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SubjectRe: [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL
On Wed 23-11-11 16:42:27, Johannes Weiner wrote:
> From: Johannes Weiner <jweiner@redhat.com>
>
> Pages have their corresponding page_cgroup descriptors set up before
> they are used in userspace, and thus managed by a memory cgroup.
>
> The only time where lookup_page_cgroup() can return NULL is in the
> page sanity checking code that executes while feeding pages into the
> page allocator for the first time.
>
> Remove the NULL checks against lookup_page_cgroup() results from all
> callsites where we know that corresponding page_cgroup descriptors
> must be allocated.

OK, shouldn't we add

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 2d123f9..cb93f64 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -35,8 +35,7 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
struct page_cgroup *base;

base = NODE_DATA(page_to_nid(page))->node_page_cgroup;
- if (unlikely(!base))
- return NULL;
+ BUG_ON(!base);

offset = pfn - NODE_DATA(page_to_nid(page))->node_start_pfn;
return base + offset;
@@ -112,8 +111,7 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
unsigned long pfn = page_to_pfn(page);
struct mem_section *section = __pfn_to_section(pfn);

- if (!section->page_cgroup)
- return NULL;
+ BUG_ON(!section->page_cgroup);
return section->page_cgroup + pfn;
}

just to make it explicit?
>
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>

Other than that
Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/memcontrol.c | 8 ++------
> 1 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d825af9..d4d139a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1894,9 +1894,6 @@ void mem_cgroup_update_page_stat(struct page *page,
> bool need_unlock = false;
> unsigned long uninitialized_var(flags);
>
> - if (unlikely(!pc))
> - return;
> -
> rcu_read_lock();
> memcg = pc->mem_cgroup;
> if (unlikely(!memcg || !PageCgroupUsed(pc)))
> @@ -2669,8 +2666,6 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
> }
>
> pc = lookup_page_cgroup(page);
> - BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
> -
> ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
> if (ret || !memcg)
> return ret;
> @@ -2942,7 +2937,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
> * Check if our page_cgroup is valid
> */
> pc = lookup_page_cgroup(page);
> - if (unlikely(!pc || !PageCgroupUsed(pc)))
> + if (unlikely(!PageCgroupUsed(pc)))
> return NULL;
>
> lock_page_cgroup(pc);
> @@ -3326,6 +3321,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
> struct page_cgroup *pc;
>
> pc = lookup_page_cgroup(page);
> + /* Can be NULL while bootstrapping the page allocator */
> if (likely(pc) && PageCgroupUsed(pc))
> return pc;
> return NULL;
> --
> 1.7.6.4
>

--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic


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