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

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Message-ID: <Z4lT7uYviShCbXt6@casper.infradead.org>
Date: Thu, 16 Jan 2025 18:46:06 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Donet Tom <donettom@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Gregory Price <gourry@...rry.net>,
	Yu Zhao <yuzhao@...gle.com>, Ritesh Harjani <ritesh.list@...il.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	David Hildenbrand <david@...hat.com>,
	Huang Ying <ying.huang@...ux.alibaba.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC] mm/swap.c: Enable promotion of unmapped MGLRU page cache
 pages

On Thu, Jan 16, 2025 at 04:40:21PM +0530, Donet Tom wrote:
> > 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?

No.  Look at the implementation of folio_memcg(), and
mem_cgroup_lruvec() for that matter.

>         } 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);
> +       }
>  }
> 
>