[v3 for-next 3/3] Documentation: block: document error injection delay feature

[Md Haris Iqbal <haris.iqbal@linux.dev>]

Document the delay_us option: the delay happens above the driver, a
delayed bio is not run through the rules again, and holding a bio back
reorders it against bios submitted later.

Cc: Christoph Hellwig <hch@lst.de>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Md Haris Iqbal <haris.iqbal@linux.dev>
---
 Documentation/block/error-injection.rst | 39 +++++++++++++++++++++----
 1 file changed, 34 insertions(+), 5 deletions(-)

diff --git a/Documentation/block/error-injection.rst b/Documentation/block/error-injection.rst
index 81f31af82e65..6f7609b91d24 100644
--- a/Documentation/block/error-injection.rst
+++ b/Documentation/block/error-injection.rst
@@ -7,9 +7,9 @@ Configurable Error Injection
 Overview
 --------
 
-Configurable error injection allows injecting specific block layer status codes
-for sector ranges of a block device.  Errors can be injected unconditionally, or
-with a given probability.
+Configurable error injection allows injecting delays and/or specific block
+layer status codes for sector ranges of a block device by adding rules.  The
+rules can be configured to trigger unconditionally, or with a given probability.
 
 To use configurable error injection, CONFIG_BLK_ERROR_INJECTION must be enabled.
 
@@ -34,15 +34,36 @@ op=<string>		block layer operation this rule applies to.  This uses
 			the XYZ for each REQ_OP_XYZ operation, e.g. READ, WRITE
 			or DISCARD. Mandatory.
 status=<string>		Status to return.  This uses XYZ for each BLK_STS_XYZ
-			code, e.g. IOERR or MEDIUM. Mandatory.
+			code, e.g. IOERR or MEDIUM.  Mandatory unless delay_us
+			is given.
 start=<number>		First block layer sector the rule applies to.
 			Optional, defaults to 0.
 nr_sectors=<number>	Number of sectors this rule applies.
 			Optional, defaults to the remainder of the device.
-chance=<number>		Only return a failure with a likelihood of 1/chance.
+chance=<number>		Only apply the rule with a likelihood of 1/chance.
 			Optional, defaults to 1 (always).
+delay_us=<number>	Hold the bio back for this many microseconds.  Without
+			status the bio is then submitted to the device as
+			usual.  With status it is failed once the delay has
+			expired.  Bios with REQ_NOWAIT set are never delayed.
+			Optional, defaults to 0 (no delay).  Values above 600
+			seconds are rejected.
 ===================	=======================================================
 
+Delays
+------
+
+A delayed bio is held above the driver, so the device never sees a slow I/O.
+A delay does not reach the blk-mq timeout handler or SCSI error handling.
+
+A bio that matched a delay rule is not evaluated against the rules again, even
+after it is split and resubmitted internally.  Removing a rule does not release
+bios it is already delaying.
+
+Holding a bio back reorders it against bios submitted later.  On zoned devices
+this breaks sequential write ordering and the block layer fails the
+out-of-order writes, so only delay reads there.
+
 Example
 -------
 
@@ -54,6 +75,14 @@ Return BLK_STS_MEDIUM for every write to /dev/nvme0n1:
 
 	$ echo 'add,op=WRITE,start=0,status=MEDIUM' > /sys/kernel/debug/block/nvme0n1/error_injection
 
+Delay every read of /dev/nvme0n1 by 10 milliseconds, then issue it normally:
+
+	$ echo 'add,op=READ,delay_us=10000' > /sys/kernel/debug/block/nvme0n1/error_injection
+
+Fail one in 100 writes with BLK_STS_TIMEOUT, but only after 30 seconds:
+
+	$ echo 'add,op=WRITE,status=TIMEOUT,chance=100,delay_us=30000000' > /sys/kernel/debug/block/nvme0n1/error_injection
+
 Remove all rules for /dev/nvme0n1:
 
 	$ echo 'removeall' > /sys/kernel/debug/block/nvme0n1/error_injection
-- 
2.53.0