Messages in this thread | | | Date | Tue, 21 Jul 2015 16:34:07 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -mm v9 1/8] memcg: add page_cgroup_ino helper |
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On Sun, 19 Jul 2015 15:31:10 +0300 Vladimir Davydov <vdavydov@parallels.com> wrote:
> This function returns the inode number of the closest online ancestor of > the memory cgroup a page is charged to. It is required for exporting > information about which page is charged to which cgroup to userspace, > which will be introduced by a following patch. > > ... >
> --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -441,6 +441,29 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page) > return &memcg->css; > } > > +/** > + * page_cgroup_ino - return inode number of the memcg a page is charged to > + * @page: the page > + * > + * Look up the closest online ancestor of the memory cgroup @page is charged to > + * and return its inode number or 0 if @page is not charged to any cgroup. It > + * is safe to call this function without holding a reference to @page. > + */ > +unsigned long page_cgroup_ino(struct page *page)
Shouldn't it return an ino_t?
> +{ > + struct mem_cgroup *memcg; > + unsigned long ino = 0; > + > + rcu_read_lock(); > + memcg = READ_ONCE(page->mem_cgroup); > + while (memcg && !(memcg->css.flags & CSS_ONLINE)) > + memcg = parent_mem_cgroup(memcg); > + if (memcg) > + ino = cgroup_ino(memcg->css.cgroup); > + rcu_read_unlock(); > + return ino; > +}
The function is racy, isn't it? There's nothing to prevent this inode from getting torn down and potentially reallocated one nanosecond after page_cgroup_ino() returns? If so, it is only safely usable by things which don't care (such as procfs interfaces) and this should be documented in some fashion.
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