6.12-stable review patch. If anyone has any objections, please let me know.

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From: Mike Yuan me@yhndnzj.com

commit 3cd1548a278c7d6a9bdef1f1866e7cf66bfd3518 upstream.

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 Link: https://patch.msgid.link/20251108190930.440685-1-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 Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/shmem.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-)

--- a/mm/shmem.c +++ b/mm/shmem.c @@ -127,8 +127,7 @@ 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 @@ -4330,7 +4329,6 @@ static int shmem_parse_one(struct fs_con "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) @@ -4480,14 +4478,15 @@ static int shmem_reconfigure(struct fs_c 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)) {