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    SubjectRe: [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL
    On Thu 24-11-11 11:05:49, Johannes Weiner wrote:
    > On Thu, Nov 24, 2011 at 10:52:51AM +0100, Michal Hocko wrote:
    > > 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?
    >
    > No, see the last hunk in this patch. It's actually possible for this
    > to run, although only while feeding fresh pages into the allocator:

    Bahh. Yes, I have noticed the hunk but then I started thinking about
    how to make the NULL case explicit and totally forgot about that.
    Sorry about the noise.

    >
    > > > @@ -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;
    >
    > We could add a lookup_page_cgroup_safe() for this DEBUG_VM-only
    > callsite as an optimization separately and remove the NULL check from
    > lookup_page_cgroup() itself. But this patch was purely about removing
    > the actively misleading checks.

    Yes, but I am not sure whether code duplication is worth it. Let's just
    stick with current form. Maybe just move the comment when it can be NULL
    to the lookup_page_cgroup directly?

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


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