On Tue 02-03-21 10:17:03, Michal Hocko wrote:
> [Cc Johannes for awareness and fixup Nick's email]
>
> On Tue 02-03-21 01:34:51, Zhou Guanghui wrote:
> > When split page, the memory cgroup info recorded in first page is
> > not copied to tail pages. In this case, when the tail pages are
> > freed, the uncharge operation is not performed. As a result, the
> > usage of this memcg keeps increasing, and the OOM may occur.
> >
> > So, the copying of first page's memory cgroup info to tail pages
> > is needed when split page.
>
> I was not aware that alloc_pages_exact is used for accounted allocations
> but git grep told me otherwise so this is not a theoretical one. Both
> users (arm64 and s390 kvm) are quite recent AFAICS. split_page is also
> used in dma allocator but I got lost in indirection so I have no idea
> whether there are any users there.
>
> The page itself looks reasonable to me.
>
> > Signed-off-by: Zhou Guanghui <zhouguangh...@huawei.com>
>
> Acked-by: Michal Hocko <mho...@suse.com>
>
> Minor nit
>
> > ---
> > include/linux/memcontrol.h | 10 ++++++++++
> > mm/page_alloc.c | 4 +++-
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e6dc793d587d..c7e2b4421dc1 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -867,6 +867,12 @@ void mem_cgroup_print_oom_group(struct mem_cgroup
> > *memcg);
> > extern bool cgroup_memory_noswap;
> > #endif
> >
> > +static inline void copy_page_memcg(struct page *dst, struct page *src)
> > +{
> > + if (src->memcg_data)
> > + dst->memcg_data = src->memcg_data;
>
> I would just drop the test. The struct page is a single cache line which
> is dirty by the reference count so another store will unlikely be
> noticeable even when NULL is stored here and you safe a conditional.
Disregard this. As Zi Yan mentioned in other reply, we need to keep the
check and take a css reference along with transfering the memcg.
--
Michal Hocko
SUSE Labs
