linux-ext4 - Re: [PATCH 1/1] iomap: allow NULL swap info bdev when activating swapfile

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Message-ID: <20251030060008.GB12727@lst.de>
Date: Thu, 30 Oct 2025 07:00:08 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>, miklos@...redi.hu, brauner@...nel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/1] iomap: allow NULL swap info bdev when activating
 swapfile

On Wed, Oct 29, 2025 at 07:38:23AM -0700, Darrick J. Wong wrote:
> > > However, in the future there could be fuse+iomap filesystems that are
> > > block device based but don't set s_bdev.  In this case, sis::bdev will
> > > be set to NULL when we enter iomap_swapfile_activate, and we can pick
> > > up a bdev from the first iomap mapping that the filesystem provides.
> > 
> > Could, or will be?  I find the way the swapfiles work right now
> > disgusting to start with, but extending that bypass to fuse seems
> > even worse.
> 
> Yes, "Could", in the sense that a subsequent fuse patch wires up sending
> FUSE_IOMAP_BEGIN to the fuse server to ask for layouts for swapfiles,
> and the fuse server can reply with a mapping or EOPNOTSUPP to abort the
> swapon.  (There's a separate FUSE_IOMAP_IOEND req at deactivation time).

Maybe spell that out.

> "Already does" in the sense that fuse already supports swapfiles(!) if
> your filesystem implements FUSE_BMAP and attaches via fuseblk (aka
> ntfs3g).

Yikes.  This is just such an amazingly bad idea.