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

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

[Mike Yuan]

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@yhndnzj.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
 mm/shmem.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b9081b817d28..1b976414d6fa 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -131,8 +131,7 @@ struct shmem_options {
 #define SHMEM_SEEN_INODES 2
 #define SHMEM_SEEN_HUGE 4
 #define SHMEM_SEEN_INUMS 8
-#define SHMEM_SEEN_NOSWAP 16
-#define SHMEM_SEEN_QUOTA 32
+#define SHMEM_SEEN_QUOTA 16
 };
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
@@ -4677,7 +4676,6 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 				       "Turning off swap in unprivileged tmpfs mounts unsupported");
 		}
 		ctx->noswap = true;
-		ctx->seen |= SHMEM_SEEN_NOSWAP;
 		break;
 	case Opt_quota:
 		if (fc->user_ns != &init_user_ns)
@@ -4827,14 +4825,15 @@ static int shmem_reconfigure(struct fs_context *fc)
 		err = "Current inum too high to switch to 32-bit inums";
 		goto out;
 	}
-	if ((ctx->seen & SHMEM_SEEN_NOSWAP) && ctx->noswap && !sbinfo->noswap) {
+
+	/*
+	 * "noswap" doesn't use fsparam_flag_no, i.e. there's no "swap"
+	 * counterpart for (re-)enabling swap.
+	 */
+	if (ctx->noswap && !sbinfo->noswap) {
 		err = "Cannot disable swap on remount";
 		goto out;
 	}
-	if (!(ctx->seen & SHMEM_SEEN_NOSWAP) && !ctx->noswap && sbinfo->noswap) {
-		err = "Cannot enable swap on remount if it was disabled on first mount";
-		goto out;
-	}
 
 	if (ctx->seen & SHMEM_SEEN_QUOTA &&
 	    !sb_any_quota_loaded(fc->root->d_sb)) {

base-commit: 0d7bee10beeb59b1133bf5a4749b17a4ef3bbb01
-- 
2.51.1

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@yhndnzj.com>
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>
> ---

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

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

[David Hildenbrand (Red Hat)]

On 08.11.25 20:09, 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@yhndnzj.com>
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>
> ---

Makes sense to me

Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David