Re: [PATCH V4 1/2] daxctl: Add support for famfs mode

Dave Jiang
2026-02-19T21:47:45Z

On 1/18/26 3:36 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
>
> Putting a daxdev in famfs mode means binding it to fsdev_dax.ko
> (drivers/dax/fsdev.c). Finding a daxdev bound to fsdev_dax means
> it is in famfs mode.
>
> The test is added to the destructive test suite since it
> modifies device modes.
>
> With devdax, famfs, and system-ram modes, the previous logic that assumed
> 'not in mode X means in mode Y' needed to get slightly more complicated
>
> Add explicit mode detection functions:
> - daxctl_dev_is_famfs_mode(): check if bound to fsdev_dax driver
> - daxctl_dev_is_devdax_mode(): check if bound to device_dax driver
>
> Fix mode transition logic in device.c:
> - disable_devdax_device(): verify device is actually in devdax mode
> - disable_famfs_device(): verify device is actually in famfs mode
> - All reconfig_mode_*() functions now explicitly check each mode
> - Handle unknown mode with error instead of wrong assumption
>
> Modify json.c to show 'unknown' if device is not in a recognized mode.
>
> Signed-off-by: John Groves <john@groves.net>
> ---
> daxctl/device.c | 126 ++++++++++++++++++++++++++++++---
> daxctl/json.c | 6 +-
> daxctl/lib/libdaxctl-private.h | 2 +
> daxctl/lib/libdaxctl.c | 77 ++++++++++++++++++++
> daxctl/lib/libdaxctl.sym | 7 ++
> daxctl/libdaxctl.h | 3 +
> 6 files changed, 210 insertions(+), 11 deletions(-)
>
> diff --git a/daxctl/device.c b/daxctl/device.c
> index e3993b1..14e1796 100644
> --- a/daxctl/device.c
> +++ b/daxctl/device.c
> @@ -42,6 +42,7 @@ enum dev_mode {
> DAXCTL_DEV_MODE_UNKNOWN,
> DAXCTL_DEV_MODE_DEVDAX,
> DAXCTL_DEV_MODE_RAM,
> + DAXCTL_DEV_MODE_FAMFS,
> };
>
> struct mapping {
> @@ -471,6 +472,13 @@ static const char *parse_device_options(int argc, const char **argv,
> "--no-online is incompatible with --mode=devdax\n");
> rc = -EINVAL;
> }
> + } else if (strcmp(param.mode, "famfs") == 0) {
> + reconfig_mode = DAXCTL_DEV_MODE_FAMFS;
> + if (param.no_online) {
> + fprintf(stderr,
> + "--no-online is incompatible with --mode=famfs\n");
> + rc = -EINVAL;
> + }
> }
> break;
> case ACTION_CREATE:
> @@ -696,8 +704,42 @@ static int disable_devdax_device(struct daxctl_dev *dev)
> int rc;
>
> if (mem) {
> - fprintf(stderr, "%s was already in system-ram mode\n",
> - devname);
> + fprintf(stderr, "%s is in system-ram mode\n", devname);
> + return 1;
> + }
> + if (daxctl_dev_is_famfs_mode(dev)) {
> + fprintf(stderr, "%s is in famfs mode\n", devname);
> + return 1;
> + }
> + if (!daxctl_dev_is_devdax_mode(dev)) {
> + fprintf(stderr, "%s is not in devdax mode\n", devname);
> + return 1;
> + }
> + rc = daxctl_dev_disable(dev);
> + if (rc) {
> + fprintf(stderr, "%s: disable failed: %s\n",
> + daxctl_dev_get_devname(dev), strerror(-rc));
> + return rc;
> + }
> + return 0;
> +}
> +
> +static int disable_famfs_device(struct daxctl_dev *dev)
> +{
> + struct daxctl_memory *mem = daxctl_dev_get_memory(dev);
> + const char *devname = daxctl_dev_get_devname(dev);
> + int rc;
> +
> + if (mem) {
> + fprintf(stderr, "%s is in system-ram mode\n", devname);
> + return 1;
> + }
> + if (daxctl_dev_is_devdax_mode(dev)) {
> + fprintf(stderr, "%s is in devdax mode\n", devname);
> + return 1;
> + }
> + if (!daxctl_dev_is_famfs_mode(dev)) {
> + fprintf(stderr, "%s is not in famfs mode\n", devname);
> return 1;
> }
> rc = daxctl_dev_disable(dev);
> @@ -711,6 +753,7 @@ static int disable_devdax_device(struct daxctl_dev *dev)
>
> static int reconfig_mode_system_ram(struct daxctl_dev *dev)
> {
> + struct daxctl_memory *mem = daxctl_dev_get_memory(dev);
> const char *devname = daxctl_dev_get_devname(dev);
> int rc, skip_enable = 0;
>
> @@ -724,11 +767,21 @@ static int reconfig_mode_system_ram(struct daxctl_dev *dev)
> }
>
> if (daxctl_dev_is_enabled(dev)) {
> - rc = disable_devdax_device(dev);
> - if (rc < 0)
> - return rc;
> - if (rc > 0)
> + if (mem) {
> + /* already in system-ram mode */
> skip_enable = 1;
> + } else if (daxctl_dev_is_famfs_mode(dev)) {
> + rc = disable_famfs_device(dev);
> + if (rc)
> + return rc;
> + } else if (daxctl_dev_is_devdax_mode(dev)) {
> + rc = disable_devdax_device(dev);
> + if (rc)
> + return rc;
> + } else {
> + fprintf(stderr, "%s: unknown mode\n", devname);
> + return -EINVAL;
> + }
> }
>
> if (!skip_enable) {
> @@ -750,7 +803,7 @@ static int disable_system_ram_device(struct daxctl_dev *dev)
> int rc;
>
> if (!mem) {
> - fprintf(stderr, "%s was already in devdax mode\n", devname);
> + fprintf(stderr, "%s is not in system-ram mode\n", devname);
> return 1;
> }
>
> @@ -786,12 +839,28 @@ static int disable_system_ram_device(struct daxctl_dev *dev)
>
> static int reconfig_mode_devdax(struct daxctl_dev *dev)
> {
> + struct daxctl_memory *mem = daxctl_dev_get_memory(dev);
> + const char *devname = daxctl_dev_get_devname(dev);
> int rc;
>
> if (daxctl_dev_is_enabled(dev)) {
> - rc = disable_system_ram_device(dev);
> - if (rc)
> - return rc;
> + if (mem) {
> + rc = disable_system_ram_device(dev);
> + if (rc)
> + return rc;
> + } else if (daxctl_dev_is_famfs_mode(dev)) {
> + rc = disable_famfs_device(dev);
> + if (rc)
> + return rc;
> + } else if (daxctl_dev_is_devdax_mode(dev)) {
> + /* already in devdax mode, just re-enable */
> + rc = daxctl_dev_disable(dev);
> + if (rc)
> + return rc;
> + } else {
> + fprintf(stderr, "%s: unknown mode\n", devname);
> + return -EINVAL;
> + }
> }
>
> rc = daxctl_dev_enable_devdax(dev);
> @@ -801,6 +870,40 @@ static int reconfig_mode_devdax(struct daxctl_dev *dev)
> return 0;
> }
>
> +static int reconfig_mode_famfs(struct daxctl_dev *dev)
> +{
> + struct daxctl_memory *mem = daxctl_dev_get_memory(dev);
> + const char *devname = daxctl_dev_get_devname(dev);
> + int rc;
> +
> + if (daxctl_dev_is_enabled(dev)) {
> + if (mem) {
> + fprintf(stderr,
> + "%s is in system-ram mode, must be in devdax mode to convert to famfs\n",
> + devname);
> + return -EINVAL;
> + } else if (daxctl_dev_is_famfs_mode(dev)) {
> + /* already in famfs mode, just re-enable */
> + rc = daxctl_dev_disable(dev);
> + if (rc)
> + return rc;
> + } else if (daxctl_dev_is_devdax_mode(dev)) {
> + rc = disable_devdax_device(dev);
> + if (rc)
> + return rc;
> + } else {
> + fprintf(stderr, "%s: unknown mode\n", devname);
> + return -EINVAL;
> + }
> + }
> +
> + rc = daxctl_dev_enable_famfs(dev);
> + if (rc)
> + return rc;
> +
> + return 0;
> +}
> +
> static int do_create(struct daxctl_region *region, long long val,
> struct json_object **jdevs)
> {
> @@ -887,6 +990,9 @@ static int do_reconfig(struct daxctl_dev *dev, enum dev_mode mode,
> case DAXCTL_DEV_MODE_DEVDAX:
> rc = reconfig_mode_devdax(dev);
> break;
> + case DAXCTL_DEV_MODE_FAMFS:
> + rc = reconfig_mode_famfs(dev);
> + break;
> default:
> fprintf(stderr, "%s: unknown mode requested: %d\n",
> devname, mode);
> diff --git a/daxctl/json.c b/daxctl/json.c
> index 3cbce9d..01f139b 100644
> --- a/daxctl/json.c
> +++ b/daxctl/json.c
> @@ -48,8 +48,12 @@ struct json_object *util_daxctl_dev_to_json(struct daxctl_dev *dev,
>
> if (mem)
> jobj = json_object_new_string("system-ram");
> - else
> + else if (daxctl_dev_is_famfs_mode(dev))
> + jobj = json_object_new_string("famfs");
> + else if (daxctl_dev_is_devdax_mode(dev))
> jobj = json_object_new_string("devdax");
> + else
> + jobj = json_object_new_string("unknown");
> if (jobj)
> json_object_object_add(jdev, "mode", jobj);
>
> diff --git a/daxctl/lib/libdaxctl-private.h b/daxctl/lib/libdaxctl-private.h
> index ae45311..0bb73e8 100644
> --- a/daxctl/lib/libdaxctl-private.h
> +++ b/daxctl/lib/libdaxctl-private.h
> @@ -21,12 +21,14 @@ static const char *dax_subsystems[] = {
> enum daxctl_dev_mode {
> DAXCTL_DEV_MODE_DEVDAX = 0,
> DAXCTL_DEV_MODE_RAM,
> + DAXCTL_DEV_MODE_FAMFS,
> DAXCTL_DEV_MODE_END,
> };
>
> static const char *dax_modules[] = {
> [DAXCTL_DEV_MODE_DEVDAX] = "device_dax",
> [DAXCTL_DEV_MODE_RAM] = "kmem",
> + [DAXCTL_DEV_MODE_FAMFS] = "fsdev_dax",
> };
>
> enum memory_op {
> diff --git a/daxctl/lib/libdaxctl.c b/daxctl/lib/libdaxctl.c
> index b7fa0de..0a6cbfe 100644
> --- a/daxctl/lib/libdaxctl.c
> +++ b/daxctl/lib/libdaxctl.c
> @@ -418,6 +418,78 @@ DAXCTL_EXPORT int daxctl_dev_is_system_ram_capable(struct daxctl_dev *dev)
> return false;
> }
>
> +/*
> + * Check if device is currently in famfs mode (bound to fsdev_dax driver)
> + */
> +DAXCTL_EXPORT int daxctl_dev_is_famfs_mode(struct daxctl_dev *dev)

Should this return bool?

> +{
> + const char *devname = daxctl_dev_get_devname(dev);
> + struct daxctl_ctx *ctx = daxctl_dev_get_ctx(dev);
> + char *mod_path, *mod_base;
> + char path[200];
> + const int len = sizeof(path);
> +
> + if (!device_model_is_dax_bus(dev))
> + return false;
> +
> + if (!daxctl_dev_is_enabled(dev))
> + return false;
> +
> + if (snprintf(path, len, "%s/driver", dev->dev_path) >= len) {
> + err(ctx, "%s: buffer too small!\n", devname);
> + return false;
> + }
> +
> + mod_path = realpath(path, NULL);
> + if (!mod_path)
> + return false;
> +
> + mod_base = basename(mod_path);
> + if (strcmp(mod_base, dax_modules[DAXCTL_DEV_MODE_FAMFS]) == 0) {
> + free(mod_path);
> + return true;
> + }
> +
> + free(mod_path);
> + return false;
> +}
> +
> +/*
> + * Check if device is currently in devdax mode (bound to device_dax driver)
> + */
> +DAXCTL_EXPORT int daxctl_dev_is_devdax_mode(struct daxctl_dev *dev)

return bool?

DJ

> +{
> + const char *devname = daxctl_dev_get_devname(dev);
> + struct daxctl_ctx *ctx = daxctl_dev_get_ctx(dev);
> + char *mod_path, *mod_base;
> + char path[200];
> + const int len = sizeof(path);
> +
> + if (!device_model_is_dax_bus(dev))
> + return false;
> +
> + if (!daxctl_dev_is_enabled(dev))
> + return false;
> +
> + if (snprintf(path, len, "%s/driver", dev->dev_path) >= len) {
> + err(ctx, "%s: buffer too small!\n", devname);
> + return false;
> + }
> +
> + mod_path = realpath(path, NULL);
> + if (!mod_path)
> + return false;
> +
> + mod_base = basename(mod_path);
> + if (strcmp(mod_base, dax_modules[DAXCTL_DEV_MODE_DEVDAX]) == 0) {
> + free(mod_path);
> + return true;
> + }
> +
> + free(mod_path);
> + return false;
> +}
> +
> /*
> * This checks for the device to be in system-ram mode, so calling
> * daxctl_dev_get_memory() on a devdax mode device will always return NULL.
> @@ -982,6 +1054,11 @@ DAXCTL_EXPORT int daxctl_dev_enable_ram(struct daxctl_dev *dev)
> return daxctl_dev_enable(dev, DAXCTL_DEV_MODE_RAM);
> }
>
> +DAXCTL_EXPORT int daxctl_dev_enable_famfs(struct daxctl_dev *dev)
> +{
> + return daxctl_dev_enable(dev, DAXCTL_DEV_MODE_FAMFS);
> +}
> +
> DAXCTL_EXPORT int daxctl_dev_disable(struct daxctl_dev *dev)
> {
> const char *devname = daxctl_dev_get_devname(dev);
> diff --git a/daxctl/lib/libdaxctl.sym b/daxctl/lib/libdaxctl.sym
> index 3098811..2a812c6 100644
> --- a/daxctl/lib/libdaxctl.sym
> +++ b/daxctl/lib/libdaxctl.sym
> @@ -104,3 +104,10 @@ LIBDAXCTL_10 {
> global:
> daxctl_dev_is_system_ram_capable;
> } LIBDAXCTL_9;
> +
> +LIBDAXCTL_11 {
> +global:
> + daxctl_dev_enable_famfs;
> + daxctl_dev_is_famfs_mode;
> + daxctl_dev_is_devdax_mode;
> +} LIBDAXCTL_10;
> diff --git a/daxctl/libdaxctl.h b/daxctl/libdaxctl.h
> index 53c6bbd..84fcdb4 100644
> --- a/daxctl/libdaxctl.h
> +++ b/daxctl/libdaxctl.h
> @@ -72,12 +72,15 @@ int daxctl_dev_is_enabled(struct daxctl_dev *dev);
> int daxctl_dev_disable(struct daxctl_dev *dev);
> int daxctl_dev_enable_devdax(struct daxctl_dev *dev);
> int daxctl_dev_enable_ram(struct daxctl_dev *dev);
> +int daxctl_dev_enable_famfs(struct daxctl_dev *dev);
> int daxctl_dev_get_target_node(struct daxctl_dev *dev);
> int daxctl_dev_will_auto_online_memory(struct daxctl_dev *dev);
> int daxctl_dev_has_online_memory(struct daxctl_dev *dev);
>
> struct daxctl_memory;
> int daxctl_dev_is_system_ram_capable(struct daxctl_dev *dev);
> +int daxctl_dev_is_famfs_mode(struct daxctl_dev *dev);
> +int daxctl_dev_is_devdax_mode(struct daxctl_dev *dev);
> struct daxctl_memory *daxctl_dev_get_memory(struct daxctl_dev *dev);
> struct daxctl_dev *daxctl_memory_get_dev(struct daxctl_memory *mem);
> const char *daxctl_memory_get_node_path(struct daxctl_memory *mem);