Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

[Michal Hocko <mhocko@suse.com>]

On Tue 09-03-21 12:32:55, Matthew Wilcox wrote:
> On Tue, Mar 09, 2021 at 10:02:00AM +0100, Michal Hocko wrote:
[...]
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 913c2b9e5c72..d44dea2b8d22 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3135,8 +3135,21 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> >  	if (memcg && !mem_cgroup_is_root(memcg)) {
> >  		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
> >  		if (!ret) {
> > +			int nr_pages = 1 << order;
> >  			page->memcg_data = (unsigned long)memcg |
> >  				MEMCG_DATA_KMEM;
> > +			
> > +			/*
> > +			 * Compound pages are normally split or freed
> > +			 * via their head pages so memcg_data in in the
> > +			 * head page should be sufficient but there
> > +			 * are exceptions to the rule (see __free_pages).
> > +			 * Non compound pages would need to copy memcg anyway.
> > +			 */
> > +			for (i = 1; i < nr_pages; i++) {
> > +				struct page * p = page + i;
> > +				p->memcg_data = page->memcg_data
> > +			}
> >  			return 0;
> 
> I would condition this loop on if (!(gfp & __GFP_COMP)), but yes, something
> along these lines.  I might phrase the comment a little differently ...
> 
> 			/*
> 			 * Compound pages are treated as a single unit,
> 			 * but non-compound pages can be freed individually
> 			 * so each page needs to have its memcg set to get
> 			 * the accounting right.
> 			 */

OK, I must have misunderstood your __free_pages fix then. I thought this
was about compound pages. Btw. again I forgot about css ref counting so
here is an updated version.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 913c2b9e5c72..ec2c705f38fa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3133,10 +3133,22 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
 
 	memcg = get_mem_cgroup_from_current();
 	if (memcg && !mem_cgroup_is_root(memcg)) {
-		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
+		int nr_pages = 1 << order;
+		ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
 		if (!ret) {
 			page->memcg_data = (unsigned long)memcg |
 				MEMCG_DATA_KMEM;
+			if (nr_pages > 1) {
+				/*
+				 * comment goes here
+				 */
+				for (i = 1; i < nr_pages; i++) {
+					struct page * p = page + i;
+					p->memcg_data = page->memcg_data
+				}
+				/* Head page reference from get_mem_cgroup_from_current */
+				css_get_many(&memcg->css, nr_pages - 1);
+			}
 			return 0;
 		}
 		css_put(&memcg->css);

-- 
Michal Hocko
SUSE Labs