rnull: enable configuration via `configfs`

Allow rust null block devices to be configured and instantiated via
`configfs`.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-13-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnull/Kconfig

@@ -4,7 +4,7 @@
 
 config BLK_DEV_RUST_NULL
 	tristate "Rust null block driver (Experimental)"
-	depends on RUST
+	depends on RUST && CONFIGFS_FS
 	help
 	  This is the Rust implementation of the null block driver. Like
 	  the C version, the driver allows the user to create virutal block

drivers/block/rnull/configfs.rs

@@ -0,0 +1,207 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::{NullBlkDevice, THIS_MODULE};
+use core::fmt::Write;
+use kernel::{
+    block::mq::gen_disk::{GenDisk, GenDiskBuilder},
+    c_str,
+    configfs::{self, AttributeOperations},
+    configfs_attrs, new_mutex,
+    page::PAGE_SIZE,
+    prelude::*,
+    str::{kstrtobool_bytes, CString},
+    sync::Mutex,
+};
+use pin_init::PinInit;
+
+pub(crate) fn subsystem() -> impl PinInit<kernel::configfs::Subsystem<Config>, Error> {
+    let item_type = configfs_attrs! {
+        container: configfs::Subsystem<Config>,
+        data: Config,
+        child: DeviceConfig,
+        attributes: [
+            features: 0,
+        ],
+    };
+
+    kernel::configfs::Subsystem::new(c_str!("rnull"), item_type, try_pin_init!(Config {}))
+}
+
+#[pin_data]
+pub(crate) struct Config {}
+
+#[vtable]
+impl AttributeOperations<0> for Config {
+    type Data = Config;
+
+    fn show(_this: &Config, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+        writer.write_str("blocksize,size,rotational\n")?;
+        Ok(writer.bytes_written())
+    }
+}
+
+#[vtable]
+impl configfs::GroupOperations for Config {
+    type Child = DeviceConfig;
+
+    fn make_group(
+        &self,
+        name: &CStr,
+    ) -> Result<impl PinInit<configfs::Group<DeviceConfig>, Error>> {
+        let item_type = configfs_attrs! {
+            container: configfs::Group<DeviceConfig>,
+            data: DeviceConfig,
+            attributes: [
+                // Named for compatibility with C null_blk
+                power: 0,
+                blocksize: 1,
+                rotational: 2,
+                size: 3,
+            ],
+        };
+
+        Ok(configfs::Group::new(
+            name.try_into()?,
+            item_type,
+            // TODO: cannot coerce new_mutex!() to impl PinInit<_, Error>, so put mutex inside
+            try_pin_init!( DeviceConfig {
+                data <- new_mutex!(DeviceConfigInner {
+                    powered: false,
+                    block_size: 4096,
+                    rotational: false,
+                    disk: None,
+                    capacity_mib: 4096,
+                    name: name.try_into()?,
+                }),
+            }),
+        ))
+    }
+}
+
+#[pin_data]
+pub(crate) struct DeviceConfig {
+    #[pin]
+    data: Mutex<DeviceConfigInner>,
+}
+
+#[pin_data]
+struct DeviceConfigInner {
+    powered: bool,
+    name: CString,
+    block_size: u32,
+    rotational: bool,
+    capacity_mib: u64,
+    disk: Option<GenDisk<NullBlkDevice>>,
+}
+
+#[vtable]
+impl configfs::AttributeOperations<0> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+
+        if this.data.lock().powered {
+            writer.write_str("1\n")?;
+        } else {
+            writer.write_str("0\n")?;
+        }
+
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        let power_op = kstrtobool_bytes(page)?;
+        let mut guard = this.data.lock();
+
+        if !guard.powered && power_op {
+            guard.disk = Some(NullBlkDevice::new(
+                &guard.name,
+                guard.block_size,
+                guard.rotational,
+                guard.capacity_mib,
+            )?);
+            guard.powered = true;
+        } else if guard.powered && !power_op {
+            drop(guard.disk.take());
+            guard.powered = false;
+        }
+
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<1> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+        writer.write_fmt(fmt!("{}\n", this.data.lock().block_size))?;
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        let text = core::str::from_utf8(page)?.trim();
+        let value = text.parse::<u32>().map_err(|_| EINVAL)?;
+
+        GenDiskBuilder::validate_block_size(value)?;
+        this.data.lock().block_size = value;
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<2> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+
+        if this.data.lock().rotational {
+            writer.write_str("1\n")?;
+        } else {
+            writer.write_str("0\n")?;
+        }
+
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        this.data.lock().rotational = kstrtobool_bytes(page)?;
+
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<3> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+        writer.write_fmt(fmt!("{}\n", this.data.lock().capacity_mib))?;
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        let text = core::str::from_utf8(page)?.trim();
+        let value = text.parse::<u64>().map_err(|_| EINVAL)?;
+
+        this.data.lock().capacity_mib = value;
+        Ok(())
+    }
+}

drivers/block/rnull/rnull.rs

@@ -1,28 +1,25 @@
 // SPDX-License-Identifier: GPL-2.0
 
 //! This is a Rust implementation of the C null block driver.
-//!
-//! Supported features:
-//!
-//! - blk-mq interface
-//! - direct completion
-//! - block size 4k
-//!
-//! The driver is not configurable.
+
+mod configfs;
 
 use kernel::{
-    alloc::flags,
-    block::mq::{
+    block::{
+        self,
+        mq::{
             self,
             gen_disk::{self, GenDisk},
             Operations, TagSet,
         },
+    },
     error::Result,
-    new_mutex, pr_info,
+    pr_info,
     prelude::*,
-    sync::{Arc, Mutex},
+    sync::Arc,
     types::ARef,
 };
+use pin_init::PinInit;
 
 module! {
     type: NullBlkModule,
@@ -35,33 +32,39 @@
 #[pin_data]
 struct NullBlkModule {
     #[pin]
-    _disk: Mutex<GenDisk<NullBlkDevice>>,
+    configfs_subsystem: kernel::configfs::Subsystem<configfs::Config>,
 }
 
 impl kernel::InPlaceModule for NullBlkModule {
     fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
         pr_info!("Rust null_blk loaded\n");
 
-        // Use a immediately-called closure as a stable `try` block
-        let disk = /* try */ (|| {
-            let tagset = Arc::pin_init(TagSet::new(1, 256, 1), flags::GFP_KERNEL)?;
-
-            gen_disk::GenDiskBuilder::new()
-                .capacity_sectors(4096 << 11)
-                .logical_block_size(4096)?
-                .physical_block_size(4096)?
-                .rotational(false)
-                .build(format_args!("rnullb{}", 0), tagset)
-        })();
-
         try_pin_init!(Self {
-            _disk <- new_mutex!(disk?, "nullb:disk"),
+            configfs_subsystem <- configfs::subsystem(),
         })
     }
 }
 
 struct NullBlkDevice;
 
+impl NullBlkDevice {
+    fn new(
+        name: &CStr,
+        block_size: u32,
+        rotational: bool,
+        capacity_mib: u64,
+    ) -> Result<GenDisk<Self>> {
+        let tagset = Arc::pin_init(TagSet::new(1, 256, 1), GFP_KERNEL)?;
+
+        gen_disk::GenDiskBuilder::new()
+            .capacity_sectors(capacity_mib << (20 - block::SECTOR_SHIFT))
+            .logical_block_size(block_size)?
+            .physical_block_size(block_size)?
+            .rotational(rotational)
+            .build(fmt!("{}", name.to_str()?), tagset)
+    }
+}
+
 #[vtable]
 impl Operations for NullBlkDevice {
     #[inline(always)]

rust/kernel/block/mq/gen_disk.rs

@@ -51,7 +51,7 @@ pub fn rotational(mut self, rotational: bool) -> Self {
 
     /// Validate block size by verifying that it is between 512 and `PAGE_SIZE`,
     /// and that it is a power of two.
-    fn validate_block_size(size: u32) -> Result {
+    pub fn validate_block_size(size: u32) -> Result {
         if !(512..=bindings::PAGE_SIZE as u32).contains(&size) || !size.is_power_of_two() {
             Err(error::code::EINVAL)
         } else {