Add support for the badblocks_once feature, which automatically clears
bad blocks after they are encountered during I/O operations. This
matches the functionality in the C null_blk driver.

When badblocks_once is enabled:
- Bad blocks are checked during I/O requests as usual
- If a bad block is encountered, the I/O is marked as failed
- The bad block range is immediately cleared from the bad blocks table
- Subsequent I/O to the same sectors will succeed

This feature is useful for testing scenarios where bad blocks are
transient or where devices can recover from bad sectors after a single
access attempt.

The feature is configurable via the configfs badblocks_once attribute
and disabled by default, maintaining compatibility with existing
behavior.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 drivers/block/rnull/configfs.rs | 31 +++++++++++++++++++++++++++++++
 drivers/block/rnull/rnull.rs    | 21 +++++++++++++++------
 2 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/drivers/block/rnull/configfs.rs b/drivers/block/rnull/configfs.rs
index 61a76addf468b..a39691b39e374 100644
--- a/drivers/block/rnull/configfs.rs
+++ b/drivers/block/rnull/configfs.rs
@@ -103,6 +103,7 @@ fn make_group(
                 discard: 10,
                 no_sched:11,
                 badblocks: 12,
+                badblocks_once: 13,
             ],
         };
 
@@ -126,6 +127,7 @@ fn make_group(
                     discard: false,
                     no_sched: false,
                     bad_blocks: Arc::pin_init(BadBlocks::new(false), GFP_KERNEL)?,
+                    bad_blocks_once: false,
                 }),
             }),
             core::iter::empty(),
@@ -186,6 +188,7 @@ struct DeviceConfigInner {
     discard: bool,
     no_sched: bool,
     bad_blocks: Arc<BadBlocks>,
+    bad_blocks_once: bool,
 }
 
 #[vtable]
@@ -222,6 +225,7 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
                 discard: guard.discard,
                 no_sched: guard.no_sched,
                 bad_blocks: guard.bad_blocks.clone(),
+                bad_blocks_once: guard.bad_blocks_once,
             })?);
             guard.powered = true;
         } else if guard.powered && !power_op {
@@ -422,3 +426,30 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
         Ok(())
     }
 }
+
+#[vtable]
+impl configfs::AttributeOperations<13> 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().bad_blocks_once {
+            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().bad_blocks_once = kstrtobool_bytes(page)?;
+
+        Ok(())
+    }
+}
diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
index 861392c5b5841..0f569c5b65f77 100644
--- a/drivers/block/rnull/rnull.rs
+++ b/drivers/block/rnull/rnull.rs
@@ -161,6 +161,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
                     discard: *module_parameters::discard.value() != 0,
                     no_sched: *module_parameters::no_sched.value() != 0,
                     bad_blocks: Arc::pin_init(BadBlocks::new(false), GFP_KERNEL)?,
+                    bad_blocks_once: false,
                 })?;
                 disks.push(disk, GFP_KERNEL)?;
             }
@@ -188,6 +189,7 @@ struct NullBlkOptions<'a> {
     discard: bool,
     no_sched: bool,
     bad_blocks: Arc<BadBlocks>,
+    bad_blocks_once: bool,
 }
 struct NullBlkDevice;
 
@@ -206,6 +208,7 @@ fn new(options: NullBlkOptions<'_>) -> Result<GenDisk<Self>> {
             discard,
             no_sched,
             bad_blocks,
+            bad_blocks_once,
         } = options;
 
         let mut flags = mq::tag_set::Flags::default();
@@ -235,6 +238,7 @@ fn new(options: NullBlkOptions<'_>) -> Result<GenDisk<Self>> {
                 memory_backed,
                 block_size: block_size.into(),
                 bad_blocks,
+                bad_blocks_once,
             }),
             GFP_KERNEL,
         )?;
@@ -393,6 +397,7 @@ struct QueueData {
     memory_backed: bool,
     block_size: u64,
     bad_blocks: Arc<BadBlocks>,
+    bad_blocks_once: bool,
 }
 
 #[pin_data]
@@ -442,12 +447,16 @@ fn queue_rq(
         if queue_data.bad_blocks.enabled() {
             let start = rq.sector();
             let end = start + u64::from(rq.sectors());
-            if !matches!(
-                queue_data.bad_blocks.check(start..end),
-                badblocks::BlockStatus::None
-            ) {
-                rq.data_ref().error.store(1, ordering::Relaxed);
-            }
+            match queue_data.bad_blocks.check(start..end) {
+                badblocks::BlockStatus::None => {}
+                badblocks::BlockStatus::Acknowledged(range)
+                | badblocks::BlockStatus::Unacknowledged(range) => {
+                    rq.data_ref().error.store(1, ordering::Relaxed);
+                    if queue_data.bad_blocks_once {
+                        queue_data.bad_blocks.set_good(range)?;
+                    }
+                }
+            };
         }
 
         // TODO: Skip IO if bad block.

-- 
2.51.2