linux-kernel - Re: [PATCH RFC] mm: ghost swapfile support for zswap

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Message-ID: <20251121114011.GA71307@cmpxchg.org>
Date: Fri, 21 Nov 2025 06:40:11 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Chris Li <chrisl@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Kairui Song <kasong@...cent.com>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>,
	Barry Song <baohua@...nel.org>, Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, pratmal@...gle.com,
	sweettea@...gle.com, gthelen@...gle.com, weixugc@...gle.com
Subject: Re: [PATCH RFC] mm: ghost swapfile support for zswap

On Fri, Nov 21, 2025 at 01:31:43AM -0800, Chris Li wrote:
> The current zswap requires a backing swapfile. The swap slot used
> by zswap is not able to be used by the swapfile. That waste swapfile
> space.
>
> The ghost swapfile is a swapfile that only contains the swapfile header
> for zswap. The swapfile header indicate the size of the swapfile. There
> is no swap data section in the ghost swapfile, therefore, no waste of
> swapfile space.  As such, any write to a ghost swapfile will fail. To
> prevents accidental read or write of ghost swapfile, bdev of
> swap_info_struct is set to NULL. Ghost swapfile will also set the SSD
> flag because there is no rotation disk access when using zswap.

Zswap is primarily a compressed cache for real swap on secondary
storage. It's indeed quite important that entries currently in zswap
don't occupy disk slots; but for a solution to this to be acceptable,
it has to work with the primary usecase and support disk writeback.

This direction is a dead-end. Please take a look at Nhat's swap
virtualization patches. They decouple zswap from disk geometry, while
still supporting writeback to an actual backend file.

Nacked-by: Johannes Weiner <hannes@...xchg.org>