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SubjectRe: [RFC][PATCH] Reduce size of swap_cgroup by CSS ID
On Fri, 06 Feb 2009 10:07:40 +0800
Li Zefan <lizf@cn.fujitsu.com> wrote:

> > +/*
> > + * A helper function to get mem_cgroup from ID. must be called under
> > + * rcu_read_lock(). Because css_tryget() is called under this, css_put
> > + * should be called later.
> > + */
> > +static struct mem_cgroup *mem_cgroup_lookup_get(unsigned short id)
> > +{
> > + struct cgroup_subsys_state *css;
> > +
> > + /* ID 0 is unused ID */
> > + if (!id)
> > + return NULL;
> > + css = css_lookup(&mem_cgroup_subsys, id);
> > + if (css && css_tryget(css))
> > + return container_of(css, struct mem_cgroup, css);
> > + return NULL;
> > +}
>
> the returned mem_cgroup needn't be protected by rcu_read_lock(), so I
> think this is better:
> rcu_read_lock();
> css = css_lookup(&mem_cgroup_subsys, id);
> rcu_read_unlock();
> and no lock is needed when calling mem_cgroup_lookup_get().
>
Hmm, maybe you're right.

> > * Returns old value at success, NULL at failure.
> > * (Of course, old value can be NULL.)
> > */
> > -struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
> > +unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
>
> kernel-doc needs to be updated
>
ok.

> > * lookup_swap_cgroup - lookup mem_cgroup tied to swap entry
> > * @ent: swap entry to be looked up.
> > *
> > - * Returns pointer to mem_cgroup at success. NULL at failure.
> > + * Returns CSS ID of mem_cgroup at success. NULL at failure.
>
> s/NULL/0/
>
yes.

Okay, thanks.

BTW, this patch is totally buggy ;( Sorry. Below is my memo.
==

- mem_cgroup can by destroyed() while there are swap_cgroup point to mem_cgroup.
If there are reference from swap_cgroup, kfree() is delayed.
- But this patch just uses css_tryget(). css_tryget() returnes false if
mem_cgroup is destroyed.
- So, refcnt for destroyed mem_cgroup will not be decreased and memory for
mem_cgroup will be never freed.

- Even if we use ID instead of pointer, the situation does not change.
we have to prevent ID to be reused....
- One choice for removing all swap_account at destroy() is forget all swap
accounts. But this may need "scan" all swap_cgroup in the system at rmdir().
This will be unacceptably slow on large swap systems.
==

I'll post v2 but it will be still tricky.

Thanks,
-Kame




















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