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

[Donet Tom <donettom@linux.ibm.com>]

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);
+       }
  }