Re: [PATCH 2/4] mm: thp: make deferred split shrinker memcg aware

[Yang Shi]

On 6/12/19 3:09 AM, Kirill A. Shutemov wrote:
> On Tue, Jun 11, 2019 at 10:06:36PM -0700, Yang Shi wrote:
> > On 6/11/19 7:47 PM, Kirill A. Shutemov wrote:
> > > On Fri, Jun 07, 2019 at 02:07:37PM +0800, Yang Shi wrote:
> > > > +	/*
> > > > +	 * The THP may be not on LRU at this point, e.g. the old page of
> > > > +	 * NUMA migration.  And PageTransHuge is not enough to distinguish
> > > > +	 * with other compound page, e.g. skb, THP destructor is not used
> > > > +	 * anymore and will be removed, so the compound order sounds like
> > > > +	 * the only choice here.
> > > > +	 */
> > > > +	if (PageTransHuge(page) && compound_order(page) == HPAGE_PMD_ORDER) {
> > > What happens if the page is the same order as THP is not THP? Why removing
> > It may corrupt the deferred split queue since it is never added into the
> > list, but deleted here.
> >
> > > of destructor is required?
> > Due to the change to free_transhuge_page() (extracted deferred split queue
> > manipulation and moved before memcg uncharge since page->mem_cgroup is
> > needed), it just calls free_compound_page(). So, it sounds pointless to
> > still keep THP specific destructor.
> >
> > It looks there is not a good way to tell if the compound page is THP in
> > free_page path or not, we may keep the destructor just for this?
> Other option would be to move mem_cgroup_uncharge(page); from
> __page_cache_release() to destructors. Destructors will be able to
> call it as it fits.

Yes, it is an option. Since __page_cache_release() is called by 
__put_single_page() too, so mem_cgroup_uncharge() has to be called in 
both __put_single_page() and the desctructor (free_compound_page() which 
is called by both THP and other compound page except HugeTLB). But, it 
sounds acceptable IMHO.

>