Re: [RFC] mm/swap.c: Enable promotion of unmapped MGLRU page cache pages

[Donet Tom]

On 1/15/25 18:55, Matthew Wilcox wrote:
> On Wed, Jan 15, 2025 at 06:06:25AM -0600, Donet Tom wrote:
> > An MGLRU page cache page is eligible for promotion when:
> >
> > 1. Memory Tiering and pagecache_promotion_enabled are enabled
> > 2. It resides in a lower memory tier.
> > 3. It is referenced.
> > 4. It is part of the working set.
> > 5. It belongs to the active list.
> > For MGLRU, the youngest generation and the youngest generation - 1
> > are treated as the active list.
> ... why do you only promote folios if they belong to a memcg?
I missed adding non memcg folios for promotion . I will add them too.
> > +promo_candid:
> > +	if (!folio_test_isolated(folio) &&
> > +		(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
> > +		numa_pagecache_promotion_enabled) {
> > +		memcg = folio_memcg(folio);
> > +		if (memcg) {
> > +			lruvec = mem_cgroup_lruvec(memcg, folio_pgdat(folio));
> > +			gen = folio_lru_gen(folio);
> > +
> > +			if ((gen < MAX_NR_GENS) && lru_gen_is_active(lruvec, gen))
> > +				promotion_candidate(folio);
> > +		}
> Should there be an 'else' clause here?  Or should it be:
>
> 		lruvec = mem_cgroup_lruvec(memcg, folio_pgdat(folio));
> 		gen = folio_lru_gen(folio);
> 		if ((gen < MAX_NR_GENS) && lru_gen_is_active(lruvec, gen))
> 			promotion_candidate(folio);
Yes, there should be an else block to handle the promotion of non memcg 
folios.

Does this approach look correct to you?

        } while (!try_cmpxchg(&folio->flags, &old_flags, new_flags));
+
+promo_candid:
+       if (!folio_test_isolated(folio) &&
+               (sysctl_numa_balancing_mode & 
NUMA_BALANCING_MEMORY_TIERING) &&
+               numa_pagecache_promotion_enabled) {
+               pgdat = folio_pgdat(folio);
+               gen = folio_lru_gen(folio);
+
+               if (mem_cgroup_disabled())
+                       lruvec = &pgdat->__lruvec;
+               else {
+                       memcg = folio_memcg(folio);
+                       lruvec = mem_cgroup_lruvec(memcg, pgdat);
+               }
+
+               if ((gen < MAX_NR_GENS) && lru_gen_is_active(lruvec, gen))
+                       promotion_candidate(folio);
+       }
 }