linux-kernel - Re: [PATCH] shmem: fix tmpfs reconfiguration (remount) when noswap is set

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Message-ID: <20251112-vorbehalt-heizsysteme-3b11ba2b7ea3@brauner>
Date: Wed, 12 Nov 2025 10:33:46 +0100
From: Christian Brauner <brauner@...nel.org>
To: Mike Yuan <me@...dnzj.com>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>, 
	Hugh Dickins <hughd@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] shmem: fix tmpfs reconfiguration (remount) when noswap
 is set

On Sat, Nov 08, 2025 at 07:09:47PM +0000, Mike Yuan wrote:
> In systemd we're trying to switch the internal credentials setup logic
> to new mount API [1], and I noticed fsconfig(FSCONFIG_CMD_RECONFIGURE)
> consistently fails on tmpfs with noswap option. This can be trivially
> reproduced with the following:
> 
> ```
> int fs_fd = fsopen("tmpfs", 0);
> fsconfig(fs_fd, FSCONFIG_SET_FLAG, "noswap", NULL, 0);
> fsconfig(fs_fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> fsmount(fs_fd, 0, 0);
> fsconfig(fs_fd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);  <------ EINVAL
> ```
> 
> After some digging the culprit is shmem_reconfigure() rejecting
> !(ctx->seen & SHMEM_SEEN_NOSWAP) && sbinfo->noswap, which is bogus
> as ctx->seen serves as a mask for whether certain options are touched
> at all. On top of that, noswap option doesn't use fsparam_flag_no,
> hence it's not really possible to "reenable" swap to begin with.
> Drop the check and redundant SHMEM_SEEN_NOSWAP flag.
> 
> [1] https://github.com/systemd/systemd/pull/39637
> 
> Fixes: 2c6efe9cf2d7 ("shmem: add support to ignore swap")
> Signed-off-by: Mike Yuan <me@...dnzj.com>
> Cc: Luis Chamberlain <mcgrof@...nel.org>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: <stable@...r.kernel.org>
> ---

Reviewed-by: Christian Brauner <brauner@...nel.org>