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

[Balbir Singh <bsingharora@gmail.com>]

On Wed, 2017-04-26 at 04:46 +0000, Naoya Horiguchi wrote:
> 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.
>

We should call mem_cgroup_uncharge() after isolate_lru_page()/put_page().
We could check if page_count() is 0 or force if required (!MF_RECOVERED &&
!MF_DELAYED). We could even skip the VM_BUG_ON if the page is poisoned.

Balbir Singh. 

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