Re: [PATCH v2 1/2] mm: Uncharge poisoned pages

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Wed, Apr 26, 2017 at 01:45:00PM +1000, Balbir Singh wrote:
> > > >  static int delete_from_lru_cache(struct page *p)
> > > >  {
> > > > +	if (memcg_kmem_enabled())
> > > > +		memcg_kmem_uncharge(p, 0);
> > > > +
> > > 
> > > The changelog is not quite clear, so we are uncharging a page using
> > > memcg_kmem_uncharge for a page in swap cache/page cache?
> > 
> > Hi Balbir,
> > 
> > Yes, in the normal page lifecycle, uncharge is done in page free time.
> > But in memory error handling case, in-use pages (i.e. swap cache and page
> > cache) are removed from normal path and they don't pass page freeing code.
> > So I think that this change is to keep the consistent charging for such a case.
> 
> I agree we should uncharge, but looking at the API name, it seems to
> be for kmem pages, why are we not using mem_cgroup_uncharge()? Am I missing
> something?

Thank you for pointing out.
Actually I had the same question and this surely looks strange.
But simply calling mem_cgroup_uncharge() here doesn't work because it
assumes that page_refcount(p) == 0, which is not true in hwpoison context.
We need some other clearer way or at least some justifying comment about
why this is ok.

- Naoya