Fault injection capabilities infrastructure
  ===========================================
  
  See also drivers/md/faulty.c and "every_nth" module option for scsi_debug.
  
  
  Available fault injection capabilities
  --------------------------------------
  
  o failslab
  
    injects slab allocation failures. (kmalloc(), kmem_cache_alloc(), ...)
  
  o fail_page_alloc
  
    injects page allocation failures. (alloc_pages(), get_free_pages(), ...)
  
  o fail_make_request

    injects disk IO errors on devices permitted by setting

    /sys/block/<device>/make-it-fail or
    /sys/block/<device>/<partition>/make-it-fail. (generic_make_request())

  o fail_mmc_request
  
    injects MMC data errors on devices permitted by setting
    debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request

  Configure fault-injection capabilities behavior
  -----------------------------------------------
  
  o debugfs entries
  
  fault-inject-debugfs kernel module provides some debugfs entries for runtime
  configuration of fault-injection capabilities.

  - /sys/kernel/debug/fail*/probability:

  
  	likelihood of failure injection, in percent.
  	Format: <percent>

  	Note that one-failure-per-hundred is a very high error rate
  	for some testcases.  Consider setting probability=100 and configure

  	/sys/kernel/debug/fail*/interval for such testcases.

  - /sys/kernel/debug/fail*/interval:

  
  	specifies the interval between failures, for calls to
  	should_fail() that pass all the other tests.
  
  	Note that if you enable this, by setting interval>1, you will
  	probably want to set probability=100.

  - /sys/kernel/debug/fail*/times:

  
  	specifies how many times failures may happen at most.
  	A value of -1 means "no limit".

  - /sys/kernel/debug/fail*/space:

  
  	specifies an initial resource "budget", decremented by "size"
  	on each call to should_fail(,size).  Failure injection is
  	suppressed until "space" reaches zero.

  - /sys/kernel/debug/fail*/verbose

  
  	Format: { 0 | 1 | 2 }

  	specifies the verbosity of the messages when failure is
  	injected.  '0' means no messages; '1' will print only a single
  	log line per failure; '2' will print a call trace too -- useful
  	to debug the problems revealed by fault injection.

  - /sys/kernel/debug/fail*/task-filter:

  	Format: { 'Y' | 'N' }
  	A value of 'N' disables filtering by process (default).

  	Any positive value limits failures to only processes indicated by
  	/proc/<pid>/make-it-fail==1.

  - /sys/kernel/debug/fail*/require-start:
  - /sys/kernel/debug/fail*/require-end:
  - /sys/kernel/debug/fail*/reject-start:
  - /sys/kernel/debug/fail*/reject-end:

  
  	specifies the range of virtual addresses tested during
  	stacktrace walking.  Failure is injected only if some caller

  	in the walked stacktrace lies within the required range, and
  	none lies within the rejected range.
  	Default required range is [0,ULONG_MAX) (whole of virtual address space).
  	Default rejected range is [0,0).

  - /sys/kernel/debug/fail*/stacktrace-depth:

  
  	specifies the maximum stacktrace depth walked during search

  	for a caller within [require-start,require-end) OR
  	[reject-start,reject-end).

  - /sys/kernel/debug/fail_page_alloc/ignore-gfp-highmem:

  	Format: { 'Y' | 'N' }
  	default is 'N', setting it to 'Y' won't inject failures into

  	highmem/user allocations.

  - /sys/kernel/debug/failslab/ignore-gfp-wait:
  - /sys/kernel/debug/fail_page_alloc/ignore-gfp-wait:

  	Format: { 'Y' | 'N' }
  	default is 'N', setting it to 'Y' will inject failures

  	only into non-sleep allocations (GFP_ATOMIC allocations).

  - /sys/kernel/debug/fail_page_alloc/min-order:

  
  	specifies the minimum page allocation order to be injected
  	failures.

  o Boot option
  
  In order to inject faults while debugfs is not available (early boot time),
  use the boot option:
  
  	failslab=
  	fail_page_alloc=

  	fail_make_request=

  	mmc_core.fail_request=<interval>,<probability>,<space>,<times>

  
  How to add new fault injection capability
  -----------------------------------------
  
  o #include <linux/fault-inject.h>
  
  o define the fault attributes
  
    DECLARE_FAULT_INJECTION(name);
  
    Please see the definition of struct fault_attr in fault-inject.h
    for details.

  o provide a way to configure fault attributes

  
  - boot option
  
    If you need to enable the fault injection capability from boot time, you can

    provide boot option to configure it. There is a helper function for it:

  	setup_fault_attr(attr, str);

  
  - debugfs entries
  
    failslab, fail_page_alloc, and fail_make_request use this way.

    Helper functions:

  	fault_create_debugfs_attr(name, parent, attr);

  
  - module parameters
  
    If the scope of the fault injection capability is limited to a
    single kernel module, it is better to provide module parameters to
    configure the fault attributes.
  
  o add a hook to insert failures

    Upon should_fail() returning true, client code should inject a failure.

  	should_fail(attr, size);

  
  Application Examples
  --------------------

  o Inject slab allocation failures into module init/exit code

  #!/bin/bash

  FAILTYPE=failslab

  echo Y > /sys/kernel/debug/$FAILTYPE/task-filter
  echo 10 > /sys/kernel/debug/$FAILTYPE/probability
  echo 100 > /sys/kernel/debug/$FAILTYPE/interval
  echo -1 > /sys/kernel/debug/$FAILTYPE/times
  echo 0 > /sys/kernel/debug/$FAILTYPE/space
  echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
  echo 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait

  faulty_system()

  {

  	bash -c "echo 1 > /proc/self/make-it-fail && exec $*"

  }

  if [ $# -eq 0 ]
  then
  	echo "Usage: $0 modulename [ modulename ... ]"
  	exit 1
  fi
  
  for m in $*
  do
  	echo inserting $m...
  	faulty_system modprobe $m

  	echo removing $m...
  	faulty_system modprobe -r $m
  done

  
  ------------------------------------------------------------------------------

  o Inject page allocation failures only for a specific module

  #!/bin/bash

  FAILTYPE=fail_page_alloc
  module=$1

  if [ -z $module ]
  then
  	echo "Usage: $0 <modulename>"
  	exit 1
  fi

  modprobe $module

  if [ ! -d /sys/module/$module/sections ]
  then
  	echo Module $module is not loaded
  	exit 1
  fi

  cat /sys/module/$module/sections/.text > /sys/kernel/debug/$FAILTYPE/require-start
  cat /sys/module/$module/sections/.data > /sys/kernel/debug/$FAILTYPE/require-end

  echo N > /sys/kernel/debug/$FAILTYPE/task-filter
  echo 10 > /sys/kernel/debug/$FAILTYPE/probability
  echo 100 > /sys/kernel/debug/$FAILTYPE/interval
  echo -1 > /sys/kernel/debug/$FAILTYPE/times
  echo 0 > /sys/kernel/debug/$FAILTYPE/space
  echo 2 > /sys/kernel/debug/$FAILTYPE/verbose
  echo 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-wait
  echo 1 > /sys/kernel/debug/$FAILTYPE/ignore-gfp-highmem
  echo 10 > /sys/kernel/debug/$FAILTYPE/stacktrace-depth

  trap "echo 0 > /sys/kernel/debug/$FAILTYPE/probability" SIGINT SIGTERM EXIT

  
  echo "Injecting errors into the module $module... (interrupt to stop)"
  sleep 1000000