linux-kernel - Re: [PATCH v4 12/19] mm, swap: use swap cache as the swap in synchronize layer

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Message-ID: <aUN1ou9otdqjSRls@MiWiFi-R3L-srv>
Date: Thu, 18 Dec 2025 11:31:46 +0800
From: Baoquan He <bhe@...hat.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Barry Song <baohua@...nel.org>, Chris Li <chrisl@...nel.org>,
	Nhat Pham <nphamcs@...il.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>,
	David Hildenbrand <david@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Youngjun Park <youngjun.park@....com>,
	Hugh Dickins <hughd@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Ying Huang <ying.huang@...ux.alibaba.com>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v4 12/19] mm, swap: use swap cache as the swap in
 synchronize layer

On 12/05/25 at 03:29am, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
> 
> Current swap in synchronization mostly uses the swap_map's
> SWAP_HAS_CACHE bit. Whoever sets the bit first does the actual
> work to swap in a folio.
> 
> This has been causing many issues as it's just a poor implementation
> of a bit lock. Raced users have no idea what is pinning a slot, so
> it has to loop with a schedule_timeout_uninterruptible(1), which is
> ugly and causes long-tailing or other performance issues. Besides,
> the abuse of SWAP_HAS_CACHE has been causing many other troubles for
> synchronization or maintenance.
> 
> This is the first step to remove this bit completely.
> 
> We have just removed all swap in paths that bypass the swap cache, and

I think we didn't remove swap in paths that bypss the swap cache, we
just add folio to swap cache in those swap in paths where swap cache is
bypassed.