent->use_hierarchy) {
> + memcg->high = PAGE_COUNTER_MAX;
> + memcg->soft_limit = PAGE_COUNTER_MAX;
> + if (parent)
> + memcg->swappiness = mem_cgroup_swappiness(parent);
> + if (parent && parent->use_hierarchy) {
> + memcg->use_hierarchy = true;
> + memcg->oom_kill_disable = parent->oom_kill_disable;
oom_kill_disable was propagated to child cgroup despite use_hierarchy
configuration. I don't see any reason to change this.
> page_counter_init(&memcg->memory, &parent->memory);
> - memcg->high = PAGE_COUNTER_MAX;
> - memcg->soft_limit = PAGE_COUNTER_MAX;
> page_counter_init(&memcg->memsw, &parent->memsw);
> page_counter_init(&memcg->kmem, &parent->kmem);
> page_counter_init(&memcg->tcpmem, &parent->tcpmem);
> -
> - /*
> - * No need to take a reference to the parent because cgroup
> - * core guarantees its existence.
> - */
> } else {
> page_counter_init(&memcg->memory, NULL);
> - memcg->high = PAGE_COUNTER_MAX;
> - memcg->soft_limit = PAGE_COUNTER_MAX;
> page_counter_init(&memcg->memsw, NULL);
> page_counter_init(&memcg->kmem, NULL);
> page_counter_init(&memcg->tcpmem, NULL);
> @@ -4296,19 +4211,30 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
> }
> mutex_unlock(&memcg_create_mutex);
>
> - ret = memcg_propagate_kmem(memcg);
> - if (ret)
> - return ret;
> + /* The following stuff does not apply to the root */
> + if (!parent) {
> + root_mem_cgroup = memcg;
> + return &memcg->css;
> + }
> +
> + error = memcg_propagate_kmem(parent, memcg);
I don't think ->css_alloc is the right place for this function: if
create_css() fails after ->css_alloc and before ->css_online, it'll call
->css_free, which won't cleanup kmem properly.
> + if (error)
> + goto fail;
>
> if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket)
> static_branch_inc(&memcg_sockets_enabled_key);
Frankly, I don't get why this should live here either. This has nothing
to do with memcg allocation and looks rather like a preparation for
online.
>
> - /*
> - * Make sure the memcg is initialized: mem_cgroup_iter()
> - * orders reading memcg->initialized against its callers
> - * reading the memcg members.
> - */
> - smp_store_release(&memcg->initialized, 1);
> + return &memcg->css;
> +fail:
> + mem_cgroup_free(memcg);
> + return NULL;
> +}
> +
> +static int
> +mem_cgroup_css_online(struct cgroup_subsys_state *css)
> +{
> + if (css->id > MEM_CGROUP_ID_MAX)
> + return -ENOSPC;
>
> return 0;
> }
> @@ -4330,10 +4256,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
> }
> spin_unlock(&memcg->event_list_lock);
>
> - vmpressure_cleanup(&memcg->vmpressure);
> -
> memcg_offline_kmem(memcg);
> -
> wb_memcg_offline(memcg);
> }
>
> @@ -4347,9 +4270,11 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
> if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg->tcpmem_active)
> static_branch_dec(&memcg_sockets_enabled_key);
>
> + vmpressure_cleanup(&memcg->vmpressure);
vmpressure->work can be scheduled after offline, so ->css_free is
definitely the right place for vmpressure_cleanup. Looks like you've
just fixed a potential use-after-free bug.
Thanks,
Vladimir
> + cancel_work_sync(&memcg->high_work);
> + mem_cgroup_remove_from_trees(memcg);
> memcg_free_kmem(memcg);
> -
> - __mem_cgroup_free(memcg);
> + mem_cgroup_free(memcg);
> }
>
> /**
--
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