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

[Christian Brauner]

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@xxxxxxxxxxx>
> Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---

Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>