Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

Michal Hocko
2013-03-26T08:43:48Z

On Tue 26-03-13 12:35:58, Glauber Costa wrote:
> >>
> >> I doubt it's a win to add 4K to kernel text size instead of adding
> >> a few extra lines of code... but it's up to you.
> >
> > I will leave the decision to Glauber. The updated version which uses
> > kmalloc for the static buffer is bellow.
> >
> I prefer to allocate dynamically here. But although I understand why we
> need to call cgroup_name, I don't understand what is wrong with
> kasprintf if we're going to allocate anyway. It will allocate a string
> just big enough. A PAGE_SIZE'd allocation is a lot more likely to fail.
>
> Now, if we really want to be smart here, we can do something like what
> I've done for the slub attribute buffers, that can actually have very
> long values.
>
> allocate a small buffer that will hold 80 % > of the allocations (256
> bytes should be enough for most cache names), and if the string is
> bigger than this, we allocate. Once we allocate, we save it in a static
> pointer and leave it there. The hope here is that we may be able to
> live without ever allocating in many systems.
>
> > +
> > + /*
> > + * kmem_cache_create_memcg duplicates the given name and
> > + * cgroup_name for this name requires RCU context.
> > + * This static temporary buffer is used to prevent from
> > + * pointless shortliving allocation.
> > + */
> The comment is also no longer true if you don't resort to a static buffer.

The buffer _is_ static (read global variable hidden with the function
scope).

> The following (untested) patch implements the idea I outlined above.
>
> What do you guys think ?

I really do not care which way to fix this.

[...]
> +static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
> + struct kmem_cache *s)
> {
> - char *name;
> - struct dentry *dentry;
> + const char *cgname; /* actual cache name */
> + char *name = NULL; /* actual cache name */
> + char buf[256]; /* stack buffer for small allocations */
> + int buf_len;
> + static char *buf_name; /* pointer to a page, if we ever need */
> + struct kmem_cache *new;
> +
> + lockdep_assert_held(&memcg_cache_mutex);
>
> rcu_read_lock();
> - dentry = rcu_dereference(memcg->css.cgroup->dentry);
> + cgname = cgroup_name(memcg->css.cgroup);
> rcu_read_unlock();

cgname is valid only within RCU read lock AFAIU.

--
Michal Hocko
SUSE Labs