linux-kernel - Re: [PATCH v2 09/12] mm, swap: use the swap table to track the swap count

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Message-ID: <aXsaNsUFCiHYrECk@yjaykim-PowerEdge-T330>
Date: Thu, 29 Jan 2026 17:28:38 +0900
From: YoungJun Park <youngjun.park@....com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>,
	Barry Song <baohua@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	David Hildenbrand <david@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	linux-kernel@...r.kernel.org, Chris Li <chrisl@...nel.org>,
	Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v2 09/12] mm, swap: use the swap table to track the swap
 count

On Wed, Jan 28, 2026 at 05:28:33PM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>

> index bfafa637c458..751430e2d2a5 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -37,6 +37,7 @@ struct swap_cluster_info {
>  	u8 flags;
>  	u8 order;
>  	atomic_long_t __rcu *table;	/* Swap table entries, see mm/swap_table.h */
> +	unsigned long *extend_table;	/* For large swap count, protected by ci->lock */

I assume using 'int *' is to save memory on 64-bit architectures (8 bytes ->
4 bytes per entry), which aligns with swp_tb_get_count() returning an int.

Regarding the extended reference table.
While I agree that a simple array is better for speed, readability and so on, the
2KB overhead (assuming SWAPFILE_CLUSTER=256) might be significant in
constrained environments when only a few entries overflow SWP_TB_COUNT_MAX.

Have you considered using a resizable hash table(example. or something others) 
instead? I am curious if this approach could be applicable 
as a future optimization after the current code is merged.

Thanks :)
Youngjun Park