Re: [PATCH -V8 14/16] hugetlb/cgroup: add charge/uncharge calls forHugeTLB alloc/free

[Michal Hocko]

On Sat 09-06-12 16:30:54, Johannes Weiner wrote:
> On Sat, Jun 09, 2012 at 06:39:06PM +0530, Aneesh Kumar K.V wrote:
> > Johannes Weiner <hannes@xxxxxxxxxxx> writes:
> > 
> > > On Sat, Jun 09, 2012 at 02:29:59PM +0530, Aneesh Kumar K.V wrote:
> > >> From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > >> 
> > >> This adds necessary charge/uncharge calls in the HugeTLB code.  We do
> > >> hugetlb cgroup charge in page alloc and uncharge in compound page destructor.
> > >> 
> > >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > >> ---
> > >>  mm/hugetlb.c        |   16 +++++++++++++++-
> > >>  mm/hugetlb_cgroup.c |    7 +------
> > >>  2 files changed, 16 insertions(+), 7 deletions(-)
> > >> 
> > >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > >> index bf79131..4ca92a9 100644
> > >> --- a/mm/hugetlb.c
> > >> +++ b/mm/hugetlb.c
> > >> @@ -628,6 +628,8 @@ static void free_huge_page(struct page *page)
> > >>  	BUG_ON(page_mapcount(page));
> > >>  
> > >>  	spin_lock(&hugetlb_lock);
> > >> +	hugetlb_cgroup_uncharge_page(hstate_index(h),
> > >> +				     pages_per_huge_page(h), page);
> > >
> > > hugetlb_cgroup_uncharge_page() takes the hugetlb_lock, no?
> > 
> > Yes, But this patch also modifies it to not take the lock, because we
> > hold spin_lock just below in the call site. I didn't want to drop the
> > lock and take it again.
> 
> Sorry, I missed that.
> 
> > > It's quite hard to review code that is split up like this.  Please
> > > always keep the introduction of new functions in the same patch that
> > > adds the callsite(s).
> > 
> > One of the reason I split the charge/uncharge routines and the callers
> > in separate patches is to make it easier for review. Irrespective of
> > the call site charge/uncharge routines should be correct with respect
> > to locking and other details. What I did in this patch is a small
> > optimization of avoiding dropping and taking the lock again. May be the
> > right approach would have been to name it __hugetlb_cgroup_uncharge_page
> > and make sure the hugetlb_cgroup_uncharge_page still takes spin_lock.
> > But then we don't have any callers for that.
> 
> I think this makes it needlessly complicated and there is no correct
> or incorrect locking in (initially) dead code :-)
> 
> The callsites are just a few lines.  It's harder to review if you
> introduce an API and then change it again mid-patchset.

Fully agreed.

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